Holt Chemistry Chapter 8 Chemical Equations and Reactions PDF

C H A P T E R 258 Copyright © by Holt, Rinehart and Winston. All rights reserved.  W ith an eruption of flames and hot gases, a space shuttle leaves the ground on its way into orbit. The brightness and warmth of the flame clearly indicates that a change is occurr...

C H A P T E R 258 Copyright © by Holt, Rinehart and Winston. All rights reserved.  W ith an eruption of flames and hot gases, a space shuttle leaves the ground on its way into orbit. The brightness and warmth of the flame clearly indicates that a change is occurring. From the jets of the shuttle itself, blue flames emerge. These flames are the result of a reaction between hydrogen and oxygen. The sight is awesome and beautiful. In this chapter, you will learn about chemical reactions, such as the ones that send a space shuttle into space. START-UPACTIVITY S A F ET Y P R E C A U T I O N S Observing a Chemical Reaction CONTENTS 8 PROCEDURE SECTION 1 1. Place about 5 g (1 tsp) of baking soda into a sealable plastic bag. 2. Place about 5 mL (1 tsp) of vinegar into a plastic film canister. Describing Chemical Secure the lid. Reactions 3. Place the canister into the bag. Squeeze the air out of the bag, and tightly seal the bag. SECTION 2 4. Use a balance to determine the total mass of the bag and the bag’s Balancing Chemical contents. Make a note of this value. Equations 5. Open the canister without opening the bag, and allow the vinegar and baking soda to mix. SECTION 3 6. When the reaction has stopped, measure and record the total mass of the bag and the bag’s contents. Classifying Chemical Reactions ANALYSIS 1. What evidence shows that a chemical reaction has taken place? SECTION 4 2. Compare the masses of the bag and its contents before and after Writing Net Ionic the reaction. What does this result demonstrate about chemical reactions? Equations Pre-Reading Questions 1 What are some signs that a chemical change may be taking place? 2 What are the reactants of a reaction? What are the products of a reaction? 3 Describe the law of conservation of mass. 4 Define the terms synthesis and decomposition, and describe what you would expect to happen in each of these types of reactions. 259 Copyright © by Holt, Rinehart and Winston. All rights reserved. S ECTI O N X 1 Describing Chemical Reactions KEY TERMS O BJ ECTIVES chemical reaction 1 List evidence that suggests that a chemical reaction has occurred and chemical equation evidence that proves that a chemical reaction has occurred. 2 Describe a chemical reaction by using a word equation and a formula equation. 3 Interpret notations in formula equations, such as those relating to states of matter or reaction conditions. Chemical Change You witness chemical changes taking place in iron that rusts, in milk that turns sour, and in a car engine that burns gasoline. The processes of diges- tion and respiration in your body are the result of chemical changes. chemical reaction A chemical reaction is the process by which one or more substances the process by which one or change into one or more new substances whose chemical and physical more substances change to properties differ from those of the original substances. In any chemical produce one or more different reaction, the original substances, which can be elements or compounds, substances are known as reactants. The substances created are called products. A common example of a chemical reaction is shown in Figure 1. Evidence of a Chemical Reaction It’s not always easy to tell that a chemical change is happening, but there are some signs to look for, which are summarized in Table 1. For example, certain signs indicate that wood burning in a campfire is undergoing a chemical change. Smoke rises from the wood, and a hissing sound is made. Energy that lights up the campsite and warms the air around the fire is released. The surface of the wood changes color as the wood burns. Eventually, all that remains of the firewood is a grey, powdery ash. In Figure 2, you can see copper reacting with nitric acid. Again, several clues suggest that a chemical reaction is taking place. The color of the solution changes from colorless to blue. The solution bubbles and fizzes as a gas forms. The copper seems to be used up as the reaction continues. Sometimes, the evidence for a chemical change is indirect. When you place a new battery in a flashlight, you don’t see any changes in the bat- Figure 1 Chemical changes occur tery. However, when you turn the flashlight on, electrical energy causes as wood burns. Two products the filament in the bulb to heat up and emit light. This release of electri- formed are carbon dioxide cal energy is a clue that a chemical reaction is taking place in the battery. and water. Although these signs suggest a change may be chemical, they do not prove that the change is chemical. 