CHAPTER 9 Chemical Reactions 9.1 Reactions and Equations The process by which atoms of one or more substances are rearranged to form different substances is called a chemical reaction. There are a number of possible indications that a chemical reaction has occurred. Among these are absorption of heat and release of energy in the form of light or heat, color change, production of an odor, and appearance of gas bubbles or a solid. Statements called equations are used to represent chemical reactions. Equations indicate the reactants, or starting substances, and the products, or substances formed during the reaction. The reactants are written to the left of an arrow that stands for yields, and the products are written to the right. Plus signs are used to separate the different reactants or products. The symbols (s), (l), (g), and (aq) indicate solid, liquid, gas, or water solution (aqueous), respectively. Equations can be in the form of word equations, in which the reactants or products are indicated by their names. Word equations can be replaced by skeleton equations, which use chemical formulas rather than words to identify reactants and products. Skeleton equations are more informative than word equations because skeleton equations identify the atoms that make up each reactant and product. Example Problem 1 Writing Word and Skeleton Equations Write a word equation and a skeleton equation for the reaction in which hydrogen gas reacts with oxygen gas to form liquid water. The word equation uses the names and physical states of the reactants and products, along with the arrow and plus symbols. hydrogen(g) oxygen(g) water(l) The skeleton equation replaces the names of the substances with their chemical formulas. H 2(g) O 2(g) H 2O(l) 81
1. Write a word equation and a skeleton equation for each of the following descriptions of chemical reactions. a. Solid lithium reacts with chlorine gas to produce solid lithium chloride. b. Nitrogen gas reacts with oxygen gas to produce nitrogen dioxide gas. c. Solid iron reacts with aqueous copper(ii) nitrate to produce solid copper and aqueous iron(ii) nitrate. Balanced chemical equations Skeleton equations do not reflect the fact that matter is conserved during a reaction. In actual chemical reactions, atoms are neither created nor destroyed but are conserved. An equation that reflects the fact that the same number of each kind of atom must appear on both sides of the arrow is called a balanced chemical equation. It uses chemical formulas to show the identities and relative amounts of the substances involved in a reaction. To balance a chemical equation, you write coefficients, which are numbers placed in front of the chemical formulas. The lowest wholenumber ratio is used. If no coefficient appears next to a formula, the number 1 is understood. The steps involved in balancing a chemical equation are as follows. Step 1 Write the skeleton equation. Step 2 Count the atoms of the elements in the reactants. Any polyatomic ions that remain unchanged can be counted as if they are elements. Step 3 Count the atoms of the elements in the products. Step 4 Change the coefficients to make the number of atoms of each element equal on both sides of the equation. Do not change subscripts. Step 5 Write the coefficients in their lowest possible ratio. Step 6 Check your work. 82
Example Problem 2 Writing a Balanced Chemical Equation Write the balanced chemical equation for the reaction in which solid sulfur (S) combines with oxygen gas to produce sulfur trioxide gas. Step 1 Write the skeleton equation. S(s) O 2(g) SO 3(g) Step 2 Count atoms of each element in the reactants. 1 S, 2 O Step 3 Count atoms of each element in the products. 1 S, 3 O Step 4 Adjust coefficients. In the formulas O 2 and SO 3, oxygen is in a 2 to 3 ratio. To balance O, 3 O 2 are needed for every 2 SO 3. Insert the coefficients 3 and 2, respectively. S(s) 3O 2(g) 2SO 3(g) Now the S atoms must be accounted for. The placement of the 2 before the SO 3 means that there are now 2 S atoms on the right. There must also be 2 on the left. Place the coefficient 2 in front of the reactant S. 2S(s) 3O 2(g) 2SO 3(g) Step 5 Write the coefficients in their lowest possible ratio. The ratio 2:3:2 cannot be written any lower and still produce whole numbers. The ratio is already the lowest possible. Step 6 Check your work. Count the number of atoms of each element on both sides of the equation and make sure they are equal. Reactants: 2 S, 6 O Products: 2 S, 6 O The numbers are equal, so the equation is correctly balanced. 83
