types of reactions

Combination and Decomposition Reactions

Recall that when hydrogen and oxygen combine, they form water and release energy. In 1937, this reaction took place with frightening force when the Hindenburg blimp, filled with hydrogen gas, exploded over New Jersey. The hydrogen gas in the blimp reacted with atmospheric oxygen to form water and a great deal of energy. Because of this tragedy, modern blimps now are filled with the nonreactive gas, helium. Such a reaction of hydrogen with oxygen is an example of a combination, or synthesis reaction. In this reaction, two or more reactants combine to produce one product. The general form of a combination reaction is as follows: X + Y → XY The reactants in a combination reaction can be either elements or compounds. But because the product of a combination reaction is always more complex than the reactants, it is always a compound.Let's take a look at a few common combination reactions: When a Group A metal cation reacts with a nonmetal, the product is an ionic compound. An example is this reaction of sodium and chlorine gas to produce sodium chloride or common table salt: 2Na + Cl2 → 2NaCl Compounds of nonmetals and oxygen, called nonmetal oxides, often combine with water to produce acids, which have low pH values. For example, carbon dioxide combines with water to produce carbonic acid. This reaction occurs in carbonated beverages such as soda pop where the carbonic acid formed is responsible for the fizz in sodas: H2O + CO2 → H2CO3 Metallic oxides combine with water to form bases, which have high pH values. For example, calcium oxide combines with water to yield calcium hydroxide: CaO + H2O → Ca(OH)2 Combination reactions also occur when metals combine with oxygen to form metal oxides. Play the video. It shows the combination reaction between sodium and oxygen, which forms the compound sodium oxide: Na + O2 → Na2O (Note: this equation is unbalanced as written.) Balance this equation now. Then return to the lesson and click Next to continue.ust as water molecules can be formed by the combination reaction of hydrogen and oxygen, they can be broken apart into oxygen and hydrogen atoms by using direct current electricity. This reaction can be chemically written as follows: 2H2O → 2H2 + O2 This reaction is the reverse of the combination reaction and is called a decomposition reaction. In a decomposition reaction, one reactant breaks down into two or more products. The general form for a decomposition reaction is this: XY → X + Y However, because the reactants are usually stable, energy in the form of heat, light, or electricity is often required for decomposition reactions to occur. Let's look at a few examples of decomposition reactions and the sources that trigger them.Light triggers a decomposition reaction when silver chloride—a light-sensitive crystalline solid—is exposed to light. Silver chloride decomposes in the presence of light to form silver metal and chlorine. Photography uses this decomposition reaction. Metallic silver produced in the reaction is deposited on the photography film to form an invisible, or latent, image. This image is developed later to form a visible image. 2AgCl → 2Ag + Cl2 Certain salts such as tin chloride can undergo electrolytic decomposition; that is, they break down into the elements, tin and chlorine, when exposed to direct current electricity: SnCl2 → Sn + Cl2 Some decomposition reactions occur spontaneously. In other words, they do not need energy. Carbonic acid, for example, decomposes spontaneously to give carbon dioxide and water. This reaction can be shown this way: H2CO3 → H2O + CO2