260 Chapter 8 Copyright © by Holt, Rinehart and Winston. All rights reserved.  Table 1 Evidence of Chemical Change Changes in energy Formation of new substances release of energy as heat formation of a gas release of energy as light formation of a precipitate (an insoluble solid) production of sound change in color reduction or increase of temperature change in odor absorption or release of electrical energy Chemical Reaction Versus Physical Change For proof of a chemical change, you need a chemical analysis to show that at least one new substance forms. The properties of the new substance— such as density, melting point, or boiling point—must differ from those of the original substances. Even when evidence suggests a chemical change, you can’t be sure immediately. For example, when paints mix, the color of the resulting paint differs from the color of the original paints. But the change is physi- cal—the substances making up the paints have not changed. When you boil water, the water absorbs energy and a gas forms. But the gas still con- Figure 2 sists of water molecules, so a new substance has not formed. Even though When copper reacts with nitric acid, several signs of a they demonstrate some of the signs of a chemical change, all changes of reaction are seen. A toxic, state, including evaporation, condensation, melting, and freezing, are brown gas is produced, and physical changes. the color of the solution changes. NO−3 H3O+ NO2 H2O Cu2+ Cu H2O NO−3 Chemical Equations and Reactions 261 Copyright © by Holt, Rinehart and Winston. All rights reserved.  Figure 3 Energy is released as the elements sodium and chlorine react to form the compound sodium chloride. Breaking down water into hydrogen and oxygen requires the input of electrical energy. Reactions and Energy Changes www.scilinks.org Chemical reactions either release energy or absorb energy as they hap- Topic: Chemical Reactions pen, as shown in Figure 3. A burning campfire and burning natural gas are SciLinks code: HW4029 examples of reactions that release energy. Natural gas, which is mainly methane, undergoes the following reaction: methane + oxygen  → carbon dioxide + water + energy Notice that when energy is released, it can be considered a product of the reaction. If the energy required is not too great, some other reactions that absorb energy will occur because they take energy from their surroundings. An example is the decomposition of dinitrogen tetroxide, which occurs at room temperature. dinitrogen tetroxide + energy  → nitrogen dioxide Notice that when energy is absorbed, it can be considered a reactant of the reaction. Reactants Must Come Together You cannot kick a soccer ball unless your shoe contacts the ball. Chemical reactions are similar. Molecules and atoms of the reactants must come into contact with each other for a reaction to take place. Think about what happens when a safety match is lighted, as shown in Figure 4. One reactant, potassium chlorate (KClO3) is on the match head. The other reactant, phosphorus, P4, is on the striking surface of the matchbox. The reaction begins when the two substances come together by rubbing the match head across the striking surface. If the reactants are kept apart, the reac- tion will not happen. Under most conditions, safety matches do not ignite by themselves. 262 Chapter 8 Copyright © by Holt, Rinehart and Winston. All rights reserved.  Figure 4 The reactants KClO3 (on the match head) and P4 (on the striking surface) must be brought together for a safety match to ignite. Constructing a Chemical Equation You know that symbols represent elements, and formulas represent compounds. In the same way, equations are used to represent chemical reactions. A correctly written chemical equation shows the chemical chemical equation formulas and relative amounts of all reactants and products. Constructing a representation of a chemical a chemical equation usually begins with writing a word equation. This reaction that uses symbols to word equation contains the names of the reactants and of the products show the