Chemistry 4
Determine the element that was oxidized and the element that was reduced and how the charge changed. Zn + 2HCl → ZnCl2 + H2 SiCl4 + 2Mg → 2MgCl2 + Si H2 + Cl2 → 2HCl
1. Oxidized: Zn (went from 0 to +2)Reduced: H (went from +1 to 0) 2. Oxidized: Mg (went from 0 to +2)Reduced: Si (went from +4 to 0) 3. Oxidized: H (went from 0 to +1)Reduced: Cl (went from 0 to −1)
Determine the oxidation number of the metal in each of the following compounds. CuNO3 Al2O3 PbO2
1.The oxidation number of Cu is +1. The ionic charge of copper in this compound is 1+, because the charge of the polyatomic ion NO3 is 1−. 2. The oxidation number of Al is +3 because the ionic charge of each aluminum in this compound 3+. The charge of oxygen is 2− multiplied by the subscript 3, which equals 6−. The charge of aluminum (3+) multiplied by the subscript 2 equals 6+. The charges (6+ and 6−) cancel out to create a neutral compound. 3. The oxidation number of Pb is +4, based on its ionic charge. The ionic charge of lead in this lead (IV) oxide compound is 4+, because the charge of oxygen is 2− multiplied by its subscript of 2. This equals 4−, which balances the 4+ oxidation number for lead.
Zinc
10 grams
Zinc Chloride + Hydrogen Gas
15 grams
Category 2
2HgO → 2Hg + O2 C6H12O6 → 6C + 6H2O
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4Fe + 3O2 - goes with - 2Fe2O3 2Fe + O2 - goes with - 2FeO CH4 + 2O2 - goes with - CO2 + 2H2O 2CH3OH + 3O2 - goes with - 2CO2 + 4H2O2 2C2H6 + 7O2 - goes with - 4CO2 + 6H2O
Hydrogen Chloride
5 grams
How does the law of conservation of mass apply to chemical reactions?
A balanced chemical equation demonstrates the conservation of mass by showing equal ratios of compounds in the reactants and the products. However, sometimes it can be difficult to observe what happens to all the atoms in a chemical reaction, especially if one of the reactants or products is a gas. Let's observe how mass is conserved in a chemical reaction.
decomposition reaction
A decomposition reaction is the opposite of a synthesis reaction; it starts with only one reactant but ends with more than one product. Most decomposition reactions require energy, usually in the form of heat or electricity, for the reaction to occur. AB → A + B AB represents a compound made from elements or polyatomic ions, which separates into other compounds or individual elements A and B. An example of a decomposition reaction is the breaking down of water into hydrogen and oxygen gas. When an electric current is used to decompose a substance, the process is called electrolysis. Water is two parts hydrogen to one part oxygen, so when water decomposes, it results in twice as much hydrogen gas (H2) as oxygen gas (O2).
Redox in single replacement
A single replacement reaction is almost always a redox reaction because it involves a neutral element that changes its electrostatic charge to combine with a new compound. Mg(s) + [H]+ [Cl]− → [Mg]2+ [Cl]− + [H]+ (equation with ions) Magnesium changes its charge from zero to 2+. Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) (balanced equation)
How do elements and compounds become oxidized or reduced?
According to the concept of electronegativity, every atom has a certain amount of attraction for the electrons shared in a covalent bond. Some atoms have a stronger attraction for these shared electrons than others. Picture a covalent bond as a sort of tug-of-war where both atoms are competing for the shared electrons in the bond, much like two children fighting over the same toy. Just as the stronger child can pull the toy closer to herself than a weaker child in a tug-of-war, the atom with the higher electronegativity has a stronger "pull" on the electrons than the atom with the lower electronegativity. When two nonmetals compete for the electrons that are "shared" in a covalent bond, the electrons spend more of their time closer to the more electronegative element. This uneven sharing of electrons in a covalent bond means that the more electronegative element becomes more "negative" while the less electronegative element becomes more "positive."
Balanced Equation
After the formulas have been written correctly, coefficients are used to balance the overall equation.
All Elements
All the oxidation numbers (multiplied by each of their subscripts) in a compound must add up to the total charge of that compound or ion. This means we can use subtraction to solve for any elements not listed in the previous oxidation number rules.
