Unit Five: Reactions

Coefficients

A chemical equation lists more than just the chemical formulas of the reactants and products. Notice the numbers next to the formulas for sodium hydroxide (NaOH) and sodium chloride (NaCl) above. What do they mean? These numbers, called coefficients, represent the number of units of each substance taking part in a reaction. When no coefficient appears before a substance in a balanced equation, a coefficient of one is assumed. In the equation above, for example, one unit of nickel(II) chloride (NiC l 2 ) combined with two units of sodium hydroxide (NaOH). The products were one unit of nickel(II) hydroxide (Ni(OH ) 2 ) and two units of sodium chloride (NaCl). Remember that according to the law of conservation of mass, atoms are rearranged but never destroyed in a chemical reaction. Notice in the equation above that there are equal numbers of each type of atom on each side of the arrow. Knowing the number of units of reactants enables chemists to add the correct amounts of reactants to a reaction. Also, these coefficients tell them exactly how much product will form. For example, you would react one unit of NiC l 2 with two units of NaOH to produce one unit of Ni(OH ) 2 and two units of NaCl. You can see this reaction in Figure 3, above.

Sharing electrons

A hydrogen atom has one electron in its outer energy level. So, it needs one electron to fill its outer energy level. An oxygen atom has six electrons in its outer energy level. It needs two electrons for its outer level to be stable with eight electrons. Hydrogen and oxygen become stable and form bonds in a different way than sodium and chlorine. Instead of gaining or losing electrons, they share them. Figure 6 shows how hydrogen and oxygen share electrons to achieve a more stable arrangement of electrons to form water. Chemical bond formation When atoms gain, lose, or share electrons, an attraction forms between the atoms, pulling them together to form a compound. This attraction is called a chemical bond. A chemical bond is the force that holds atoms together in a compound.

Damage from nuclear decay

Alpha, beta, and gamma radiation can all be dangerous to human tissue. Biological molecules inside your body are large and easily damaged. A single alpha or beta particle or gamma ray can damage many fragile biological molecules. Damage from radiation can cause cells to function improperly, leading to illness and disease. Transmutation Another result of nuclear decay is transmutation. Transmutation occurs during alpha and beta decay. Transmutation is the process of changing one element into a different element. During alpha decay, a nucleus loses two protons and two neutrons. As a result, the resulting nucleus has two fewer protons and two less neutrons. The nucleus transmutes into an entirely different element. The new element has an atomic number two less than that of the original element. The mass number of the new element is four less than that of the original element. The top panel of Figure 7 shows a transmutation caused by alpha decay. During beta decay, a neutron emits an electron and becomes a proton. The resulting nucleus has one more proton. It becomes the element with an atomic number one greater than that of the original element. However, the total number of protons and neutrons does not change during beta decay. Therefore, the mass number of the new element is the same as that of the original element. The bottom panel of Figure 7 shows a transmutation caused by beta decay.

Conservation of Energy in

Chemical Reactions You learned in an earlier chapter that energy can change from one form to another, but the total amount of energy never changes. This principle is usually stated as the law of conservation of energy. Does the total amount of energy remain constant in chemical reactions, too? Consider the exergonic burning of methane (C H 4 ), the major component of natural gas, as described by the following equation. Note that energy is included as a product. C H 4 (g) + 2 O 2 (g) → C O 2 (g) + 2 H 2 O(g) + energy During this process, some of the chemical energy of the reactants is released as thermal energy and light. However, the sum of the energy released and the chemical energy of the products is exactly equal to the chemical energy of the reactants in an exergonic chemical reaction. chemical energy = chemical energy + energy released of reactants of products So the total amount of energy before and after the reaction remains the same. Similarly, the total amount of energy remains the same in endergonic chemical reactions. Summarizing, the law of conservation of energy applies to chemical reactions, as well as to other types of energy transformations

Lavoisier and the Conservation of Mass

Chemical reactions take place all around you, and even within you. A chemical reaction is a change in which one or more substances are converted into new substances. The starting substances that react are called reactants. The new substances produced are called products. This relationship between reactants and products can be written as follows: reactants → products. By the 1770s, the pseudoscience of alchemy was starting to be replaced by chemistry. While alchemy imitated science, alchemists did not provide science-based explanations about the natural world. However scientists, such as the French chemist Antoine Lavoisier, studied chemical reactions using scientific methods. As a result of such study, Lavoisier established that the total mass of the products always equals the total mass of the reactants. This principle is demonstrated in Figure 1. The mystery of exactly what happens when substances change made Lavoisier curious. In one experiment, Lavoisier placed a carefully measured mass of solid mercury(II) oxide into a sealed container. When he heated this container, he noted a dramatic change. The red powder transformed into a silvery liquid that he recognized as mercury metal, and a gas was produced. When he determined the mass of the liquid mercury and the gas, their combined masses were exactly the same as the mass of the red powder he started with. Lavoisier's experiment also established that the gas produced by heating mercury(II) oxide was a component of air. He did this by heating mercury metal with air and saw that a portion of the air combined with the mercury metal to produce red mercury(II) oxide. mercury(II) oxide oxygen plus mercury 10.0 g = 0.7 g + 9.3 g Notice that the mass of the mercury(II) oxide reactant is equal to the combined masses of the mercury metal and the oxygen gas products. Hundreds of experiments carried out in Lavoisier's laboratory confirmed that matter is not created or destroyed, but conserved in a chemical reaction. This important principle became known as the law of conservation of mass. This means that the total starting mass of all reactants is equals the total final mass of all products.

Naming Binary Covalent Compounds

Covalent compounds are those formed between elements that are nonmetals. Some pairs of nonmetals can form more than one compound with each other. For example, nitrogen and oxygen can combine to form a number of different compounds, including N 2 O, NO, N O 2 , and N 2 O 5 . In the system of naming using oxidation numbers that you have just learned, each of these compounds would be called nitrogen oxide. From that name, you would not know the composition of the compound. So, a different system of naming must be used for covalent compounds. Using prefixes Scientists use the Greek prefixes in Table 6 to indicate how many atoms of each element are in a binary covalent compound. The nitrogen and oxygen compounds N 2 O, NO, N O 2 , and N 2 O 5 would be named dinitrogen monoxide, nitrogen monoxide, nitrogen dioxide, and dinitrogen pentoxide, respectively. Notice that the last vowel of the prefix is dropped when the second element begins with a vowel, as in monoxide and pentoxide. The prefix mono- is always omitted from the name of the first element of the compound. For example, CO is carbon monoxide as opposed to monocarbon monoxide.

Writing Chemical Formulas

Does the table in Figure 16 look like it has anything to do with chemistry? It is an early table of the elements made for alchemy, a practice from the Middle Ages that laid the foundations for modern chemistry. Alchemists used symbols like these to write the formulas of substances created when individual elements combined. Before you can write a chemical formula, you must have all the needed information at your fingertips. What will you need to know? Oxidation numbers To write a chemical formula, you need to know the elements involved and the number of electrons they gain, lose, or share to become stable. This last piece of information is what chemists refer to as an element's oxidation number. An oxidation number is a positive or negative number that indicates how many electrons an atom has gained, lost, or shared to become stable. For ionic compounds, the oxidation number is the same as the charge on the ion. For example, a sodium ion has a charge of 1+ and an oxidation number of 1+. A chloride ion has a charge of 1- and an oxidation number of 1-.

Mass and Energy

During the fusion reactions that take place inside the Sun, no matter is ejected from the Sun. However, the total mass of all the particles in the reaction is greater before the reaction than after the reaction. How can there be less mass after a reaction if no matter leaves the Sun? Isn't mass conserved? According to the theory of special relativity, a tremendous amount of energy is the same thing as a small amount of mass. Mass is energy, and energy is mass. Figure 12 shows this relationship. One gram of mass is enough energy to launch the Washington Monument into orbit. However, unless incredibly large energies are involved, the mass-energy relationship is almost impossible to observe. With nuclear fission and fusion reactions, such tremendous energies are involved that the total amount of matter after the reaction can be noticeably different from the total amount of matter before the reaction. This change in the total amount of matter occurs even when the total number of protons plus neutrons does not change. Converting between mass and energy is just a conversion of units. It is similar to converting from miles per hour to meters per second. To convert from units of mass to units of energy, multiply by the speed of light in a vacuum squared (c2 ). Mass-Energy Equation units of energy (joules) = [units of mass (kg)] × [speed of light in a vacuum (m/s)]2 E = mc2 The speed of light in a vacuum is 300,000,000 m/s. The example problem and practice problems on the next page will help you to further explore mass-energy equivalence.