relationship between the reactants and the products separated by an arrow. The arrow means “forms” or “produces.” Then, the chemical formulas are substituted for the names. Finally, the equation is balanced so that it obeys the law of conservation of mass. The numbers of atoms of each element must be the same on both sides of the arrow. Writing a Word Equation or a Formula Equation The first step in writing a chemical equation is to write a word equation. To write the word equation for a reaction, you must write down the names of the reactants and separate the names with plus signs. An arrow is used to separate the reactants from the products. Then, the names of the prod- ucts are written to the right of the arrow and are separated by plus signs. The word equation for the reaction of methane with oxygen to form car- bon dioxide and water is written as follows: methane + oxygen  → carbon dioxide + water To convert this word equation into a formula equation, use the for- mulas for the reactants and for the products. The formulas for methane, oxygen, carbon dioxide, and water replace the words in the word equation to make a formula equation. The word methane carries no quantitative meaning, but the formula CH4 means a molecule of methane. This change gives the unbalanced formula equation below. The question marks indi- cate that we do not yet know the number of molecules of each substance. ?CH4 + ?O2  → ?CO2 + ?H2O Chemical Equations and Reactions 263 Copyright © by Holt, Rinehart and Winston. All rights reserved.  Equations and Reaction Information A chemical equation indicates the amount of each substance in the reac- tion. But it can also provide other valuable information about the sub- stances or conditions, such as temperature or pressure, that are needed for the reaction. Equations Are Like Recipes Imagine that you need to bake brownies for a party. Of course, you would want to follow a recipe closely to be sure that your brownies turn out right. You must know which ingredients to use and how much of each ingredient to use. Special instructions, such as whether the ingredients should be chilled or at room temperature when you mix them, are also provided in the recipe. Chemical equations have much in common with a recipe. Like a recipe, any instructions shown in an equation can help you or a chemist Figure 5 be sure the reaction turns out the way it should, as shown in Figure 5. A The equation for the balanced equation indicates the relative amounts of reactants and prod- reaction between baking ucts in the reaction. As discussed below, even more information can be soda and vinegar provides shown by an equation. a lot of information about the reaction. Na+ CO2 HCO−3 C2H3O−2 Na+ H3O+ HC2H3O2 C2H3O−2 H2O H2O NaHCO3(s) + HC2H3O2(aq) → NaC2H3O2(aq) + CO2(g) + H2O(l) 264 Chapter 8 Copyright © by Holt, Rinehart and Winston. All rights reserved. Equations Can Show Physical States and Reaction Conditions The recipe for brownies will specify whether each ingredient should be used in a solid or liquid form. The recipe also may state that the batter should bake at 400°F for 20 min. Additional instructions tell what to do if you are baking at high elevation. Chemical equations are similar. Equations for chemical reactions often list the physical state of each react- ant and the conditions under which the reaction takes place. Look closely at the equation that represents the reaction of baking soda with vinegar. NaHCO3(s) + HC2H3O2(aq)  → NaC2H3O2(aq) + CO2(g) + H2O(l) Baking soda, sodium hydrogen carbonate, is a solid, so the formula is fol- lowed by the symbol (s). Vinegar, the other reactant, is acetic acid dis- solved in water—an aqueous solution. Sodium acetate, one of the products, remains in aqueous solution. So, the formulas for vinegar and sodium acetate are followed by the symbol (aq). Another product, carbon dioxide, is a gas and is marked with the symbol (g). Finally, water is pro- duced in the liquid state, so its formula is followed by the symbol (l). When information about the conditions of the reaction is desired, the arrow is a good place to show it. Several symbols are used to show the conditions under which a reaction happens. Consider the preparation of ammonia in a commercial plant. 