What are the characteristics of acid-base reactions?
An application of a double displacement reaction is seen during the neutralization of an acid using a base. This is also called an acid-base reaction. View the video below to learn about acid-base reactions. In this acid-base reaction, an acid (HCl) reacts with a base (NaOH) to form a salt (NaCl) and water. Notice that one of the ions in the acid is a hydrogen ion, H+, and that one of the ions in the base is a hydroxide ion, OH−. Remember, most acids contain H+and most bases contain OH−. When an acid and a base react in this double replacement reaction, these two ions combine to form HOH, or H2O. The other ions involved in the reaction combine to form the ionic salt.
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An example of a decomposition reaction is the breaking down of water into hydrogen and oxygen gas. When an electric current is used to decompose a substance, the process is called electrolysis. Water is two parts hydrogen to one part oxygen, so when water decomposes, it results in twice as much hydrogen gas (H2) as oxygen gas (O2).
Batteries
Batteries (electrochemical cells) produce electricity by separating oxidation-reduction reactions into separate chambers that are connected by wires or other conductors. When electrons are shared between the reactants, the flow of electrons powers our laptops, MP3 players, and other battery-powered devices.
CaSO4
Ca2+ ion = +2, O = −2 Determine what the oxidation number of S must be to have the entire compound add up to zero. +2 + 4(−2) + S = 0S = 8 − 2S = +6
chemical reaction
Chemical reactions involve bonds being broken and formed as atoms rearrange to make new substances. A chemical reaction starts with a reactant (the initial substance before a chemical change) and ends with a product (the new substance created after a chemical change). Chemical reactions are often described and represented using chemical equations. Think of chemical equations as a shorthand chemical sentences. They allow us to describe the chemical reaction without having to write a long paragraph about the ratios of reactants and products in the reaction.
Are there equal amounts of each element on both sides of the equation?
Count the number of atoms of each element on both sides of the equation. Only place new coefficients to balance a chemical equation; subscripts do not change. Balanced: H2CO3 → CO2 + H2O Not balanced: H2CO3 → CO2 + H2
Combustion reactions always involve molecular oxygen (O2) as a reactant in the following manner:
CxHx + O2 → H2O + CO2 (organic compounds) A + O2 → AOx (inorganic compounds)
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Did you notice a pattern in the chemical equations for each category of reaction? The reactions in the activity represent two types of chemical reactions: one where two reactants form one product (Category 1 in the chart) and one where one reactant breaks down into multiple products (Category 2 in the chart).
Insoluble Product
Double replacement reactions form a product that is insoluble in water, but it bubbles out of the solution as a gas instead of forming a solid precipitate. In the reaction below, hydrogen sulfide gas is formed as one of the products. FeS(aq) + 2HCl(aq) → H2S(g) + FeCl2(aq)
Are the atoms accurately bonded?
Double-check the bonds shown in the chemical equation. If the subscripts in the compound or molecule do not balance to a neutral molecule, the products of the chemical equation are incorrect. Example: 4Na + O2 → 2Na2O Na2O is correct because O has an ion charge of 2—which is made neutral by two Na ions with a 1+ charge.
Element Was Oxidized
Element Was Oxidized Lost electrons during a reaction Acts in the reactant that is the reducing agent Mg → Mg2+ + 2e- Has an increase in its oxidation number
Element Was Reduced
Element Was Reduced Gained electrons during a reaction Acts in the reactant that is the oxidizing agent Has a decrease in its oxidation number O2 + 4e- → 2O2-
Rules for Assigning Oxidation Numbers
Even though the atoms involved in a covalent molecule do not have charges like an ion, we assign them positive and negativeoxidation numbers to represent this "uneven sharing" that occurs. The following rules for assigning oxidation numbers will be useful when you need to determine if a reaction involves oxidation and reduction.
Covalent Compounds
For covalent compounds, pretend the compound is ionic with the more electronegative element forming the negative ion (anion). For example: Fluorine is always −1 in a compound, oxygen is almost always −2, and hydrogen is +1 in covalent compounds. Examples: CCl4 ( C = +4, Cl = −1) NH3 ( N = −3, H = +1 )
Beginning Reactions
For double replacement reactions, the positive ions of the reactants switch positions to create the products.