Nuclear Fission

Energy is released whenever a nucleus emits nuclear radiation. However, the amount of energy released during a nuclear decay is very small compared to the amount of energy that can be released during nuclear fission. Recall that nuclear fission is the process of splitting a nucleus into two or more smaller nuclei. Scientists can do this by bombarding the larger nucleus with neutrons. Figure 8 shows a nuclear fission reaction for uranium-235. Other isotopes, including plutonium-239 and uranium-233 also undergo nuclear fusion. The nuclear bomb that was dropped on Hiroshima during World War II was powered by the fission of uranium-235. Chain reactions The products of a fission reaction usually include several neutrons in addition to the smaller nuclei. These neutrons can then strike other nuclei in the sample, causing them to split as well. These reactions then release more neutrons, causing additional nuclei to split. A chain reaction is a series of repeated fission reactions caused by the release of additional neutrons in every fission. A chain reaction is shown in Figure 9. If a chain reaction is uncontrolled, an enormous amount of energy is released in an instant. This is what happens when a nuclear fission bomb is detonated. However, a chain reaction can be controlled by adding materials that absorb neutrons. If enough neutrons are absorbed, the reaction will be controlled. This is how a nuclear power plant operates.

Formulas

Every element has a chemical symbol. For example, the chemical symbol Na represents the element sodium, and the symbol Cl represents the element chlorine. When written as NaCl, these symbols make up the formula for the compound sodium chloride. A chemical formula shows what elements a compound contains and the exact number of the atoms of each element in a unit of that compound. Another compound with which you are familiar is H 2 O, which is more commonly known as water. This formula contains the symbols H for the element hydrogen and O for the element oxygen. Notice the subscript number 2 written after the H for hydrogen. Subscript means "written below." A subscript written after a symbol tells how many atoms of that element are in a single unit of the compound. If a symbol has no subscript, the unit contains only one atom of that element. A unit of H 2 O contains two hydrogen atoms and one oxygen atom. Look at the formulas for each compound listed in Table 1. What elements combine to form each compound? How many atoms of each element are required to form each of the compounds?

Chemical Reactions—Energy

Exchanges Crowds often gather to watch a rocket launch. Hundreds of kilograms of solid and liquid rocket fuel are converted into a gas, pushing the enormous rocket skyward. The combustion of rocket fuel is an example of a rapid chemical reaction. Most chemical reactions proceed more slowly, but all chemical reactions release or absorb energy. This energy can take many forms, such as thermal energy, light, sound, or electricity. The thermal energy produced by a wood fire and the light emitted by a glow stick are examples of energy released by chemical reactions. Chemical bonds are the source of this energy. When most chemical reactions take place, some chemical bonds in the reactants are broken, which requires energy called activation energy. In order for products to be produced, new bonds must form. Bond formation releases energy. Reactions such as dynamite combustion, shown in Figure 13, require much less energy to break chemical bonds than the energy released when new bonds are formed. The result is a release of energy and an explosion

Volume and pressure

For chemical reactions involving gases, volume and pressure are important considerations because they relate to the concentrations of the reacting gases. For example, decreasing the volume of a flask containing gases while the temperature remains constant increases the concentrations of the gases. Just as with liquid solutions, increasing the concentrations of gases increases the rate at which the particles collide with each other and with the walls of the container. The pressure inside the flask increases. More importantly, the reaction rate increases as well because the reacting gas particles collide with each other more frequently. The effect of increased pressure and decreased volume on gas particles is demonstrated in Figure 20. Reading Check Compare and contrast the effects of increased concentration of liquid reactants and decreased volume of gaseous reactants.

Combined Elements

Have you ever noticed the color of the Statue of Liberty? Why is it green? Was it painted that way? No, the Statue of Liberty was not painted. It is made of the metal copper, which is an element. Pennies have a coating made of copper. But copper isn't green. Uncombined, elemental copper is a bright, shiny copper color. So, again, we ask: Why is the Statue of Liberty green? Compounds The matter around you includes pure elements such as copper, sulfur, and oxygen. But, like many other elements, these elements combine chemically to form compounds when the conditions are right. For example, the copper in the Statue of Liberty combines with oxygen and sulfur in the air to form a number of different compounds, including copper sulfate. Copper sulfate is one of the compounds responsible for the bluegreen patina seen on the Statue of Liberty. Figure 1 compares copper sulfate, in the patina on the Statue of Liberty, with elemental copper and elemental sulfur. Although it is formed from these elements, copper sulfate looks very different from either copper or sulfur and has its own unique properties. New properties A compound formed when elements combine often has properties that aren't anything like those of the individual elements. For example, potassium iodide, shown in Figure 2, is a compound made from the elements potassium and iodine. Potassium is a shiny, soft, silvery metal that reacts violently with water. Iodine is a blue-black solid that turns to a purple gas at room temperature. Would you have guessed that these elements combine to make a white, crystalline salt?

Understanding Chemical Equations

Have you watched someone cook food outdoors on a charcoal grill? When charcoal burns, as shown in Figure 6, heat is liberated by the chemical reaction between carbon in the charcoal and oxygen in the air. Molecules of carbon dioxide gas are produced. Note in the balanced equation for this reaction that one oxygen molecule (O2) is required for each carbon atom and that one carbon dioxide molecule (CO2) is produced. Also, we know from the periodic table that the average mass of a carbon atom is 12.01 amu and the average mass of an oxygen atom is 16.00 amu. Therefore, the average mass of an O2 molecule is 32.00 amu (2 × 16.00 amu) and the average mass of a CO2 molecule is 44.01 amu (12.01 amu + (2 × 16.00 amu)). C(s) + O 2 (g) → C O 2 (g) 1 atom 1 molecule 1 molecule 12.01 amu 32.00 amu 44.01 amu In the laboratory, selecting a single carbon atom (mass 12.01 amu) and reacting it with a single oxygen molecule (mass 32.00 amu) is virtually impossible. Instead, chemists use masses in grams, rather than amu. For example, 12.01 grams of carbon reacts with 32.00 grams of oxygen, the same ratio of masses as in the balanced equation. That's because the number of carbon atoms in 12.01 grams of carbon must be very nearly equal to the number of oxygen molecules in 32.00 grams of oxygen. In fact, 12.01 grams of carbon contains 6.02 × 1 0 23 carbon atoms and 32.00 grams of oxygen contains 6.02 × 1 0 23 oxygen molecules. Reading Check Explain why chemists use masses in grams instead of amu. Moles Because the number of particles involved in most chemical reactions is so large, chemists use a counting unit called the mole (mol). One mole is the amount of a substance that contains 6.02 × 1 0 23 particles of that substance. The reaction between one mole of carbon and one mole of oxygen, yielding one mole of carbon dioxide is summarized in

Forces in the Nucleus

How are the protons and neutrons that make up a nucleus held together so tightly? Positive electric charges repel each other, so why do the protons that are part of a nucleus not push each other away? The strong force is the force that causes protons and neutrons to be attracted to each other. Figure 3 illustrates the strong force between protons and neutrons. The strong force is one of the four basic forces in nature. It is about 100 times stronger than the electromagnetic force. The attractive forces between all of the protons and neutrons that make up a nucleus keep the nucleus together. However, protons and neutrons have to be extremely close together to be attracted by the strong force. The strong force is a short-range force that quickly becomes extremely weak as protons and neutrons get farther apart. The electromagnetic force is a long-range force, so protons that are farther apart are still repelled by the electric force, as shown in Figure 4.

Half-Life

How can you tell when a radioactive isotope is going to decay? Suppose you shake a box filled with hundreds of pennies. You then remove every penny that comes up tails. You would remove about half the pennies every time you did this. You could not predict exactly which pennies would come up tails each time. However, you could predict approximately how many pennies would come up tails after each shake. You could also predict how many times you can repeat this process before you have removed all of the pennies from the box. Radioactive decay works in a similar way. You cannot know when a specific radioactive nucleus will decay. However, you can accurately predict approximately how many radioactive nuclei will decay in a given amount of time. If a large number of radioactive nuclei are present in a sample, it is possible to consider the half-life of that sample. Half-life is the amount of time it takes for half the nuclei in a sample of an isotope to decay. Half-life is similar to the amount of time between shakes for the pennies in the shoe box. The radioactive nucleus is called the parent nucleus. The nucleus left after the isotope decays is called the daughter nucleus. Figure 17 shows the proportion of decaying nuclei left after each half-life. Notice that one half of the original parent nuclei remain after one half-life. One quarter of the original parent nuclei remain after two half-lives. Half-lives vary widely. The half-life of polonium-214 is less than a thousandth of a second. Carbon-14 has a half-life of slightly less than 6,000 years. Uranium-238 has a half-life of 4.5 billion years. Scientists can use their knowledge of the half-life of an isotope to calculate the ages of rocks, fossils, and artifacts.