350°C, 25 000 kPa  N2(g) + 3H2(g) ←        → 2NH3(g)  catalyst The double arrow indicates that reactions occur in both the forward and reverse directions and that the final result is a mixture of all three sub- stances. The temperature at which the reaction occurs is 350°C. The pressure at which the reaction occurs, 25 000 kPa, is also shown above the arrow. A catalyst is used to speed the reaction, so the catalyst is men- tioned, too. Other symbols used in equations are shown in Table 2. Table 2 State Symbols and Reaction Conditions Symbol Meaning (s), (l), (g) substance in the solid, liquid, or gaseous state (aq) substance in aqueous solution (dissolved in water)  → “produces” or “yields,” indicating result of reaction  → ←  reversible reaction in which products can reform into reactants; final result is a mixture of products and reactants ∆ → or → heat reactants are heated; temperature is not specified → Pd name or chemical formula of a catalyst, added to speed a reaction Refer to Appendix A to see more symbols used in equations. Chemical Equations and Reactions 265 Copyright © by Holt, Rinehart and Winston. All rights reserved.  When to Use the Symbols Although chemical equations can be packed with information, most of the ones you will work with will show only the formulas of reactants and prod- ucts. However, sometimes you need to know the states of the substances. Recognizing and knowing the symbols used will help you understand these equations better. And learning these symbols now will make learning new information that depends on these symbols easier. 1 Section Review CRITICAL THINKING 11. Describe evidence that burning gasoline in UNDERSTANDING KEY IDEAS an engine is a chemical reaction. 12. Describe evidence that chemical reactions 1. What is a chemical reaction? take place during a fireworks display. 2. What is the only way to prove that a chemi- 13. The directions on a package of an epoxy cal reaction has occurred? glue say to mix small amounts of liquid 3. When water boils on the stove, does a from two separate tubes. Either liquid alone chemical change or a physical change take does not work as a glue. Should the liquids place? be considered reactants? Explain your 4. Give four examples of evidence that suggests answer. that a chemical change probably is occurring. 14. When sulfur is heated until it melts and 5. When propane gas, C3H8, is burned with then is allowed to cool, beautiful yellow oxygen, the products are carbon dioxide and crystals form. How can you prove that this water. Write an unbalanced formula equation change is physical? for the reaction. 15. Besides the reactant, what is needed for 6. Assume that liquid water forms in item 5. the electrolysis experiment that breaks Write a formula equation for the reaction that down water? shows the physical states of all compounds. 16. Write the word equation for the electrolysis 7. What does “Mn” above the arrow in a for- of water, and indicate the physical states mula equation mean? and condition(s) of the reaction. 8. What symbol is used in a chemical equation 17. For each of the following equations, write to indicate “produces” or “yields”? a sentence that describes the reaction, 9. Solid silicon and solid magnesium chloride including the physical states and reaction form when silicon tetrachloride gas reacts conditions. with magnesium metal. Write a word equa- a. Zn(s) + 2HCl(aq)  → ZnCl2(aq) + H2(g) tion and an unbalanced formula equation. b. CaCl2(aq) + Na2CO3(aq)  → Include all of the appropriate notations. CaCO3(s) + 2NaCl(aq) 10. Magnesium oxide forms from magnesium c. NaOH(aq) + HCl(aq)  → metal and oxygen gas. Write a word equa- NaCl(aq) + H2O(l) tion and an unbalanced formula equation. ∆ Include all of the appropriate notations. d. CaCO3(s) → CaO(s) + CO2(g) 266 Chapter 8 Copyright © by Holt, Rinehart and Winston. All rights reserved. S ECTI O N 2 Balancing Chemical Equations KEY TERMS O BJ ECTIVES coefficient 1 Relate the conservation of mass to the rearrangement of atoms in a chemical reaction. 