Combustion of Hydrocarbons

The fifth category of chemical reactions is the combustion reaction. When an organic substance combines with oxygen to produce heat and light, a combustion of a hydrocarbon reaction occurs. Many important combustion reactions occur with organic molecules called hydrocarbons, which are made up of only carbon and hydrogen, and produce carbon dioxide and water as products. Much of the energy used in your everyday life comes from combustion reaction of molecules with oxygen. For example, propane (C3H8) in a camp stove combines with oxygen to generate heat for cooking. This reaction can be written as follows: C3H8 + 5O2 → 3CO2 + 4H2O Gasoline (C8H18) reacts with oxygen, and when burned, it generates enough energy to power a car. This reaction can be written as follows: C8H18 + O2 → CO2 + H2O (Note that this equation is unbalanced as written.)The previous examples showed how carbon dioxide is produced when you burn a substance in the presence of oxygen. However, carbon dioxide isn't always the product. If hydrocarbons burn in insufficient amounts of oxygen, carbon monoxide (CO) forms instead of carbon dioxide. Formation of carbon monoxide results from incomplete combustion of carbon when there is not enough oxygen. While carbon dioxide is relatively nontoxic and is generated by animals and used by plants, carbon monoxide is a poisonous gas even at very low concentrations. For this reason, combustion reactions should always take place in well-ventilated areas. For example, wood combusts with oxygen, just as a hydrocarbon does. If you burn wood in a fireplace with a closed flue, the room may fill with toxic carbon monoxide due to incomplete combustion. Incomplete combustion can also produce soot, which consists of carbon particles. A typical campfire will produce carbon dioxide, carbon monoxide, and soot, depending on the availability of oxygen in a given part of the campfire. Combustion reactions also occur in the human body during metabolism. The energy produced during these reactions is necessary for the functioning, growth, and reproduction of most living organisms. Carbohydrates are molecules associated with living organisms. In contrast to hydrocarbons, carbohydrates contain carbon, hydrogen, and also oxygen. Glucose and other sugars are carbohydrates as are many other complex molecules. Carbohydrates react with plentiful oxygen in organisms to produce carbon dioxide and water. This reaction occurs much more slowly than the burning of hydrocarbons, though. The reaction of carbohydrates with oxygen in organisms is called metabolism instead of combustion, but the products are the same. The equation for the combustion of glucose is as follows: C6H12O6 + 6O2 → 6CO2 + 6H2O Fats and proteins are also metabolized in organisms to produce carbon dioxide and water, but proteins also produce nitrogen compounds. Metabolism of substances in your body with insufficient oxygen results in the lower energy production along with potentially toxic products, such as ethanol and lactic acid.

Double-Replacement and Acid-Base Reactions

The fourth category of chemical reactions is the double-replacement reaction. This reaction is also called double-displacement reaction and has this general form: AX + BY → BX + AY This type of reaction involves the exchange of cations between two compounds. For example, if you mix solutions of lead nitrate and potassium chloride, lead cations bond to chloride anions and settle on the bottom as a white solid. Potassium cations and nitrate anions combine to form potassium nitrate, which exists in aqueous state (by its nature, it is a soluble ionic substance). The white solid that settles to the bottom is lead chloride and is referred to as a precipitate (by its nature, it's an insoluble ionic substance). This reaction is a double-displacement reaction because an exchange of atoms or ions occurs between compounds and is written as follows: Pb(NO3)2(aq) + 2KCl(aq) → PbCl2(s) + 2KNO3(aq) Double-displacement reactions can be of three types depending on the nature of the products: a product could be a solid that precipitates out of the solution, a product could be a gas, or it could be water. Let's look at some examples of double-replacement reactions and the products. The reaction between aqueous solutions of sodium sulfide and cadmium nitrate is an example of a double-replacement reaction forming a precipitate. A yellow precipitate of cadmium sulfide is formed. This is the equation for this reaction: Na2S(aq) + Cd(NO3)2(aq) → CdS(s) + 2NaNO3(aq) Gas is formed as a product in a double-replacement reaction between hydrochloric acid and sodium sulfide. Hydrogen sulfide gas is produced and bubbles out of the solution, while sodium chloride remains in the solution. This is the equation for this reaction: 2HCl(aq) + Na2S(aq) → H2S(g) + 2NaCl(aq) Water is formed as a product in the double-replacement reaction between sulfuric acid and aluminum hydroxide. In this reaction, aluminum sulfate salt is also generated: 3H2SO4(aq) + 2Al(OH)3(aq) → Al2(SO4)3(aq) + 6H2O(l) This double-replacement reaction is an example of an acid-base neutralization reaction. In short, double-displacement reactions involving an acid and a base resulting in the formation of salt and water are acid-base neutralization reactions: acid + base → salt + water

Oxidation-Reduction Reactions

The sixth category of chemical reactions is the oxidation-reduction reaction, better known as the redox reaction. Rusting is a very common redox reaction. If an iron nail is exposed to air for a long time, it combines with oxygen in the air, in the presence of moisture, to form iron(III) oxide, rust. The rusting of iron can be written as follows: 4Fe + 3O2 → 2Fe2O3 This reaction is an oxidation reaction where iron reacts with oxygen to form an oxide. Although rusting is a reaction of iron with oxygen, it is not a combustion of a hydrocarbon reaction since it does not involve burning, and water and carbon dioxide are not formed as products. In fact, combustion is a type of oxidation reaction because it too produces oxide of carbon, CO2. Conversely, when a compound loses oxygen, a reduction reaction takes place. For example, iron(III) oxide can be reduced to metallic iron by heating it in the presence of carbon: 2Fe2O3 + 3C → 4Fe + 3CO2 Note that as iron oxide is reduced, carbon is oxidized to carbon dioxide. In oxidation-reduction reactions, when one compound is oxidized, the other is reduced. But, oxidation and reduction do not always involve oxygen. Let's see how such reactions are described.