Unfilled and filled energy levels

How do the electron dot diagrams represent elements, and how does that relate to their abilities to make compounds? Hydrogen and helium, the elements in period 1 of the periodic table, can hold a maximum of two electrons in their outer energy levels. Hydrogen contains one electron in its lone energy level. The dot diagram for hydrogen has a single dot next to its symbol. This means that hydrogen's outer energy level is not full. It is more stable when it is part of a compound. In contrast, helium's outer energy level contains two electrons as represented by its electron dot diagram with two dots—a pair of electrons—next to its symbol. Helium already has a full outer energy level by itself and is chemically stable. As a result, helium rarely forms compounds. Compare the electron dot diagrams of helium and hydrogen below. H He Outer levels—getting their fill How does hydrogen, or any other element, change the number of outer electrons to become stable? Atoms with unstable outer energy levels can lose, gain, or share electrons to obtain a stable outer energy level. They do this by combining with other atoms that also have partially complete outer energy levels. As a result, each achieves stability. Gaining and losing electrons Figure 5 shows electron dot diagrams for sodium (Na) and chlorine (Cl). When they combine, sodium loses one electron and chlorine gains one electron. You can see from the electron dot diagram that chlorine now has a stable outer energy level similar to a noble gas. As shown in Figure 5 on the previous page, sodium had only one electron in its outer energy level, which it lost to combine with chlorine in sodium chloride. As a result, the next-to-thelast energy level becomes the new outermost energy level. Sodium now has an outer energy level that is stable with eight electrons. When the outer electron of sodium is removed, a complete inner energy level is revealed and becomes the outer energy level. Sodium and chlorine are now stable because of the exchange of an electron.

Writing Equations

If you wanted to describe the chemical reaction shown in Figure 2, you might write something like this: Nickel(II) chloride, dissolved in water, plus sodium hydroxide, dissolved in water, produces solid nickel(II) hydroxide plus sodium chloride, dissolved in water. This series of words is rather cumbersome, but all of the information is important. The same is true of descriptions of most chemical reactions. Many words are needed to state all the important information about reactions. As a result, scientists developed a shorthand method to describe chemical reactions. A chemical equation is a way to describe a chemical reaction using chemical formulas and other symbols. Some of the symbols used in chemical equations are listed in Table 1. The chemical equation for the reaction shown in Figure 2 looks like this: NiC l 2 (aq) + 2NaOH(aq) → Ni(OH ) 2 (s) + 2NaCl(aq) It is much easier to quickly and clearly identify what is happening by writing the information in this form. Chemical equations quickly convey information such as the states of matter of the reactants and products. Later, you will learn how chemical equations make it easier to calculate the quantities of reactants that are needed and of products that are formed. Symbol Meaning → produces or yields + plus (s) solid (l) liquid (g) gas (aq) aqueous—a substance is dissolved in water heat The reactants are heated. light The reactants are exposed to light. elec. An electric current is applied to the reactants.

Radioactivity

In most nuclei, the strong force is able to keep the nucleus permanently together. These nuclei are stable. When the strong force is not large enough to hold a nucleus together tightly, the nucleus can decay. When a nucleus decays, it emits matter and energy. Radioactivity is the process of nuclei decaying and emitting matter and energy. All nuclei that contain more than 83 protons are radioactive. However, some nuclei that contain fewer than 83 protons are also radioactive, such as carbon-14. In addition, no nuclei with more than 92 protons is stable enough to occur naturally. Instead, people must synthesize these elements, usually in a lab. Figure 6 shows which elements have measurable percentages of radioactive isotopes.

Balancing Equations

Lavoisier's mercury(II) oxide reaction, shown in Figure 4, can be written as: HgO(s) heat Hg(l) + O 2 (g) Notice the number of mercury atoms is the same on both sides of the equation but that the number of oxygen atoms is not: Numbers and Kinds of Atoms HgO(s) → O 2(g) + Hg(l) Hg 1 1 O1 2 In this equation, one oxygen atom appears on the reactant side of the equation and two appear on the product side. But according to the law of conservation of mass, one oxygen atom cannot just become two. Nor can you simply add the subscript 2 and write Hg O 2 instead of HgO. The formulas Hg O 2 and HgO do not represent the same compound. In fact, Hg O 2 does not exist. The formulas in a chemical equation must accurately represent the compounds that react. Fixing this equation requires a process called balancing. Balancing an equation doesn't change what happens in a reaction—it simply changes the way the reaction is represented.

Polar and nonpolar molecules

Look again at the molecule of hydrogen chloride (HCl) in Figure 12. The atom holding the electrons more closely will always have a slightly negative charge. This polar bond results in the molecule being polar. A polar molecule is one in which the unequal sharing of electrons results in a slightly positive end and a slightly negative end, although the overall molecule is neutral. On the other hand, a nonpolar molecule is a molecule that does not have oppositely charged ends. Polarity and geometry Just because a molecule is nonpolar, that doesn't mean its electrons are all shared equally. When a molecule contains just two atoms, such as HCl, it is easy to see how the polar bond creates a polar molecule. When a molecule contains three or more atoms, however, like the molecule of H 2 O shown in Figure 14, you need to consider both the polarity of the bonds and the shape of the molecule to determine whether the molecule is polar or nonpolar. Figure 15, on the next page, shows how determining polarity is dependent on multiple factors. Polar bonds can be found in nonpolar molecules.

Oxidation numbers and the periodic table

Many elements have one common oxidation number, as shown in Figure 17. Notice how they fit with the periodic table groupings. Many of the transition elements can have more than one oxidation number, as shown in Table 2. When naming these compounds, the oxidation number is expressed in the name with a roman numeral. For example, the oxidation number of iron in iron(III) oxide is 3+. Binary ionic compounds The easiest compounds to write formulas for are binary compounds, which are composed of two elements. When writing formulas, remember that although the individual ions that make up ionic compounds carry charges, the compound itself is neutral. For example, lithium fluoride is composed of a lithium ion with a 1+ charge and a fluoride ion with a 1- charge. One of each ion put together makes a neutral compound with the formula LiF. Some compounds require more figuring. Aluminum oxide contains an aluminum ion with a 3+ charge and an oxide ion with a 2- charge. To determine the overall positive and negative charge, you must find the least common multiple of 3 and 2, which is 6. In order to have a 6+ charge, you need two aluminum ions. In order to have a 6- charge, you need three oxygen ions. This gives the neutral compound A l 2 O 3

More Energy Out

Many of the reactions with which you are most familiar involve the release of energy. Chemical reactions that release energy are called exergonic reactions. In these reactions, the activation energy required to break the original bonds is less than the energy that is released when new bonds form. As a result, some form of energy, such as light or thermal energy, is given off by the reaction. The abdomen of a firefly, as shown in Figure 14, glows as a result of an exergonic reaction that produces visible light. Thermal energy released In many reactions, the energy given off is thermal energy. This is the case with some heat packs that are used to treat muscle aches and other problems. When the energy given off is primarily in the form of thermal energy, the reaction is called an exothermic reaction. Wood burning and the explosion of dynamite are exothermic reactions. Iron rusting is also exothermic, but, under typical conditions, the reaction proceeds so slowly that it's difficult to detect any temperature change. Reading Check Infer Why is a log fire considered to be an exothermic reaction? Exothermic reactions provide most of the power used in homes and industries, as shown in Figure 15. Fossil fuels contain carbon. These fuels, such as coal, petroleum, and natural gas, combine with oxygen to yield carbon dioxide gas and energy. Unfortunately impurities in these fuels, such as sulfur, burn as well, producing pollutants such as sulfur dioxide. Sulfur dioxide combines with water in the atmosphere, producing acid rain.

How could a chemical engineer apply Le Châtelier's principle to maximize the production of ammonia (N H 3 ) in the following equilibrium system?

N 2 (g) + 3 H 2 (g) ⇌ 2N H 3 (g) + energy Changing concentration Suppose that the manufacturing process is engineered to remove ammonia (N H 3 ) as it is formed. The concentration of ammonia decreases, which causes the rate of the reverse reaction to decrease. As a result, the forward reaction is temporarily faster than the reverse reaction, described as a shift to the right. More ammonia is formed. Changing temperature Suppose that the engineer lowers the temperature by removing energy. The equilibrium responds by reacting to release energy and raise the temperature. A shift to the right occurs and more ammonia is formed. Changing volume and pressure Because the reaction vessel contains gases, decreasing the volume increases the pressure. If possible, the equilibrium responds to reduce the pressure. The pressure can be reduced by decreasing the number of gas molecules. Because the product (N H 3 ) side of the equation has fewer gas molecules (2) than the reactant side (4), the equilibrium shifts to the right. More ammonia is formed.