How do you analyze chemical equations to identify a synthesis or decomposition reaction?
For synthesis and decomposition reactions, recognizing the correct products and their appropriate ratios to the reactants takes some practice. To help, just ask yourself the following questions when you are reviewing chemical equations.
H2CO3
H2CO3 Each hydrogen atom has an oxidation number of +1, and each oxygen atom has an oxidation number of −2. Multiply these oxidation numbers by the subscripts on the appropriate elements, and then determine what the oxidation number of carbon must be for the total to add up to zero. 2(+1) + 3(−2) + C = 0C = +4
formula
HCl + NaOH → NaCl + H2O acid + base → salt + water H+ + Cl− + Na+ + OH− → Na+ + Cl− + H+ + OH−
What are the key characteristics of a combustion reaction?
In a combustion reaction, a compound or element reacts with oxygen, releasing a large amount of energy in the form of light and heat. We use combustion reactions every day to release energy in ways that are useful to us (for cooking, for fuel, etc.). Combustion reactions are exothermic, which means energy is released by the reaction.
Combustion reaction
In a combustion reaction, oxygen is the reactant. Many elements or compounds can react with oxygen and water molecules in the air. For example, iron rusts because its electrons transfer from atoms of iron to oxygen atoms. This is an example of another type of reaction called an oxidation-reduction (redox) reaction. A combustion reaction is a special type of redox reaction, but not all redox reactions are combustion reactions. So, what makes them similar, and what makes them different? Just as with classifying other chemical reactions, you can search for the pattern in their chemical reactions that makes them similar.
Redox absent in double replacement
In a double replacement reaction, ions do not change their charges. They do not follow the pattern of redox reactions. [Ag]+ [NO3]− + [Na]+ [Cl]− → [Ag]+ [Cl]− + [Na]+ [NO3]− The charges of the ions do not change. AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
What are the characteristics of double replacement reactions?
In a double replacement reaction, the ions of two different compounds in an aqueous solution exchange places to form two new compounds. Because the reactants are dissolved in water, they can move and collide together to form new products. One of the products formed is typically a solid (or precipitate), a gas (bubbles), or water. The other product usually remains dissolved in the solution.
What is the difference between an oxidizing agent and a reducing agent?
In a redox reaction, one reactant is always an reducing agent and another reactant is the oxidizing agent.
What are the characteristics of single replacement reactions?
In a single replacement reaction, one element replaces another element in a compound to form a new substance. Single replacement reactions have the same number of reactants as products.
synthesis reaction
In a synthesis reaction (sometimes called composition reactions), two or more reactants on the left side of the arrow form one new product on the right side of the arrow. Chemists often use the following generic equation to represent a synthesis reaction: A + B → AB The letters A and B represent elements, compounds, or polyatomic ions, and AB represents a compound made from A and B. For example, calcium oxide reacts with water to produce calcium hydroxide. In this equation, the metal cation (Ca) retains the same charge throughout the reaction, which is 2+, but the hydroxide polyatomic ion (OH) has a 1− charge. Therefore, it takes two hydroxide ions to balance the charge in calcium hydroxide.
What are the characteristics of synthesis and decomposition reactions?
In chemistry, the words synthesis and decomposition describe two types of chemical reactions. View the video below to learn about a synthesis reaction and a decomposition reaction. Be sure to place important notes from the video in your chemistry journal.
Human Body
In humans, sugars, fats, and proteins are oxidized in redox reactions that provide the energy necessary for life.
formula
In the formula of a double replacement reaction below, the ions "trade partners" over the course of the chemical reaction. AB + CD → AD + CB A, B, C, D in the reactants represent ions. A and C are positive ions and B and D are negative ions. AD and CB represent ionic or molecular compounds that are formed.
It is important that chemical equations represent the law of conservation of mass by always having equal numbers of each element on both sides of the equation; chemists call this "balanced."