2 Write and interpret a balanced chemical equation for a reaction, and relate conservation of mass to the balanced equation. Reactions Conserve Mass A basic law of science is the law of conservation of mass. This law states Topic Link that in ordinary chemical or physical changes, mass is neither created nor destroyed. If you add baking soda to vinegar, they react to release carbon Refer to the “The Science of Chemistry” chapter for more dioxide gas, which escapes into the air. But if you collect all of the prod- information about the law of ucts of the reaction, you find that their total mass is the same as the total conservation of mass. mass of the reactants. Reactions Rearrange Atoms This law is based on the fact that the products and the reactants of a reac- tion are made up of the same number and kinds of atoms. The atoms are just rearranged and connected differently. Look at the formula equation for the reaction of sodium with water. ?Na + ?H2O  → ?NaOH + ?H2 The same types of atoms appear in both the reactants and products. However, Table 3 shows that the number of each type of atom is not the same on both sides of the equation. To show that a reaction satisfies the law of conservation of mass, its equation must be balanced. Table 3 Counting Atoms in an Equation Reactants Products Balanced? Unbalanced formula equation Na + H2O NaOH + H2 Sodium atoms 1 1 yes Hydrogen atoms 2 3 no Oxygen atoms 1 1 yes Chemical Equations and Reactions 267 Copyright © by Holt, Rinehart and Winston. All rights reserved.  Balancing Equations To balance an equation, you need to make the number of atoms for each element the same on the reactants’ side and on the products’ side. But there is a catch. You cannot change the formulas of any of the substances. For example, you could not change CO2 to CO3. You can only place num- coefficient bers called coefficients in front of the formulas. A coefficient multiplies a small whole number that the number of atoms of each element in the formula that follows. For appears as a factor in front of a example, the formula H2O represents 2 atoms of hydrogen and 1 atom of formula in a chemical equation oxygen. But 2H2O represents 2 molecules of water, for a total of 4 atoms of hydrogen and 2 atoms of oxygen. The formula 3Ca(NO3)2 represents 3 calcium atoms, 6 nitrogen atoms, and 18 oxygen atoms. Look at Skills Toolkit 1 as you balance equations. 1 SKILLS Balancing Chemical Equations 1. Identify reactants and products. If no equation is provided, identify the reactants and products and write an unbalanced equation for the reaction. (You may find it helpful to write a word equation first.) If not all chemicals are described in the problem, try to predict the missing chemicals based on the type of reaction. 2. Count atoms. Count the number of atoms of each element in the reactants and in the products, and record the results in a table. Identify elements that appear in only one reactant and in only one product, and balance the atoms of those elements first. Delay the balancing of atoms (often hydrogen and oxygen) that appear in more than one reactant or product. If a polyatomic ion appears on both sides of the equation, treat it as a single unit in your counts. 3. Insert coefficients. Balance atoms one element at a time by inserting coefficients. Count atoms of each element frequently as you try different coefficients. Watch for elements whose atoms become unbal- anced as a result of your work. Try the odd-even technique (explained later in this section) if you see an even number of a particular atom on one side of an equation and an odd number of that atom on the other side. 4. Verify your results. Double-check to be sure that the numbers of atoms of each element are equal on both sides of the equation. 268 Chapter 8 Copyright © by Holt, Rinehart and Winston. All rights reserved.  SAM P LE P R O B LE M A Balancing an Equation Balance the equation for the reaction of iron(III) oxide with hydrogen to form iron and water. 1 Identify reactants and products. Iron(III) oxide and hydrogen are the reactants. Iron and water are the products. The unbalanced formula equation is Fe2O3 + H2  → Fe + H2O 2 Count atoms. Reactants Products Balanced? Unbalanced formula equation Fe2O3 + H2 Fe + H2O Iron atoms 2 1 no Oxygen atoms 3 1 no Hydrogen atoms 2 2 yes 3 Insert coefficients. PRACTICE HINT Add a coefficient of 2 in front of Fe to balance the iron atoms. One way to know what Fe2O3 + H2  → 2Fe + H2O coefficient to use is to find a lowest common Add a coefficient of 3 in front of H2O to balance the oxygen atoms. multiple. In this example, there were six hydrogen Fe2O3 + H2  → 2Fe + 3H2O atoms in the products Now there are two hydrogen atoms in the reactants and six in the and two in the reactants. products. Add a coefficient of 3 in front of H2. The lowest common multiple of 6 and 2 is 6, Fe2O3 + 3H2  → 2Fe + 3H2O so a coefficient of 3 in the reactants balances 4 Verify your results. the atoms. There are two iron atoms, three oxygen atoms, and six hydrogen atoms on both sides of the equation, so it is balanced. P R AC T I C E Write a balanced equation for each of the following. 