Naming

Name binary ionic compounds with these rules. 1. Write the name of the positive ion. 2. Using Table 2, check to see whether the positive ion forms more than one oxidation number. If so, determine the oxidation number of the ion from the formula of the compound. Remember, the overall charge of the compound is zero, and the negative ion has only one possible charge. Write the charge of the positive ion using roman numerals in parentheses after the ion's name. 3. Write the root name of the anion. The root is the first part of the element's name. Chlorine is chlor-; oxygen is ox-. 4. Add the ending -ide to the root, as shown in Table 3. Chlorine becomes chloride and oxygen becomes oxide. Subscripts are not part of the name for ionic compounds

Compounds With Complex Ions

Not all compounds are binary. Many common compounds contain more than two atoms. Baking soda—used in cooking, as a medicine, and for brushing your teeth—has the formula NaHC O 3 . This is an example of an ionic compound that is not binary. Some compounds, including baking soda, contain polyatomic ions. The prefix poly- means "many," so the term polyatomic means "having many atoms." A polyatomic ion is a positively or negatively charged, covalently bonded group of atoms. Polyatomic ions as a whole contain two or more elements. Even though polyatomic ions contain more than one element, they act like individual ions in forming compounds. The polyatomic ion in baking soda is the hydrogen carbonate ion (HC O 3 - ), which is commonly called bicarbonate. Writing formulas Table 4 lists several common polyatomic ions. To write formulas for compounds containing these ions, follow the rules for binary compounds, with one addition. When more than one polyatomic ion is needed to balance the charges of the ions, write parentheses around the polyatomic ion before adding the subscript. How would you write the formula of barium chlorate? First, identify the symbol of the positive ion. Barium has a symbol of Ba and forms a 2+ ion, Ba2+. Next, identify the negative chlorate ion. Table 4 shows that it is Cl O 3 - . Finally, you need to balance the charges of the ions to make the compound neutral. It will take two chlorate ions with a 1- charge to balance the 2+ charge of the barium ion. Because the chlorate ion is polyatomic, you use parentheses before adding the subscript. Therefore, the formula is Ba(Cl O 3 ) 2 . Another example of naming complex compounds is shown in Table 5. Naming First, write the name of the positive ion. Then write the name of the negative ion. What is the name of Sr(OH ) 2 ? Begin by writing the name of the positive ion, strontium. Then find the name of the polyatomic ion OH- . Table 4 lists it as hydroxide. Thus, the name is strontium hydroxide.

Nuclear Fusion

Nuclear fusion reactions can release even more energy than nuclear fission reactions can. Recall that nuclear fusion is the process of two or more nuclei combining to form a nucleus of larger mass. Figure 10 shows an example of nuclear fusion that occurs inside the Sun. Fission splits nuclei apart. Fusion fuses nuclei together. Temperature and fusion For nuclear fusion to occur, nuclei must get close to each other. However, all nuclei have positive electric charge. Therefore, they repel each other. In order to fuse, the nuclei need to have enough kinetic energy to overcome this repulsion. Remember that the kinetic energy of atoms increases as their temperature increases. Only at temperatures of millions of degrees Celsius are nuclei moving so fast that they can get close enough for fusion to occur. These extremely high temperatures are found in the centers of stars, including the Sun. Every atom that exists, other than hydrogen-1, was originally constructed through nuclear fusion. This nuclear fusion occurs in the cores of stars and during the explosions of those stars. Nuclear fusion also occurred in the first moments after the Big Bang. Thermal energy and temperature are extremely high in each of these situations. The Sun and fusion The Sun emits more than 3.8 × 1026 J of energy every second. This radiant energy is first extracted from hydrogen nuclei in the Sun's core through a series of nuclear fusion reactions. The most important series of fusion reactions that occurs in the Sun is shown in Figure 11. This series is also common to other stars that have masses similar to the mass of the Sun. In more massive stars, another series of reactions is more common.

Writing formulas

Once you've found the oxidation numbers and their least common multiple, you can write formulas for binary ionic compounds by using the rules below. 1. Write the symbol of the element that has the positive oxidation number or charge. • All metals have positive oxidation numbers. Hydrogen often does. 2. Write the symbol of the element with the negative oxidation number. • Nonmetals have negative oxidation numbers. Hydrogen does occasionally, when bonded to a metal. 3. Find the least common multiple of the charges of each ion. • The charge (without the sign) of one ion becomes the subscript of the other ion. Reduce the subscripts to the smallest whole numbers that retain the ratio of the ions.

Oxidation-Reduction Reactions

One characteristic that is common to many chemical reactions is the tendency of the substances involved to lose or gain electrons. Chemists use the term oxidation to describe the loss of electrons and reduction to describe the gain of electrons. Chemical reactions involving electron transfer of this sort often involve oxygen, which is very reactive, pulling electrons from metallic elements. Corrosion of metal is a visible result, as shown in Figure 12. The cause and effect of oxidation and reduction can be taken one step further by describing the substances after the electron transfer. The substance that gains an electron or electrons becomes more negative, and we say it is reduced. On the other hand, the substance that loses an electron or electrons then becomes more positive, and we say it is oxidized. Reduction is the partner to oxidation; the two always work as a pair, which is commonly referred to as redox.a

Synthesis reactions

One of the easiest reaction types to recognize is a synthesis reaction. In a synthesis reaction, two or more substances combine to form another substance. The generalized formula for this reaction type is A + B → AB. The reaction in which hydrogen gas ( H 2 ) combines with chlorine gas (C l 2 ) to form hydrogen chloride (HCl) is an example of a synthesis reaction. H 2 (g) + C l 2 (g) light 2HCl(g) This synthesis reaction requires the addition of light. It can be explosive in direct sunlight. Another synthesis reaction with which you may be familiar is the combination of oxygen ( O 2 ) with iron (Fe) in the presence of water to form hydrated iron(III) oxide (Fe2O3 ), which is known as rust. Decomposition reactions A decomposition reaction is just the reverse of a synthesis. Instead of two substances coming together to form a third, a decomposition reaction occurs when one substance breaks down, or decomposes, into two or more substances. The general formula for this type of reaction can be expressed as AB → A + B. Most decomposition reactions require the input of heat, light, or electricity. For example, hydrogen peroxide ( H 2 O 2 ), shown in Figure 8, will slowly decompose in the presence of light, producing oxygen gas ( O 2 ) and water ( H 2 O). 2 H 2 O 2 (l) light O 2 (g) + 2 H 2 O(l) Single displacement The chemical reaction in which one element replaces another element in a compound is called a single-displacement reaction. Single-displacement reactions— sometimes called single-replacement reactions—are described by the general equation A + BC → AC + B. Here you can see that atom A displaces atom B to produce a new molecule, AC. A single displacement reaction is illustrated in Figure 9, where a copper wire is put into a solution of silver nitrate. Because copper is a more active metal than silver, it replaces the silver, forming a blue copper(II) nitrate solution. The silver, which is not soluble, forms crystals on the wire. Cu(s) + 2AgN O 3 (aq) → Cu(N O 3 ) 2 (aq) + 2Ag(s) Sometimes single-displacement reactions can cause problems. For example, if iron-containing vegetables such as spinach are cooked in aluminum pans, aluminum can displace iron from the vegetable. This causes a black deposit of iron to form on the sides of the pan. For this reason, it is better to use stainless steel or enamel cookware when cooking spinach. We can predict which metal will replace another using the diagram shown in Figure 10, which lists metals according to how reactive they are. A metal will replace any less active metal. Notice that silver and gold are two of the least active metals on the list. That is why these elements often occur as deposits of the pure element. More reactive metals often occur as compounds. Double displacement The positive ion of one compound replaces the positive ion of the other to form two new compounds in a double-displacement reaction—sometimes called a double-replacement reaction. You know that a doubledisplacement reaction is taking place if a precipitate, water, or a gas forms when two ionic compounds in solution are combined. A precipitate is an insoluble compound that comes out of solution during this type of reaction. The generalized formula for this reaction is AB + CD → AD + CB. Reading Check Classify Which reaction produces a precipitate? The reaction of copper(II) chloride with sodium hydroxide is an example of this type of reaction. A precipitate—copper(II) hydroxide—forms, as shown in Figure 11. 2NaOH(aq) + CuC l 2 (aq) → Cu(OH ) 2 (s) + 2NaCl(aq)

Describing the Nucleus

Recall that atoms are composed of protons, neutrons, and electrons. The nucleus of an atom is composed of protons and neutrons. Protons have a positive electric charge. Neutrons have no electric charge. So, the number of protons in a nucleus determines that nucleus's total charge. Negatively charged electrons are attracted to the positively charged nucleus and swarm around it. An electron has a charge that is equal but opposite to a proton's charge. Atoms contain the same number of protons as electrons. Size of the nucleus The protons and neutrons that make up a nucleus are packed together tightly. The region outside the nucleus where the electrons are located is large compared to the size of the nucleus. As Figure 1 helps show, the nucleus occupies only a tiny fraction of the space in an atom. If an atom were enlarged so that it was 1 km in diameter, its nucleus would have a diameter of only a few centimeters. But the nucleus has almost all of the atom's mass. Neutrons are slightly more massive than protons. However, both protons and neutrons are almost 2,000 times as massive as electrons.