It is important that chemical equations represent the law of conservation of mass by always having equal numbers of each element on both sides of the equation; chemists call this "balanced."
chemical equation
Just as you are reading this sentence, you read a chemical equation from left to right. The reacting substances are to the left of an arrow (represented with letters AB and C), and the newly formed products are to the right of an arrow (represented by the letters A and BC). These reactants and products are usually represented in chemical equations by their chemical formulas. The phases of each substance can be shown in subscripts: (s) for solid, (l) for liquid, and (g) for gas. Compounds that have been dissolved in water to form a solution are given the subscript (aq) for aqueous solution.
How do you determine oxidation numbers within chemical equations?
Let's review how oxidation numbers are assigned in a balanced chemical equation. Take a look at the combustion of methane.
LEO the lion says GER
Losing Electrons is Oxidation Gaining Electrons is Reduction
What are characteristics of redox reactions?
Many reactions, such as combustion, synthesis, decomposition, and single replacement reactions, can also fall into another category of chemical reactions called oxidation-reduction reactions, or redox reactions. Oxidation-reduction reactions sometimes create positive and negative ions that form ionic bonds. Oxidation occurs when an element loses electrons while reductionoccurs when an element accepts or gains electrons.
mass
Mass remains constant during a chemical change.
Category 1
Mg + Br2 → MgBr2 2Ca + O2 → 2CaO
Monatomic Ions
Monatomic ions have an oxidation number equal to their charge as an ion (when alone or when in a compound). Examples: NaCl ( Na = +1, Cl = −1) CaBr2 (Ca = +2, Br = −1) Ag+ (Ag = +1) Al3+ ( Al = +3)
Neutral Elements
Neutral elements in their standard state (not in a compound) always have an oxidation number of zero. Let's take a look at the following examples (all have an oxidation number of zero). O2 Na F2 S8 P4
Inorganic Compounds
Not all combustion reactions involve organic compounds made of carbon and hydrogen, which means the products are not always carbon dioxide and water. Other substances, both compounds and elements, can burn in oxygen. These reactions can often be categorized as synthesis reactions, but they are considered combustion reactions as well because a substance is reacting with oxygen and releasing energy. 4Fe + 3O2 → 2Fe2O3
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Not all oxidation-reduction reactions form ionic bonds. Elements that are covalently bonded can be "oxidized" if they have less control over their electrons than before their "oxidation." The same goes for reduction, except they have more control of their electrons after "reduction." The combustion reaction below is an example of an oxidation-reduction reaction that breaks and forms covalent bonds. The oxygen molecule (O2) attracts electrons more strongly than carbon or hydrogen. This is because the electronegativity of oxygen is higher than that of carbon and hydrogen. CH4 + 2O2 → CO2 + 2H2
How are single and double replacement reactions different from one another?
One type of chemical reaction, called a double replacement reaction, works the same way. The couples "traded" partners during the dance and began dancing as different couples than the originals. In a double replacement reaction, two ionic compounds trade partners to form two different compounds at the end of the reaction.
Organic Compounds
Organic compounds are compounds that contain carbon covalently bonded with other elements. When a hydrocarbon (a compound containing only carbon and hydrogen) combusts, the products are usually carbon dioxide and water. The burning of propane, a gas that can be used for cooking on gas grills or for heating homes, is one example of the combustion of a hydrocarbon. The balanced equation for this combustion can be seen below. C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g) Another common combustion reaction takes place inside the cells of your body. This reaction involves the combustion of an organic compound called glucose, made of carbon, hydrogen, and oxygen. Cells combine glucose (C6H12O6) from food with oxygen to release energy according to the equation below. This is cellular respiration.
OIL RIG
Oxidation Is Loss (of electrons) Reduction Is Gain (of electrons)
Oxidation
Oxidation is defined as the loss of electrons in a chemical reaction. Many metals lose electrons when they react with oxygen to make an ionic metal oxide compound. Gaining oxygen (after a loss of electrons) is how the term "oxidized" got its name. Usually, an increase in the charge of an element indicates oxidation. In the reaction of sodium and chlorine, neutral sodium was oxidized when it lost an electron to chlorine. This increased the initial charge of zero to +1 to make a positive Na+ ion. Na(s) + Charge 0 Cl2(g) Charge 0 2NaCl(s) Charge of Na: +1
How are redox reactions identified within all chemical reactions?