Single-Replacement Reactions

The third category into which chemical reactions fall is the single-replacement reaction; this reaction is also known as a single-displacement reaction. In a single-replacement reaction, one element takes the place of another element in a compound. A single-replacement reaction has this general form: AX + B → BX + A A so-called violent single-replacement reaction occurs when a piece of metallic potassium is placed in a dish of water. Potassium displaces one of the hydrogen atoms in a water molecule producing hydrogen gas and aqueous potassium hydroxide, as shown in the equation. In this case, potassium takes the place of one of the hydrogen atoms in a water molecule. 2K(s) + 2H20(l) → 2KOH(aq) + H2(g) A less dramatic single-replacement reaction takes place when a strip of zinc metal is placed in a solution of copper sulfate. The zinc replaces the copper ion in the solution and forms zinc sulfate, as shown in the equation. The copper that is displaced is deposited on the zinc strip. CuSO4(aq) + Zn(s) → ZnSO4(aq) + Cu(s) It's important to note that not all elements can displace other elements in single-displacement reactions. For example, on the previous screen, zinc displaced copper from a solution of copper sulfate. However, the reverse is not possible: copper cannot spontaneously displace zinc from a solution containing zinc ions. In fact, chemists have experimentally determined which metals displace other metals. The results can be displayed on a chart, showing the hierarchy of displacements. Any element in the chart displaces the elements below it and is displaced by the elements above it. This arrangement of metal elements is known as the activity series and shows the relative reactivity of each metal element. Thus, you can see that lithium will displace any metal on the chart, and any metal will displace gold. You may have noticed that the element hydrogen is on the activity chart with the metals even though hydrogen is not a metal. It is placed there because in replacement reactions, hydrogen behaves like a metal; that is, it donates electrons during bond formation The activity series can be used to predict the outcomes of reactions. For example, in the single-replacement reaction demonstrated earlier, the displacement of copper and the formation of zinc sulfate could have been predicted because zinc is higher on the chart than copper and thus is more reactive. So, if you were to place a copper wire into a solution of silver nitrate, would the copper displace the silver? Looking at the activity series, you can see that it would. Silver metal will be deposited and aqueous copper(II) nitrate would form, giving the solution a bluish tint. Below is the equation for this reaction: 2AgNO3(aq) + Cu(s) → Cu(NO3)2(aq) + 2Ag(s) Now can you guess what would happen if you placed silver into copper(II) nitrate solution? In this case, a look at the activity series tells you that no reaction will take place because silver is less reactive than copper.Take a look at the activity series of common metals. Observe how the reactivity with water changes. Highly reactive metals such as lithium and potassium react with cold water. Less reactive metals such as magnesium and aluminum react with steam whereas metals less reactive than these react only with acids. You may want to save or print the activity series document for future reference. Single-replacement reactions also occur when nonmetal elements displace other nonmetal elements. In halogens, reactivity follows specific trends down the group in the periodic table. Recall the trends of other properties such as electronegativity and ionization energy in the periodic table. Halogens have decreasing reactivity going down the group with fluorine being the most reactive and iodine being the least reactive. A halogen can displace any halogen ion that appears below it: bromine can displace iodine, but it cannot displace chlorine. Here are the reactions: Br2(l) + 2NaI(aq) → 2NaBr(aq) + I2(s) Br2(l) + 2NaCl(aq) → no reaction Other nonmetals also show trends in displacement. With the exception of the nonreactive noble gases, nonmetals on the right of the periodic table or higher on the chart tend to displace those that are lower in the chart or are located on the left of the periodic table. Knowing this, you can predict that sulfur will not displace oxygen from zinc oxide because sulfur is lower on the periodic table than oxygen.