Beta particles and the weak force

So far, you have learned about three fundamental forces in nature. They are the gravitational force, the electromagnetic force, and the strong force. The fourth and final fundamental force is called the weak force. The weak force causes beta decay. Like the strong force, the weak force is very short-ranged. The weak force is also weaker than all the other fundamental forces except for gravity. However, the weak force can cause neutrons to decay into protons when the neutron to proton ratio in a nucleus is too high. When this decay happens, the nucleus emits an electron. A beta particle is a high-energy electron that is emitted when a neutron decays into a proton. Beta particles are much faster and more penetrating than alpha particles. It takes a sheet of aluminum 3-mm thick to absorb most beta radiation. Table 2 summarizes the properties of beta particles. Gamma rays Gamma rays are extremely high-energy electromagnetic waves. Lead bricks or other heavy materials are necessary to stop gamma rays. They are usually emitted from a nucleus during alpha decay and beta decay. Gamma rays have no mass and no charge and travel at the speed of light. A nucleus loses energy, but no particles, during gamma decay. Table 3 summarizes the properties of gamma rays.

Molecules

Some atoms of nonmetals are unlikely to lose or gain electrons. For example, group 14 elements would have to either gain or lose four electrons to achieve a stable outer energy level. The loss of this many electrons takes a great deal of energy. Each time an electron is removed, the nucleus holds the remaining electrons even more tightly. Therefore, these atoms become more chemically stable by sharing electrons, rather than by becoming ions. The attraction that forms between atoms when they share electrons is known as a covalent bond. The neutral particle that forms as a result of electron sharing is called a molecule. The covalent bond A single covalent bond is composed of two shared electrons. Usually, one shared electron comes from each atom in the bond. A water molecule contains two single bonds, as shown in Figure 10. In each bond, a hydrogen atom contributes one electron to the bond, and the oxygen atom contributes the other. The two electrons are shared, forming a single bond. The result of this type of bonding is a stable outer energy level for each atom in the molecule. Multiple covalent bonds A covalent bond can also contain more than one pair of electrons. In the diatomic molecule of oxygen ( O 2 ), each oxygen atom has six electrons in its outer energy level and needs to gain two electrons to become stable. It can do this by sharing two of its electrons with another oxygen atom. When each atom contributes two electrons to the bond, the bond contains four electrons, or two pairs of electrons. Each pair of electrons represents a bond. Therefore, two pairs of electrons represent two bonds, called a double bond. Each oxygen atom is stable with eight electrons in its outer energy level. Similarly, a bond that contains three shared pairs of electrons is a triple bond. The diatomic molecule N 2 is an example shown in Figure 11. Covalent bonds form between nonmetallic elements. Many covalent compounds are solids or gases at room temperature

Reaction Rates

Some chemical reactions, such as the combustion of rocket fuel, take place rapidly and release tremendous amounts of energy in a matter of seconds. Other reactions, such as the rusting of steel, proceed so slowly that you hardly notice any change from one week to the next. How can you express the rate of a chemical reaction? The reaction rate is the rate at which reactants change into products. Consider the synthesis reaction, shown in Figure 18, described by the following chemical equation. 2 H 2 (l) + O 2 (l) → 2 H 2 O(g) + energy A chemist might choose one of several ways to state the rate of this reaction. Three examples are the rate at which one of the two reactants is used up, the rate at which water is produced, and the rate at which energy is released. Because many factors influence the rates of chemical reactions, the chemist would also state the conditions under which the reaction occurred.

Compounds with Added Water

Some compounds have water molecules as part of their structures. These compounds are called hydrates. A hydrate is a compound that has water chemically attached to its atoms and written into its chemical formula. Common hydrates The term hydrate comes from a word that means "water." When a solution of cobalt chloride evaporates, pink crystals that contain six water molecules for each unit of cobalt chloride are formed. The formula for this compound is CoC l 2 · 6 H 2 O and is called cobalt chloride hexahydrate. You can remove water from these crystals by heating them. The resulting blue compound is called anhydrous cobalt chloride. The word anhydrous means "without water." When anhydrous (blue) CoC l 2 is exposed to water, even from the air, it will revert back to its hydrated state. The plaster of paris shown in Figure 18 also forms a hydrate when water is added. It is made from calcium sulfate dihydrate, which is also known as gypsum. When writing a formula for a hydrate, the water is shown after a " · ". Following the dot, write the number of water molecules attached to the compound. For example, calcium sulfate dihydrate (gypsum) is written CaS O 4 · 2 H 2 O. Notice that when naming hydrates, you use the same prefixes listed in Table 6 to indicate the number of water molecules.

Types of equilibria

Some equilibria involve physical changes, rather than chemical reactions. When opposing physical changes take place at equal rates, a state of physical equilibrium exists. In a sealed bottle of soda, for example, C O 2 molecules are continually escaping from the solution. At the same time—and at an identical rate—CO2 molecules are reentering the solution. This state of physical equilibrium is shown in Figure 23. C O 2 (aq) ⇌ C O 2 (g) Likewise, when opposing chemical reactions take place at equal rates, a state of chemical equilibrium exists. An example is the chemical equilibrium established in the Haber process, used to manufacture ammonia (N H 3 ) by reacting nitrogen ( N 2 ) with hydrogen ( H 2 ). The Haber-process equilibrium reactions are described by the following equation. N 2 (g) + 3 H 2 (g) ⇌ 2N H 3 (g) + energy When this reaction is at equilibrium, ammonia is constantly being formed. At the same time and at the same rate, nitrogen and hydrogen molecules are being reformed.

Catalysts and inhibitors

Some reactions proceed too slowly to be useful. To speed up such a reaction, a catalyst can be added. A catalyst is a substance that speeds up a chemical reaction without being permanently changed itself. When you add a catalyst to a reaction, the mass of the product that is formed remains the same, but it will form more rapidly. The catalyst remains unchanged and often is recovered and reused. Catalysts are used to speed many reactions in industry, such as the process of polymerization to make plastics and fibers. In order to break down food, your body utilizes special catalysts, called enzymes. At times, it is worthwhile to prevent certain reactions from occurring. For example, foods often spoil because they react with oxygen from the air. Substances called inhibitors are used to slow down the rates of chemical reactions or prevent a reaction from happening at all. Food preservatives are inhibitors that prevent the reactions that lead to the spoilage of certain foods. One thing to remember when thinking about catalysts and inhibitors is that they do not change the amount of product produced. They only change the rate of production.

More Energy In

Sometimes a chemical reaction requires more energy to break bonds than is released when new ones are formed. These reactions are called endergonic reactions. The energy absorbed can be in the form of light, thermal energy, or electricity. Electricity is often used to supply energy to endergonic reactions. For example, an electric current passed through water produces hydrogen and oxygen, shown in Figure 16. Also, aluminum metal is obtained from its ore using the following endergonic reaction: 2A l 2 O 3 (l) elec. 4Al(l) + 3 O 2 (g) In these cases, electricity provides the energy necessary for the reactions. Thermal energy absorbed When the energy needed to keep a reaction going is in the form of thermal energy, the reaction is called an endothermic reaction. The terms exothermic and endothermic are not just related to chemical reactions. They can also describe physical changes. If you ever had to soak a swollen ankle in an Epsom salt solution, you probably noticed that when you mixed the Epsom salt in water, the solution became cold. The dissolving of Epsom salt absorbs thermal energy. Thus, it is a physical change that is endothermic. Cooking requires the addition of thermal energy to bring about chemical changes in the food. When baking cookies, you might add baking soda (NaHC O 3 ) to the dough mixture. Through an endothermic reaction, the baking soda breaks down into sodium carbonate (N a 2 C O 3 ), carbon dioxide gas (C O 2 ), and water vapor ( H 2 O). As these gases are released, tiny pockets form in the dough and the cookies puff up, as shown in Figure 17.