2. Write a balanced chemical equation for the following reactions, making sure coefficients are in their lowest possible ratio. a. Solid potassium reacts with liquid water to produce hydrogen gas and a water solution of potassium hydroxide. b. Calcium chloride and sodium carbonate in water solution produce solid calcium carbonate and a water solution of sodium chloride. c. Liquid bromine and solid lithium iodide react to form solid lithium bromide and solid iodine. 9.2 Classifying Chemical Reactions Reactions can be classified into various types. In a synthesis reaction, two or more substances react to produce a single product. An example is the reaction between the elements potassium and bromine to form potassium bromide. 2K(s) Br 2(l) 2KBr(s) Synthesis reactions can also occur between compounds or between a compound and an element. In a combustion reaction, oxygen combines with a substance and produces light and heat. The burning of methane in oxygen is an example of a combustion reaction. CH 4(g) 2O 2(g) CO 2(g) 2H 2O(g) Certain reactions may be examples of both synthesis and combustion. An example is the burning of carbon. C(s) O 2(g) CO 2(g) A decomposition reaction is one in which a single compound breaks down into two or more elements or new compounds. An example is the breakdown of aluminum oxide into aluminum and oxygen gas. 2Al 2O 3(s) 4Al(s) 3O 2(g) 84
3. Identify each of the following skeleton equations as representing a synthesis reaction, a combustion reaction, both synthesis and combustion, or decomposition reaction. Balance each equation if necessary. a. N 2(g) + H 2(g) NH 3(g) b. CaCO 3(s) CaO(s) + O 2(g) c. Se(s) + O 2(g) SeO 3(g) d. C 2H 4(g) + O 2(g) CO 2(g) + H 2O(g) Replacement reactions In a single-replacement reaction, atoms of an element replace the atoms of another element in a compound. An example of such a reaction is the replacement by zinc metal of silver in silver nitrate water solution. Zn(s) + 2AgNO 3(aq) Zn(NO 3) 2(aq) + 2Ag(s) Not all metals will replace one another. Only a metal that is more reactive chemically will replace a chemically less reactive metal. The order of such reactivity is called an activity series, as shown below. Any metal in an activity series will replace the metals below it in their compounds. In a similar fashion, any elemental halogen will replace another halogen from its compounds if the second halogen lies below it in the periodic table. Thus, fluorine is more active than chlorine, which is more active than bromine, which is more active than iodine. 85
Example Problem 3 Using an Activity Series to Predict Single-Replacement Reactions Use the above activity series to predict the single-replacement reaction that will occur between aluminum metal and lead(ii) nitrate (Pb(NO 3) 2) in a water solution and write its balanced chemical equation. Aluminum is above lead in the activity series and therefore is more active. It will replace lead in its compounds in water solution. A singlereplacement reaction will take place to form the nitrate of aluminum, a group 13 metal. The skeleton equation is shown below. Al(s) + Pb(NO 3) 2(aq) Al(NO 3) 3(aq) + Pb(s) Adding coefficients, the balanced chemical equation is shown below. 2Al(s) + 3Pb(NO 3) 2(aq) 2Al(NO 3) 3(aq) + 3Pb(s) 4. Predict whether a single-replacement reaction will occur between the following pairs of possible reactants. If so, write a balanced chemical equation for the reaction. a. chlorine gas and aqueous potassium iodide b. magnesium metal and aqueous copper(ii) sulfate c. copper metal and iron(iii) chloride in water solution d. lead metal and aqueous silver nitrate In another type of replacement reaction, called a double-replacement reaction, there is an exchange of positive ions between two compounds, often in water solution. Often, a solid of low solubility is produced during the reaction and settles out of the solution. Such a solid is called a precipitate. An example of a double-replacement reaction that produces the precipitate lead(ii) chloride is the following. Pb(NO 3) 2(aq) + 2NaCl(aq) 2NaNO 3(aq) + PbCl 2(s) 5. Write a balanced chemical equation for each of the following double-replacement reactions. 86
a. A water solution of zinc bromide (ZnBr 2) and a water solution of potassium hydroxide form a water solution of potassium bromide and a precipitate of zinc hydroxide. b. A water solution of copper(ii) sulfate and a water solution of barium chloride produce a water solution of copper(ii) chloride and solid barium sulfate. c. A precipitate of iron(iii) carbonate and a water solution of sodium nitrate are formed when a water solution of iron(iii) nitrate and a water solution of sodium carbonate are mixed. 9.3 Reactions in Aqueous Solutions When a substance dissolves in water, a solution forms. A solution is a homogeneous mixture because it has a constant composition throughout. A solution contains one or more substances called solutes dissolved in the solvent. The solvent is the most plentiful substance in a solution. An aqueous solution is a solution in which the solvent is water. When dissolved to form aqueous solutions, the ions of ionic compounds separate. Some molecular compounds also produce dissolved ions in water. If hydrogen ions are produced, the substance is called an acid. For example, the gaseous molecular compound hydrogen chloride (HCl) forms H and Cl ions in aqueous solution, which is called hydrochloric acid. When aqueous solutions that contain ions are mixed, the ions may react in a double-replacement reaction. The product is typically a solid precipitate, water, or a gas. An example of a double-replacement reaction that produces a precipitate occurs when aqueous solutions of sodium chloride and silver nitrate are mixed to form a precipitate of solid silver chloride. NaCl(aq) + AgNO 3(aq) NaNO 3(aq) + AgCl(s) To show all of the particles in solution as they really exist, a complete ionic equation can be written. Na (aq) + Cl (aq) + Ag (aq) + NO 3 (aq) Na (aq) + NO 3 (aq) + AgCl(s) The sodium and nitrate ions are on both sides of the equation. Such ions that do not participate in a reaction are called spectator ions. An ionic equation that does not show spectator ions but only the particles that 87