1 P4 + O2  → P2O5 BLEM PROLVING 2 C3H8 + O2  → CO2 + H2O SOKILL S 3 Ca2Si + Cl2  → CaCl2 + SiCl4 4 Silicon reacts with carbon dioxide to form silicon carbide, SiC, and silicon dioxide. Chemical Equations and Reactions 269 Copyright © by Holt, Rinehart and Winston. All rights reserved.  Balanced Equations Show Mass Conservation The balanced equation for the reaction of sodium with water is 2Na + 2H2O  → 2NaOH + H2 Each side of the equation has two atoms of sodium, four atoms of hydro- gen, and two atoms of oxygen. The reactants and the products are made up of the same atoms so they must have equal masses. So a balanced equation shows the conservation of mass. Never Change Subscripts to Balance an Equation If you needed to write a balanced equation for the reaction of H2 with O2 to form H2O, you might start with this formula equation: H2 + O2  → H2O To balance this equation, some people may want to change the formula of the product to H2O2. H2 + O2  → H2O2 Although the equation is balanced, the product is no longer water, but hydrogen peroxide. Look at the models and equations in Figure 6 to under- stand the problem. The first equation was balanced correctly by adding coefficients. As expected, the model shows the correct composition of the water molecules formed by the reaction. The second equation was incor- rectly balanced by changing a subscript. The model shows that the change of a subscript changes the composition of the substance.As a result, the sec- ond equation no longer shows the formation of water, but that of hydrogen peroxide. When balancing equations, never change subscripts. Keep this in mind as you learn about the odd-even technique for balancing equations. Figure 6 Use coefficients to balance an equation. Never change subscripts. 2H2 + O2 2H2O H2 + O2 H2O2 270 Chapter 8 Copyright © by Holt, Rinehart and Winston. All rights reserved.  SAM P LE P R O B LE M B The Odd-Even Technique The reaction of ammonia with oxygen produces nitrogen monoxide and water vapor. Write a balanced equation for this reaction. 1 Identify reactants and products. The unbalanced formula equation is NH3 + O2  → NO + H2O 2 Count atoms. Reactants Products Balanced? Unbalanced formula equation NH3 + O2 NO + H2O Nitrogen atoms 1 1 yes Hydrogen atoms 3 2 no Oxygen atoms 2 2 yes The odd-even technique uses the fact that multiplying an odd number by 2 always results in an even number. 3 Insert coefficients. PRACTICE HINT A 2 in front of NH3 gives an even number of H atoms. Add coefficients to Watch for cases in which NO and H2O to balance the H atoms and N atoms. all atoms in an equation are balanced except 2NH3 + O2  → 2NO + 3H2O one, which has an odd number on one side of For oxygen, double all coefficients to have an even number of O atoms the equation and an on both sides and keep the other atoms balanced. even number on the other side. Multiplying 4NH3 + 2O2  → 4NO + 6H2O all coefficients by 2 will result in an even Change the coefficient for O2 to 5 to balance the oxygen atoms. number of atoms for the unbalanced atoms 4NH3 + 5O2  → 4NO + 6H2O while keeping the rest 4 Verify your results. balanced. There are four nitrogen atoms, twelve hydrogen atoms, and ten oxygen atoms on both sides of the equation, so it is balanced. P R AC T I C E Write a balanced chemical equation for each of the following. 1 C2H2 + O2  → CO2 + H2O BLEM PROLVING 2 Fe(OH)2 + H2O2  → Fe(OH)3 SOKILL S 3 FeS2 + Cl2  → FeCl3 + S2Cl2 Chemical Equations and Reactions 271 Copyright © by Holt, Rinehart and Winston. All rights reserved.  Polyatomic Ions Can Be Balanced as a Unit So far, you’ve balanced equations by balancing individual atoms one at a time. However, balancing some equations is made easier because groups of atoms can be balanced together. This is especially true in the case of polyatomic ions, such as NO−3. Often a polyatomic ion appears in both the reactants and the products without changing. The atoms within such ions are not rearranged during the reaction. The polyatomic ion can be counted as a single unit that appears on both sides of the equation. Of course, when you think that you have finished balancing an equation, checking each atom by itself is still helpful. Look at Figure 7. The sulfate ion appears in both the reactant sulfuric acid and in the product aluminum sulfate. You could look at the sulfate ion as a single unit to make balancing the equation easier. Looking at the balanced equation, you can see that there are three sulfate ions on the reactants’ side and three on the products’ side. In balancing the equation for the reaction between sodium phosphate and calcium nitrate, you can consider the nitrate ion and the phosphate ion each to be a unit. The resulting balanced equation is Figure 7 2Na3PO4 + 3Ca(NO3)2  → 6NaNO3 + Ca3(PO4)2 In the reaction of aluminum with sulfuric acid, sulfate Count the atoms of each element to make sure that the equation is balanced. ions are part of both the reactants and the products. Al H2 SO2− 4 SO2− 4 Al3+ H2O H3O+ H2O 2Al(s) + 3H2SO4(aq) → Al2(SO4)3(aq) + 3H2(g) 272 Chapter 8 Copyright © by Holt, Rinehart and Winston. All rights reserved.  