Detecting Nuclear Radiation

Special equipment is necessary to detect and study nuclear radiation. One such piece of equipment is a Geiger counter. A Geiger counter, as shown in Figure 13, has a tube with a positively charged wire running through the center of a negatively charged metal tube. This tube is filled with a low-density gas. When radiation enters the tube at one end, it knocks electrons from the gas molecules. These electrons then knock more electrons off other gas molecules, producing an electron avalanche. The positive wire attracts these electrons, resulting in a current in that wire. This current is amplified and produces a clicking sound. The number of clicks each second indicates the intensity of radiation. Often, scientists want to do more than just detect nuclear radiation. Unlike a Geiger counter, a wire chamber can track the trajectories of subatomic particles as well as detect those particles' presence. A wire chamber works much like a Geiger counter. However, a wire chamber contains an array of positively charged wires instead of just one wire.

Isotopes

The atoms of an element all have the same number of protons in their nuclei. For example, the nuclei of all carbon atoms have six protons. However, naturally occurring carbon nuclei can have six, seven, or eight neutrons. Isotopes are nuclei that have the same number of protons but different numbers of neutrons. Each element has many different isotopes. For example, the element carbon has three isotopes that occur naturally. The atoms of all isotopes of an element have the same chemical properties. However, each isotope has its own nuclear properties. For example, some carbon isotopes are radioactive, but other carbon isotopes are not radioactive. Figure 2 shows two isotopes of helium. Nucleus numbers You can describe a nucleus with its numbers of protons and neutrons. The atomic number is the number of protons that are a part of a nucleus. The total mass of all the protons and neutrons that make up a nucleus is nearly the same as the mass of the atom. As a result, the number of protons plus neutrons is called the mass number. Reading Check Define atomic number. You can represent a nucleus with its atomic number, mass number, and the symbol of the element to which it belongs. The representation for the nucleus of the most common isotope of carbon is shown below as an example. mass number → 12 C ← element symbol This isotope is called carbon-12. The number of neutrons equals the mass number minus the atomic number. The number of neutrons that are a part of carbon-12 is 12 - 6 = 6. The nucleus of carbon-12 is composed of six protons and six neutrons. Look at Figure 2 again. How many protons does helium-4 have? How many neutrons does helium-4 have? What is the total number of protons plus neutrons?

Choosing coefficients

The balancing process involves changing coefficients in a reaction in order to achieve a balanced chemical equation, which is a chemical equation with the same number of atoms of each element on both sides of the arrow. In the equation for Lavoisier's experiment, the number of mercury atoms is balanced, but one oxygen atom is on the left and two are on the right. Oxygen can be balanced by placing a 2 before the HgO on the left. Then, placing a 2 before Hg on the right balances mercury. Numbers and Kinds of Atoms 2HgO(s) → O 2(g) + 2Hg(l) Hg 2 2 O2 2 Try your balancing act Magnesium (Mg) burns with such a brilliant white light that it is often used in fireworks, as shown in Figure 5. Burning leaves a white powder called magnesium oxide (MgO). To write a balanced chemical equation, follow these steps. Step 1 Write a chemical equation for the reaction of magnesium with oxygen. Recall that oxygen is a diatomic molecule. Mg(s) + O 2 (g) → MgO(s) Step 2 Count the atoms in reactants and products. The magnesium atoms are balanced, but the oxygen atoms are not. Numbers and Kinds of Atoms Mg(s) + O 2(g) → MgO(s) Mg 1 1 O 21 Step 3 Choose coefficients that balance the equation. Remember, never change subscripts of a correct formula to balance an equation. Try putting a coefficient of 2 before MgO. Mg(s) + O 2 (g) → 2MgO(s) Step 4 Recheck the numbers of each atom on each side of the equation and adjust coefficients again if necessary. Now two Mg atoms are on the right side and only one is on the left side. So a coefficient of 2 is needed for Mg to balance the equation. 2Mg(s) + O 2 (g) → 2MgO(s)

Nuclei with few protons

The left panel of Figure 5 shows the forces in a small nucleus, one with relatively few protons. If a nucleus has only a few protons and neutrons, they are all close enough together to be attracted to each other by the strong force. Because only a few protons are present, the total electric force repelling the protons from each other is small. As a result, the net forces between the protons and the neutrons hold the nucleus together tightly. Nuclei with many protons Some nuclei, such as uranium nuclei, are composed of many protons and neutrons. In these cases, each proton or neutron is attracted to only a few neighbors by the strong force, as shown in the right panel of Figure 5. The other protons and neutrons are too far away. Therefore, the strong force holding a proton or neutron in place in a large nucleus is about the same as for a small nucleus. However, all of the protons in a large nucleus exert a repulsive electric force on each other. Thus, the electric repulsive force on a proton in a nucleus with many protons is large. As a result, a nucleus with many protons is held together less tightly than a nucleus with fewer protons. Neutron to proton ratios There are no repulsive electric forces between neutrons. Why are there no atomic nuclei composed completely of neutrons? Such an atomic nucleus would not have a stable ratio of neutrons to protons. In less massive elements, an isotope is stable when the ratio is about 1:1. In more massive elements, an isotope is stable when the ratio of neutrons to protons is about 3:2. Nuclei with too many neutrons compared to the number of protons are unstable. Larger nuclei have a higher neutron to proton ratio because neutrons contribute to the attractive strong force but not to the repulsive electric force within the nucleus.

Molar mass

The mass in grams of one mole of a substance is called its molar mass. Just as the mass of a dozen eggs is different from the mass of a dozen watermelons, different substances have different molar masses. The atomic mass of titanium (Ti), for example, is 47.87 amu, and the molar mass is 47.87 g/mol. By comparison, the atomic mass of sodium (Na) is 22.99 amu, and its molar mass is 22.99 g/mol. For a compound such as nitrogen dioxide (NO2), the molar mass is the sum of the masses of its component atoms. The nitrogen dioxide (NO2) molecule contains one nitrogen atom (1 × 14.01 amu) and two oxygen atoms (2 × 16.00 amu = 32.00 amu). So NO2 has a molar mass of 46.01 g/mol (14.01 g/mol + 32.00 g/mol = 46.01 g/mol). Mole-mass conversions Given the mass of a substance, you can use the molar mass as a conversion factor to calculate the number of moles. The following example uses this method to calculate the number of moles in 50.00 g of N O 2 . 50.00 g N O 2 × 1 mol N O _2 46.01 g N O 2 = 1.087 mol N O 2 Similarly, given the number of moles of a substance, you can use the molar mass as a conversion factor to calculate the mass. What is the mass of 0.2020 mol of N O 2 ?

Radiometric dating

The rock shown in Figure 18 is from the Moon and is more than 4 billion years old. How can scientists learn the age of something that is thousands, millions, or even billions of years old? Scientists use many different methods to determine the ages of samples. One of their most effective methods of dating samples involves an understanding of radioactivity and half-life. Scientists call this method radiometric dating. First, scientists measure the amounts of radioactive isotope and daughter isotope in a sample. Then, they calculate the number of half-lives that need to pass to give the measured amounts. The number of half-lives can then be multiplied by the length of each half-life. This gives the amount of time that has passed since the isotope began to decay. This is usually close to the total amount of time that has passed since the object was formed. Different isotopes are useful in dating various types of materials. Carbon-14 can be used to date the fossils of once-living organisms that are tens of thousands of years old. However, carbon-14 cannot be used to date materials that were not once a part of a living organism or materials that are more than about 60,000 years old. Uranium-235, which has a much longer halflife, can be used to date rocks and minerals that are billions of years old. Other isotopes that are used in radiometric dating are potassium-40, rubidium-87, and samarium-147.