Oxidation-reduction reactions do not have a set pattern because they can occur in many different types of reactions. The only pattern they follow is the losing and gaining of electrons. Redox reactions always occur together; if one reactant in a chemical reaction is oxidized, then another reactant must be reduced. Combustion, synthesis, single replacement, and decomposition reactions can be redox reactions, but not always.
Oxidizing Agent
Oxidizing Agent: A reactant that causes another substance to be oxidized (not being oxidized itself). The oxidizing agent is the reactant that is reduced.
Oxygen
Oxygen always has an oxidation number of −2 when combined with another element, except in peroxides, when it is −1. Examples: H2O (O = −2) CaO (O = −2) H2O2 (O = −1 because peroxide)
MnO4−
Oxygen has an oxidation number of −2 in all compounds, unless it's a peroxide polyatomic ion. Determine the oxidation number of the manganese atom if the total sum must add up to the charge of the polyatomic ion, −1. 4(−2) + Mn = −1 Mn = +7
PO43−
PO43− O is −2 in all compounds (unless it's a peroxide polyatomic ion). So, 4 × −2 = −8 total from the oxygen atoms. If the entire polyatomic ion must add up to its charge, −3, what is the oxidation number of the P atom? 4(−2) + P = −3P= −3 − (−8)P= +5
Reducing Agent
Reducing Agent: A reactant that causes another substance to be reduced (not being reduced itself). The reducing agent is the reactant that is oxidized.
Reduction
Reduction is defined as the gain of electrons in a chemical reaction. When an atom or particle gains a negative electron, its overall charge is lowered, or "reduced." Usually, a decrease in charge of an element indicates reduction. In the reaction of sodium and chlorine, neutral chlorine got reduced when it gained electrons from the sodium. This decreased the initial charge of zero to -1 to make a negative Cl- ion. Na(s) Charge: 0 Cl2(g) Charge: 0 2NaCl(s) Charge of Cl: -1
Do the names of the compounds match the formulas in the chemical equation?
Reference the naming system to know what products are expected in a chemical reaction. For example, when synthesizing sodium carbonate, you can use the name to know this is an ionic bond between sodium (Na+) and the polyatomic ion carbonate (CO32−). The 2− charge on carbonate needs two sodium ions to make an electrically neutral compound of Na2CO3.
important
Remember we are only allowed to change the coefficients in front of a formula, not the subscripts within the formula, when balancing a chemical equation. Changing the subscripts in a formula changes the identity of the compounds. The equation represents an observed chemical reaction, so the formulas themselves cannot be changed.
facts
Remember, matter cannot be created or destroyed during a chemical reaction. All the atoms that were present at the beginning of the process are still there at the end, just rearranged. The law of conservation of mass also applies to physical changes. For example, if you melt 150 grams of ice, you will get 150 grams of water—no more, no less. Similarly, if you combine 10 grams of zinc with 5 grams of hydrogen chloride and it becomes zinc chloride and hydrogen gas, the total mass of the hydrogen gas and zinc chloride (including any leftover/unused reactants) will be 15 grams.
Molecular Product
Some double replacement reactions can form a molecular product. An example of a double replacement reaction forms water as one of its products. In this reaction, hydrochloric acid (HCl) reacts with an aqueous solution of sodium hydroxide (NaOH) to form an aqueous solution of sodium chloride (NaCl) and water. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
Does the chemical equation follow the pattern for synthesis and decomposition reactions?
Synthesis reactions show one combined product from all the reactants: A + B → AB. Decomposition reactions show multiple products from the breakdown of one reactant: AB → A + B.
The total mass of the baking soda, vinegar, and bottle is 782 grams. What do you think the mass of the products in the bottle will be after the reaction?
The answer is: A. less than 782 g
The total mass of the balloon, bottle, vinegar, and baking soda before the reaction is 784 grams. If the balloon is used to seal off the bottle during the reaction, what do you think the final mass will be after the reaction is complete?
The answer is: B. exactly 784 g
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The general form for a single replacement reaction is shown below. It shows one partner "cutting in" and replacing the other. A + BC → AC + B A is a neutral element and BC is an ionic compound. Positive ion A replaces the positive ion B in the ionic compound. A and C bond together, leaving B as a neutral element by itself.