participate in a reaction is called a net ionic equation. In the case of the reaction above, the net ionic equation from which the sodium and nitrate ions have been removed is as follows. Cl (aq) + Ag (aq) AgCl(s) Example Problem 4 Writing Ionic Equations Write the balanced chemical equation for the reaction between aqueous solutions of strontium nitrate and potassium sulfate, which forms the precipitate strontium sulfate. Then write the complete ionic and net ionic equations. Write the correct skeleton equation. Sr(NO 3) 2(aq) + K 2SO 4(aq) KNO 3(aq) + SrSO 4(s) Use coefficients to produce the balanced chemical equation. Sr(NO 3) 2(aq) + K 2SO 4(aq) 2KNO 3(aq) + SrSO 4(s) Write the complete ionic equation. Sr 2 (aq) + 2NO 3 (aq) + 2K (aq) + SO 4 2 (aq) 2K (aq) + 2NO 3 (aq) + SrSO 4(s) Cross out the spectator ions, which are those that are on both sides of the equation. 2 2 Sr (aq) 2NO (aq) 2K (aq) SO (aq) 3 4 That leaves the net ionic equation. Sr 2 (aq) + SO 4 2 (aq) SrSO 4(s) 2K (aq) 2NO 3 (aq) SrSO 4 (s) 6. Write balanced chemical, complete ionic, and net ionic equations for each of the following reactions. a. Aqueous solutions of lead(ii) nitrate and ammonium chloride are mixed, forming a precipitate of lead(ii) chloride. b. Aqueous solutions of aluminum chloride and sodium carbonate are combined, producing solid aluminum carbonate. 88
Reactions that form water or a gas Some double-replacement reactions in aqueous solution produce water or a gas (or both) rather than a precipitate. In such cases, the water or gas is shown as a product in the net ionic equation, as are the ions that produced it. The remaining ions are eliminated as spectator ions. The following example problem illustrates this concept. Example Problem 5 Writing Equations for a Reaction That Produces Water When hydrochloric acid and potassium hydroxide solutions are mixed, water results, together with an aqueous solution of potassium chloride. Write the balanced chemical equation, a complete ionic equation, and a net ionic equation for this reaction. The balanced chemical equation is the same as the skeleton equation. HCl(aq) + KOH(aq) H 2O(l) + KCl(aq) Write the complete ionic equation, which includes all of the ions. H (aq) + Cl (aq) + K (aq) + OH (aq) H 2O(l) + K (aq) + Cl (aq) Remove the spectator ions to produce the net ionic equation. H (aq) + OH (aq) H 2O(l) 7. Write balanced chemical, complete ionic, and net ionic equations for the reactions between the following substances, which produce water. a. nitric acid (HNO 3) and aqueous barium hydroxide b. sulfuric acid (H 2SO 4) and aqueous sodium hydroxide c. phosphoric acid (H 3PO 4) and aqueous lithium hydroxide Example Problem 6 Writing Equations for a Reaction That Produces a Gas Write balanced chemical, complete ionic, and net ionic equations for the reaction between aqueous solutions of sodium sulfide and hydrochloric acid. Hydrogen sulfide gas is produced, along with an aqueous solution of sodium chloride. 89
Write the skeleton equation with the correct formulas. Na 2S(aq) + HCl(aq) NaCl(aq) + H 2S(g) Add coefficients to write a balanced chemical equation. Na 2S(aq) + 2HCl(aq) 2NaCl(aq) + H 2S(g) Write the complete ionic equation, which includes all the ions. 2Na (aq) + S 2 (aq) + 2H (aq) + 2Cl (aq) 2Na (aq) + 2Cl (aq) + H 2S(g) Remove the spectator ions to produce the net ionic equation. S 2 (aq) + 2H (aq) H 2S(g) 8. Write balanced chemical, complete ionic, and net ionic equations for the reactions between the following substances, which produce a gas. a. hydrochloric acid and aqueous sodium cyanide, with production of hydrogen cyanide gas (HCN) b. sulfuric acid (H 2SO 4) and aqueous rubidium sulfide, with production of hydrogen sulfide gas Chapter 9 Review 9. Define chemical reaction. Where are the reactants and products shown in an equation for a chemical reaction, and how are the physical states indicated? 10. Compare and contrast word equations, skeleton equations, and balanced chemical equations. 11. Briefly list the steps used in balancing a chemical equation. 12. Contrast synthesis, combustion, and decomposition reactions. 13. Contrast single-replacement and double-replacement reactions. 14. Define aqueous solution. What name is given to a solid that forms when two aqueous solutions are mixed? 15. How do complete ionic and net ionic equations differ? 16. Name three typical types of products that may be formed in doublereplacement reactions. 90