Equilibrium

Think of a one-way street, a street on which vehicles may travel in only one direction. Now examine the general formula for a chemical reaction (reactants → products). Do you notice a similarity? The single reaction arrow may lead you to suppose that every chemical reaction proceeds in only one direction, from reactants to products. Under certain conditions, some reactions do exactly that. And when such a reaction continues until at least one reactant is completely consumed, the reaction is said to "go to completion." The decomposition of potassium chlorate (KCl O 3 ) into potassium chloride (KCl) and oxygen ( O 2 ) is just such a reaction. 2KCl O 3 (s) → 2KCl(s) + 3 O 2 (g) Unlike reactions that go to completion, many reactions under certain conditions can occur in both directions. These reactions are said to be "reversible." A reversible reaction is one that can occur in both the forward and reverse directions. Think of a reversible reaction as a two-way street, shown in Figure 22. Vehicles may proceed in both directions at the same time. When a reversible reaction's forward and reverse reactions take place at exactly the same rate, a state of balance, or equilibrium, exists. Equilibrium (plural equilibria) is a state in which forward and reverse reactions or processes proceed at equal rates. An equilibrium state is indicated with double reaction arrows, as shown below. Chemists generally call the left-to-right reaction the forward reaction and the right-to-left reaction the reverse reaction. reactants ⇌ products

Iodine tracers in the thyroid

Tracers can be used to detect problems in your thyroid, which helps regulate several body processes, including growth. Iodine accumulates in the thyroid. Iodine-131, a radioactive isotope of iodine, emits gamma rays. A patient can ingest a capsule that contains iodine-131, which can be easily absorbed by the patient's thyroid. Gamma rays from the iodine-131 penetrate the skin. Doctors can detect these gamma rays, producing an image like the one shown in Figure 15. If the detected radiation is not intense, then the thyroid has not properly absorbed the iodine-131. This could be due to the presence of a tumor. Reading Check Describe a medical use for iodine-131. Cancer treatments When a person has cancer, a group of cells in that person's body grows out of control. Cancer is a harmful and often fatal disease. The left panel of Figure 16 shows two cancerous cells. The right panel of Figure 16 shows a cancer patient undergoing radiation therapy. Doctors can use radiation to stop some types of cancerous cells from growing and dividing. Remember that radiation can ionize nearby atoms. If a source of radiation is placed near cancer cells, atoms in those cells can be ionized. If the ionized atoms are in a critical molecule, such as DNA or RNA, then the molecule may no longer function properly. The cell then may stop growing or may even die. Noncancerous cells can also be damaged during radiation therapy. For this reason, doctors must be careful to focus the radiation on the cancer cells as much as possible. However, radiation therapy still often harms healthy cells. Cancer patients often experience severe side effects when they receive radiation therapy

Transfer of electrons

What happens when potassium and iodine atoms come together? A neutral atom of potassium has one electron in its outer energy level. This is not a stable outer energy level. Recall that for most elements a stable outer energy level contains eight electrons. When it forms a compound with iodine, potassium loses the one electron from its fourth level. With the fourth level gone, the third level is a complete outer energy level. Although the complete outer energy level means the atom is now stable, because it has lost an electron, it is no longer neutral. The potassium atom has become an ion. When a potassium atom loses an electron, the atom becomes a positively charged ion because there is one electron fewer in the atom than there are protons in the nucleus. The 1+ charge of the potassium cation is shown as a superscript written after the element's symbol, K+ , to indicate its charge. Superscript means "written above." Reading Check Explain What part of an ion's symbol indicates its charge? An iodine atom has seven electrons in its outer energy level. It needs one more electron in order to have a stable outer energy level. During the reaction with potassium, the iodine atom gains an electron, giving its outer energy level eight electrons. This atom is no longer neutral because it has gained an extra negative particle. It now has a charge of 1- and is called an iodide anion, written as I- . The compound formed between potassium and iodine is called potassium iodide. The electron dot diagrams for the process are shown in Figure 8. As they lose or gain electrons, the two atoms become stable ions. Notice that the resulting compound has a neutral charge because the 1+ positive charge of K+ and the 1- negative charge of I- cancel each other.

Nuclear Decays

When a nucleus decays, particles and energy are emitted from it. We call this emission nuclear radiation. Three types of nuclear radiation are alpha, beta, and gamma radiation. Alpha and beta radiation are composed of particles. Gamma radiation is composed of electromagnetic waves. Alpha particles When the strong force is not strong enough to hold a nucleus together, that nucleus emits alpha particles. An alpha particle is a particle that is composed of two protons and two neutrons. An alpha particle is the same as a helium-4 nucleus. Reading Check Identify the components of an alpha particle. Alpha particles are extremely massive when compared with other nuclear radiation. For example, an alpha particle is about 7,000 times more massive than a beta particle. Alpha particles also have twice as much charge as beta particles. Because of their high mass and charge, alpha particles interact with other matter frequently. As a result, alpha particles transfer energy to their surroundings very quickly as they travel through solids, liquids, and gases. Alpha particles are the least penetrating form of nuclear radiation. A sheet of paper will stop most alpha particles. Table 1 summarizes the properties of alpha particles.

Factors affecting equilibria

When a state of equilibrium exists, the forward and reverse reactions are taking place at equal rates. The net amounts of both reactants and products remain constant. In the previous example of the sealed bottle of soda, what would happen if the bottle were opened? As you can assume, the system would no longer be at equilibrium and, for a time, physical changes would proceed toward the right in the following equation: C O 2 (aq) ⥂ C O 2 (g) Chemical reactions at equilibrium can likewise change. An equilibrium system may be subjected to stresses that speed up or slow down one of the opposing reactions. Instead of remaining constant, the net amounts of reactants and products favor one of the directions of the reaction. The equilibrium becomes temporarily unbalanced. But in time, the forward and reverse reactions again reach a state of balance. A new equilibrium state is established, now with changed amounts of reactants and products. When a stress is imposed on an equilibrium system, the equilibrium responds to the stress according to a general rule known as Le Châtelier's (luh SHAHT uhl yays) principle. Le Châtelier's principle states that if a stress is applied to a system at equilibrium, the equilibrium shifts in the direction that opposes the stress. A stress is any kind of change that disturbs the equilibrium. Common stresses include the following: changing concentration by adding or removing a reactant or product; changing temperature by adding or removing heat, as shown in Figure 24; and changing volume and pressure. When a forward or reverse reaction rate increases or decreases in response to a stress, the equilibrium is said to "shift."

Equal sharing

When electrons are shared in covalent bonds by similar or identical atoms, such as in the N 2 or O 2 molecules just discussed, the electron charge is shared equally across the bond. A nonpolar bond is a covalent bond in which electrons are shared equally by both atoms. Reading Check Describe the atoms involved in a nonpolar bond. Unequal sharing In some molecules, however, electrons are not shared equally, and the electron charge is concentrated more on one end of the molecule than the other. Why aren't electrons always shared equally? Different types of atoms exert different levels of attraction for the electrons in a covalent bond. The strength of the attraction of each atom to its electrons is related to the size of the atom, the charge of the nucleus, and the total number of electrons the atom contains. Recall how a magnet has a stronger pull when it is right next to a piece of metal. In a similar way, a nucleus has a stronger attraction to electrons nearby. In addition, you know that a strong magnet holds metal more firmly than a weak magnet. Similarly, more positive charge in a nucleus attracts electrons more strongly. Partial charges One example of unequal sharing in a covalent bond is found in a molecule of hydrogen chloride (HCl), shown in Figure 12. Chlorine atoms have a stronger attraction for electrons than hydrogen atoms have. As a result, the shared electrons will spend more time near the chlorine atom. The chlorine atom has a partial negative charge, which is represented by a lower-case Greek letter delta with a negative superscript ( δ - ). Because the electrons spend less time near the hydrogen atom, it has a partial positive charge, represented by a δ + . Tug-of-war When electrons are shared unequally, the bond is said to be polar. The term polar means "having opposite ends." A polar bond is a bond in which electrons are shared unequally, resulting in a slightly positive end and a slightly negative end. It might help you to visualize a polar bond as the rope in a tug-ofwar, as shown in Figure 13. Think of the shared electrons as being in the space in the center. As the two dogs in the tug-ofwar pull, the middle of the rope ends up closer to the stronger dog. Similarly, each atom in a bond attracts the electrons that they share, but the electrons will be held more closely to the atom with the stronger pull.