Determine the oxidation number of nitrogen (N) in each of the following compounds or ions. NH4+ NaNO3 NO NO2−
The oxidation number of N in this compound is −3. (If each H has an oxidation number of +1 and the overall charge of the ion is 1+, the N has an oxidation number of −3.) The oxidation number of N in this compound is +5. (If each O has an oxidation number of −2 and the overall charge of the nitrate polyatomic ion is −1, the N has an oxidation number of +5. Check this by adding up the oxidation numbers of the entire compound, including +1 for Na, to see if they add up to zero.) The oxidation number of N in this compound is +2. (If each O has an oxidation number of −2 and the overall compound is neutral, then the N must have an oxidation number of +2 to add up to zero.) The oxidation number of N in this compound is +3. (If each O has an oxidation number of −2 and the overall charge of the ion is −1, then the N has an oxidation number of +3.)
Precipitate Video
The video below shows another example of a double replacement reaction. In it, clear aqueous solutions of lead nitrate (Pb(NO3)2) and potassium iodide (KI) combine. When one of the products of a double replacement reaction is not very soluble in water, it will not stay dissolved in the aqueous solution. The reaction produces a bright yellow solid precipitate, lead iodide (PbI2), and a clear aqueous solution of potassium nitrate (KNO3). The equation for this reaction is: Pb(NO3)2(aq) + 2KI(aq) → 2KNO3(aq) + PbI2(s)
Types of combustion
There are two types of combustion reactions to explore. Any time anything burns, it is a combustion reaction. For something to burn, oxygen is needed. It is important to note that fire is not always the result of a combustion reaction, even though a compound is burning.
Are all the elements in the reactants also present in the products?
This helps you determine if the equation is written correctly. Example: Yes: H2O → H2 + O2 No: H2O → H2
Rusting
This is a redox reaction in which oxygen oxidizes (takes electrons from) iron to form iron (III) oxide, otherwise known as rust. Iron is a very strong metal, whereas rust is a brittle ionic compound that flakes off of old cars and nails. What a difference a few electrons and bonds can make!
Photosynthesis
This reaction stores energy from the sun in plants by converting carbon dioxide and water into sugar. This is a very important redox reaction in our food chain!
Oxidation and Reduction: Overview
This type of reaction involves the exchange of electrons from one substance to another. When a reaction has one or more electrons transfer from one particle to another, it is called an oxidation-reduction reaction, or redox reaction for short. Sodium chloride, for example, can be formed by the reaction of sodium metal with chlorine gas. Na(s) + Cl2(g) → 2NaCl(s)
How are combustion reactions analyzed and balanced?
To identify the reactants and products of combustion reactions, examine a reaction for its pattern. Remember, combustion reactions can also be synthesis reactions, which is the combining of reactants to make one product. Here are the patterns of reactants and products for combustion reactions: CxHx + O2 → H2O + CO2 (organic compounds) A + O2 → AOx (inorganic compounds)
2H2(g) + O2(g) → 2H2O(l)
Two molecules of hydrogen gas combine with one molecule of oxygen gas to produce two molecules of water. When an atom or molecule has no coefficient, it is understood to be one.
Ion Breakdown
When the ions trade partners, they must combine to form neutral compounds. This means each ion is given an appropriate subscript to keep charges balanced in each chemical formula.
You Can't Have One Without the Other
You Can't Have One Without the Other Oxidation and reduction go hand in hand. One substance cannot be oxidized unless another substance is reduced. If one element gains electrons (reduction), it is because another element lost them (oxidation). An element will not lose electrons (oxidation) unless another element takes them (reduction). Na(s)+ Charge: 0 Cl2(g) Charge: 0 2NaCl(s) Charge of Na: +1 Charge of Cl: -1
Exothermic
a chemical or physical change that releases energy to the surroundings
Activity series
a list of elements ranked according to their reactivity, or the ease with which they undergo a chemical reaction Chemists often use a list called an activity series to predict whether a single replacement reaction will occur. An activity series lists the elements in order of reactivity. In a single replacement reaction, a reaction will occur if the replacement element is more reactive (higher on the activity list) than the element it is trying to replace.