SAM P LE P R O B LE M C Polyatomic Ions as a Group Aluminum reacts with arsenic acid, HAsO3, to form H2 and aluminum arsenate. Write a balanced equation for this reaction. 1 Identify reactants and products. The unbalanced formula equation is Al + HAsO3  → H2 + Al(AsO3)3 2 Count atoms. PRACTICE HINT Reactants Products Balanced? If you consider poly- atomic ions as single Unbalanced formula equation Al + HAsO3 H2 + Al(AsO3)3 units, be sure to count Aluminum atoms 1 1 yes the atoms of each Hydrogen atoms 1 2 no element when you double-check your Arsenate ions 1 3 no work. Because the arsenate ion appears on both sides of the equation, consider it a single unit while balancing. 3 Insert coefficients. Change the coefficient of HAsO3 to 3 to balance the arsenate ions. Al + 3HAsO3  → H2 + Al(AsO3)3 Double all coefficients to keep the other atoms balanced and to get an even number of hydrogen atoms on each side. 2Al + 6HAsO3  → 2H2 + 2Al(AsO3)3 Change the coefficient of H2 to 3 to balance the hydrogen atoms. 2Al + 6HAsO3  → 3H2 + 2Al(AsO3)3 4 Verify your results. There are 2 aluminum atoms, 6 hydrogen atoms, 6 arsenic atoms, and 18 oxygen atoms on both sides of the equation, so it is balanced. P R AC T I C E Write a balanced equation for each of the following. 1 HgCl2 + AgNO3  → Hg(NO3)2 + AgCl BLEM PROLVING 2 Al + Hg(CH3COO)2  → Al(CH3COO)3 + Hg SOKILL S 3 Calcium phosphate and water are produced when calcium hydroxide reacts with phosphoric acid. Chemical Equations and Reactions 273 Copyright © by Holt, Rinehart and Winston. All rights reserved.  Practice Makes Perfect You have learned a few techniques that you can use to help you approach balancing equations logically. But don’t think that you are done. The more you practice balancing equations, the faster and better you will become. The best way to discover more tips to help you balance equations is to practice a lot! As you learn about the types of reactions in the next section, be aware that these types can provide tips that make balancing equations even easier. 2 Section Review 6. Balance the following equations. a. (NH4)2Cr2O7  → Cr2O3 + N2 + H2O b. NH3 + CuO  → N2 + Cu + H2O UNDERSTANDING KEY IDEAS c. Na2SiF6 + Na  → Si + NaF 1. What fundamental law is demonstrated in d. C4H10 + O2  → CO2 + H2O balancing equations? 2. What is meant by a balanced equation? 3. When balancing an equation, should you CRITICAL THINKING adjust the subscripts or the coefficients? 7. Use diagrams of particles to explain why four atoms of phosphorus can produce only PRACTICE PROBLEMS two molecules of diphosphorus trioxide, even when there is an excess of oxygen 4. Write each of the following reactions as atoms. a word equation, an unbalanced formula 8. Which numbers in the reactants and equation, and finally as a balanced equation. products in the following equation are a. When heated, potassium chlorate decom- coefficients, and which are subscripts? poses into potassium chloride and oxygen. 2Al + 3H2SO4  → Al2(SO4)3 + 3H2 b. Silver sulfide forms when silver and sulfur, S8, react. 9. Write a balanced equation for the forma- c. Sodium hydrogen carbonate breaks down tion of water from hydrogen and oxygen. to form sodium carbonate, carbon diox- Use the atomic mass of each element to ide, and water vapor. determine the mass of each molecule in 5. Balance the following equations. the equation. Use these masses to show that the equation demonstrates the law of a. ZnS + O2  → ZnO + SO2 conservation of mass. b. Fe2O3 + CO  → Fe + CO2 10. A student writes the equation below as the c. AgNO3 + AlCl3  → AgCl + Al(NO3)3 balanced equation for the reaction of iron d. Ni(ClO3)2  → NiCl2 + O2 with chlorine. Is this equation correct? Explain. Fe(s) + Cl3(g)  → FeCl3(s) 274 Chapter 8 Copyright © by Holt, Rinehart and Winston. All rights reserved.