The ionic bond

When ions combine in this way, a bond is formed. An ionic bond is the force of attraction between the opposite charges of the ions in an ionic compound. The number of positive charges must equal the number of negative charges in order to form a compound with a neutral charge. The formation of magnesium chloride (MgC l 2 ) is another example of ionic bonding. When magnesium reacts with chlorine, a magnesium atom loses the two electrons in its outer energy level and becomes the positively charged ion M g 2+ . At the same time, two chlorine atoms gain one electron each to become negatively charged chloride ions, C l - . In this case, a magnesium atom has two electrons to donate, but a single chlorine atom needs to accept only one electron. Therefore, it takes two chlorine atoms, as shown in Figure 9, to accept the two electrons that the magnesium ion donates. Reading Check Explain What is the charge of an ionic compound? Zero net charge As with potassium iodide, the result of these ionic bonds is a neutral compound. The compound as a whole is neutral because the sum of the charges on the ions is zero. The positive charge of the magnesium ion is exactly equal to the negative charge of the two chloride ions. In other words, when atoms form an ionic compound, their electrons are shifted between the individual atoms, but the overall number of protons and electrons of the combined atoms remains equal and unchanged. Therefore, the compound is neutral. Ionic bonds are usually formed between metals and nonmetals. Ionic compounds are often formed by elements across the periodic table from each other. They are typically crystalline solids with high melting points.

Ions

When you participate in a sport, you might talk about gaining or losing an advantage. To gain an advantage, you want to have a better time or score than your opponent. It is important that you keep practicing because you don't want to lose that advantage. Gaining or losing an advantage happens as you try to meet a standard for your sport. Atoms, too, lose or gain to meet a standard—stability. They do not lose or gain an advantage. Instead, atoms lose or gain electrons. An atom that has gained or lost an electron is called an ion. An ion is a charged particle that has either more or fewer electrons than protons. When an atom loses electrons, it becomes a positively charged ion known as a cation. When an atom gains electrons, it becomes a negatively charged ion known as an anion. The electric forces between oppositely charged particles can hold ions together. Some of the most common compounds are made by the loss and gain of just one electron. These compounds contain an element from group 1 and an element from group 17 on the periodic table. Some examples are sodium chloride (NaCl), commonly known as table salt, and potassium iodide (KI), an ingredient in iodized salt. Figure 7 shows the importance of iodine in nutrition.

Surface area

Which dissolves more quickly: granulated sugar or a sugar cube? As you probably guessed, the answer is granulated sugar because the individual grains of sugar have much greater total surface area compared to the sugar cube. Dissolving sugar is a physical change, but increased surface area also increases the rate of chemical reactions. Operators of grain elevators must take measures to ensure that grain dust and oxygen in the air do not combine in a combustion reaction. Even on a scorching-hot day, there is little danger that whole grains of wheat or kernels of corn will react rapidly with oxygen in the air. However, the fine particles that make up grain dust can react explosively on a hot day, as shown in Figure 21. The larger total surface area of the grain dust greatly increases the rate at which reacting particles collide. With more collisions per unit time, the rate of the combustion reaction increases dramatically.

Chemical Bond Formation

Why do atoms form compounds? The answer to that question is in the last column on the periodic table. The six noble gases in group 18 seldom form compounds. In fact, the first compound containing a noble gas was not made until 1962. Why? Atoms of noble gases are unusually stable. The reason for this stability lies in the arrangement of the electrons. Electron dot diagrams To understand the stability of atoms, it is helpful to show the electrons in the outer energy level of an atom—its valence electrons—in electron dot diagrams. Electron dot diagrams contain the chemical symbol for an element surrounded by dots representing its valence electrons. How do you know how many dots to make in electron dot diagrams? For groups 1, 2, and 13 through 18, you can use a periodic table or the portion of it shown in Figure 3. Look at the ring depicting the outer energy level of each of the elements. Group 1 elements each have one outer electron. The elements in group 2 have two. Group 13 elements have three, group 14 have four, and so on to group 18, the noble gases, which each have eight. The unique noble gases An atom is chemically stable when its outer energy level is complete. The outer energy levels of helium and hydrogen are stable with two electrons. The outer energy levels of all other elements are stable with eight. The noble gases are stable because they each have a full outer energy level. Figure 4 shows electron dot diagrams of some of the noble gases. Notice that eight dots surround Kr, Ne, Xe, Ar, and Rn, and two dots surround He.

Factors Affecting Reaction Rates

You already know that sugar dissolves faster in hot water than it does in cold water. Sugar dissolving in water is not a chemical reaction. However, the rates of most chemical reactions, too, vary with temperature. Chemists use a commonsense idea to explain why reaction rates depend upon temperature and other factors, such as concentration and surface area. This idea is called the collision model. The collision model states that atoms, ions, and molecules must collide in order to react. Understanding the collision model will help to explain why changing the conditions of a chemical reaction can have an effect on the reaction rate. Temperature You normally store perishable foods such as milk, eggs, and vegetables in a refrigerator. That's because lowering the temperature decreases the rates of the chemical reactions that cause spoilage. Conversely, increasing the temperature of chemical reactions generally increases their reaction rates. Why does temperature affect reaction rate? Recall that the temperature of a substance is a measure of the average kinetic energy of all of its particles. At higher temperatures, therefore, reacting particles move faster and collide more frequently. A higher collision frequency alone, however, does not completely explain the increase in reaction rate. Because the particles are moving faster at higher temperatures, they collide with greater energy. As a result, a greater percentage of collisions result in a reaction between colliding particles. Reading Check Explain the effect of increased temperature in terms of the collision model. Concentration Another way you can change the rate of a chemical reaction is by changing the concentration of one or more of the reactants. Concentration describes the number of particles of a substance per unit volume. Chemists usually express concentration as moles of a substance per liter (mol/L). Consider the two test tubes shown in Figure 19. Each tube has a magnesium (Mg) ribbon immersed in a solution of hydrochloric acid (HCl). The difference between the two tubes is the concentration of the acid solution. As the magnesium and hydrochloric acid react, hydrogen gas (H2 ) is released as a product, so you can compare the rates of the two reactions by how rapidly bubbles are formed. Why does magnesium ribbon react faster with the more concentrated hydrochloric acid? The more concentrated acid solution contains more reacting particles per unit volume, resulting in more opportunities for collisions between reacting particles. As a result, the reaction rate is greater

Background Radiation

You might be surprised to learn that humans have been bathed in radiation for millions of years. This radiation, called background radiation, is not produced by humans. Instead, it is emitted mainly by radioactive isotopes found in Earth's rocks, soils, and atmosphere. This background radiation is low-level but is still detectable. Building materials, such as bricks, wood, and stones, contain traces of radioactive materials. Traces of naturally occurring radioactive isotopes are also in our food, water, and air. Background radiation is even emitted from inside our own bodies. For example, our bodies contain the isotopes carbon-14 and potassium-40. Both of these isotopes are radioactive. Sources of background radiation Background radiation comes from several sources, as shown in Figure 14. The most common source of background radiation, radon gas, can seep into houses and basements from surrounding soil and rocks. In addition, some background radiation comes from high-speed particles that strike Earth's atmosphere from outer space. These high-speed particles are called cosmic rays. The amount of background radiation that a person receives can vary greatly. The amount depends on the types of rocks underground, types of materials used to construct the person's home, and the elevation at which the person lives, among other things. However, some amount of background radiation is present for everyone and has been present throughout history and prehistory. Using Nuclear Radiation in Medicine It would be easier to find a friend in a crowded area if she told you that she would be wearing a red hat. In a similar way, scientists can find one molecule in a large group of molecules if it is "wearing" something unique. A molecule cannot wear a red hat; however, it can be found easily if it has a radioactive atom in it. A radioactive atom emits radiation that doctors can detect. A tracer is a radioactive isotope that doctors use to locate molecules in an organism. Doctors use tracers to follow where particular molecules go in a human body and to study how organs function. This might seem harmful, but the radiation levels are too low to be harmful or dangerous. Agricultural scientists also use tracers in agriculture to monitor the uptake of nutrients and fertilizers. Common tracers include technetium-99m and iodine-131. These are useful tracers because they emit gamma rays that medical imaging equipment can easily detect.

Types of Reactions

You might have noticed that there are all sorts of chemical reactions. In fact, there are literally millions of chemical reactions that occur every day, and scientists have described many of them and continue to describe more. In order to organize these reactions into manageable areas of study, chemists have defined five main categories of chemical reactions: combustion, synthesis, decomposition, single displacement, and double displacement. Combustion reactions If you have ever observed something burning, you have observed a combustion reaction, such as the one shown in Figure 7. As mentioned previously, Lavoisier was one of the first scientists to accurately describe combustion. He deduced that the process of burning (combustion) involves the combination of a substance with oxygen. Our definition states that a combustion reaction occurs when a substance reacts with oxygen to produce energy in the form of heat and light. The combustion reaction shown in Figure 7 creates flames of heat and light as carbon in the wood reacts with oxygen in the air to form carbon dioxide (C O 2 ). Many combustion reactions also will fit into other categories of reactions. For example, the reaction between carbon and oxygen also is a synthesis reaction.