Oxidation numbers
a number assigned to an atom of an element that represents the number of electrons lost or gained
coefficient
a numerical or constant quantity placed before each compound in a chemical equation to balance it
decomposition reaction
a reaction in which a single compound reacts to form more than one product
Oxidation-reduction (redox) reaction
a reaction in which electrons are exchanged from one substance to another, also called redox
synthesis reaction
a reaction in which two or more reactants combine to form one product
chemical equations
a representation, using formulas and symbols, of a chemical reaction
Precipitate
a solid that separates from a solution, usually an insoluble product formed from a chemical reaction in an aqueous solution
aqueous solution
a solution in which the solvent is water
product
a substance formed after a chemical reaction
reactant
a substance that takes part in and undergoes change during a reaction.
Single replacement reaction
a type of reaction in which one element replaces a similar element within a compound In a single displacement reaction, one element switches places with another element in a compound. The reactants in a single displacement reaction are always an element and a compound, and the products are a different element and a different compound. (Three pairs of atoms bonded, one pair is two small green spheres, and the other two pairs consist of a grey and red sphere each. After the reaction, the one green sphere bonds with one grey sphere and the two leftover red spheres form a bond.) For example, in this reaction the element zinc takes the place of hydrogen in the compound hydrochloric acid. When the zinc and hydrogen switch places, the products are zinc chloride and hydrogen gas. (A large sphere representing zinc comes close to two smaller pairs of spheres that are green and blue representing hydrochloric acid. After the reaction, the two green spheres bond with the zinc leaving the two blue spheres as a bonded pair.)
Double replacement reaction
a type of reaction in which the ions of two compounds exchange places in an aqueous solution to form two new compounds In a double displacement reaction, atoms from two different compounds switch places. The reactants are always two compounds, and the products are always two different compounds. Double displacement reactions are like two pairs of dancers switching partners. (One molecule consists of two brown and three red spheres bonded. There are six pairs of green and light blue spheres bonded. After the reaction, there are five molecules. Two consist of a brown sphere with three green spheres, and three molecules consists of a red sphere bonded to two light blue spheres.) With double replacement reactions occurring in an aqueous solution, a reaction is only noticeable when a change in appearance or state occurs. The following examples illustrate some observable changes that can occur during a double replacement reaction.
Combustion reaction
any reaction in which a substance reacts with oxygen gas
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most reactive lithium (Li) potassium (K) barium (Ba) sodium (Na) calcium (Ca) magnesium (Mg) aluminum (Al) zinc (Zn) cobalt (Co) nickel (Ni) lead (Pb) copper (Cu) silver (Ag) gold (Au) least reactive
A reaction between CH4 (methane) and O2 (Oxygen) with an arrow pointing to the right to the products, CO2 (carbon dioxide) and H2O (water). The C in CH 4 is labeled with a negative 4 charge. The H in CH4 is labeled with a positive 1 charge for each H.
n methane, hydrogen gets an oxidation number of +1 because that is its oxidation number in any covalent compound. Carbon's oxidation number in methane must be −4, to balance out the four hydrogen atoms (each −1) to give an overall sum of zero for the oxidation numbers of this neutral compound.
Redox reactions
reactions that involve the transfer of electrons
law of conservation of mass
the law that explains that mass cannot be created or destroyed within a closed system
Electrolysis
the process in which an electric current is used to decompose a substance
Reduction
when a substance gains one or more electrons during a reaction, therefore attaining a lower, more negative oxidation number
Oxidation
when a substance loses one or more electrons during a reaction, therefore attaining a higher, more positive oxidation number
Because every chemical equation is different, there are no exact steps to follow that will make sure the equation balances. However, here are some helpful tips to help you make sure an equation is balanced.
✔ Count up the number of atoms for each element. Balance each element one at a time. ✔ Adding a number in front of a compound affects the number of each element in that compound. Check how this new coefficient affects each element. ✔ Sometimes an element is found in more than one compound on the same side of an equation. Try balancing the other elements first. ✔ Balancing equations is a puzzle. Use a pencil because you may have to change a coefficient more than once. ✔ If a polyatomic ion appears on both sides of the equation, balance the ion as a single unit. ✔ Double check that every element is balanced on both sides of the equation.
