1. Example 7.1 Evidence of a Chemical Reaction
Which changes involve a chemical reaction? Explain your answers.
(a) ice melting upon warming
(b) an electric current passing through water, resulting in the formation of hydrogen and oxygen gas that appear as bubbles rising in the water
(c) iron rusting
(d) bubbles forming when a soda can is opened
SOLUTION
(a) not a chemical reaction; melting ice forms water, but both the ice and water are composed of water molecules.
(b) chemical reaction; water decomposes into hydrogen and oxygen, as evidenced by the bubbling.
(c) chemical reaction; iron changes into iron oxide, changing color in the process.
(d) not a chemical reaction; even though there is bubbling, it is just carbon dioxide coming out of the liquid.
SKILLBUILDER 7.1 Evidence of a Chemical Reaction
Which changes involve a chemical reaction? Explain your answers.
(a) butane burning in a butane lighter
(b) butane evaporating out of a butane lighter
(c) wood burning
dry ice subliming
Answers: (a) Chemical reaction; heat and light are emitted. (b) Not a chemical reaction; gaseous and liquid
butane are both butane. (c) Chemical reaction; heat and light are emitted. (d) Not a chemical reaction; solid
dry ice is made of carbon dioxide, which sublimes (evaporates) as carbon dioxide gas.
For More Practice Example 7.16; Problems 25, 26, 27, 28, 29, 30.

2. Example 7.2 Writing Balanced Chemical Equations
Write a balanced equation for the reaction between solid silicon dioxide and solid carbon to produce solid silicon carbide and carbon monoxide gas.
SOLUTION
SiO2(s) + C(s)  SiC(s) + CO(g)
Begin with Si
1 Si atom  1 Si atom
Si is already balanced.
Balance O next
2 O atoms  1 O atom
To balance O, put a 2 before CO(g).
SiO2(s) + C(s)  SiC(s) + 2 CO(g)
2 O atoms  2 O atoms
Balance C
1 C atom  1 C + 2 C = 3 C atoms
To balance C, put a 3 before C(s).
SiO2(s) + 3 C(s)  SiC(s) + 2 CO(g)
3 C atoms  1 C + 2 C = 3 C atoms
1. Write a skeletal equation by writing chemical formulas for each of the reactants and products. Review Chapter 5 for nomenclature rules. (If a skeletal equation is provided, skip this step and go to Step 2.)
2. If an element occurs in only one compound on both sides of the equation, balance it first. If there is more than one such element, balance metals before nonmetals.
3. If an element occurs as a free element on either side of the chemical equation, balance it last. Always balance free elements by adjusting the coefficient on the free element.

3. Example 7.2 Writing Balanced Chemical Equations
Continued
4. If the balanced equation contains coefficient fractions, change these into whole numbers by multiplying the entire equation by the appropriate factor.
5. Check to make certain the equation is balanced by summing the total number of each type of atom on both sides of the equation.
This step is not necessary in this example. Proceed to Step 5.
SiO2(s) + 3 C(s)  SiC(s) + 2 CO(g)
Reactants Products
1 Si atom  1 Si atom
2 O atoms  2 O atoms
3 C atoms  3 C atoms
The equation is balanced.
SKILLBUILDER 7.2
Write a balanced equation for the reaction between solid chromium(III) oxide and solid carbon to produce solid chromium and carbon dioxide gas.
Answer: 2 Cr2O3(s) + 3 C(s)  4 Cr(s) + 3 CO2(g).

4. Example 7.3 Writing Balanced Chemical Equations
Write a balanced equation for the combustion of liquid octane (C8H18), a component of gasoline, in which it combines with gaseous oxygen to form gaseous carbon dioxide and gaseous water.
1. Write a skeletal equation by writing chemical formulas for each of the reactants and products. Review Chapter 5 for nomenclature rules. (If a skeletal equation is provided, skip this step and go to Step 2.)
2. If an element occurs in only one compound on both sides of the equation, balance it first. If there is more than one such element, balance metals before nonmetals.
SOLUTION
C8H18(l) + O2(g)  CO2(g) + H2O(g)
Begin with C
C8H18(l) + O2(g)  CO2(g) + H2O(g)
8 C atoms  1 C atom
To balance C, put an 8 before CO2(g).
C8H18(l) + O2(g)  8 CO2(g) + H2O(g)
8 C atoms  8 C atom
Balance H next
18 H atoms  2 H atoms
To balance H, put a 9 before H2O(g).
C8H18(l) + O2(g)  8 CO2(g) + 9 H2O(g)
18 H atoms  18 H atoms

5. Example 7.3 Writing Balanced Chemical Equations
Continued
Balance O
C8H18(l) + O2(g)  8 CO2(g) + 9 H2O(g)
2 O atoms  16 O + 9 O = 25 O atoms
3. If an element occurs as a free element on either side of the chemical equation, balance it last. Always balance free elements by adjusting the coefficient on the free element.
4. If the balanced equation contains coefficient fractions, change these into whole numbers by multiplying the entire equation by the appropriate factor.

6. Example 7.3 Writing Balanced Chemical Equations
Continued
5. Check to make certain the equation is balanced by summing the total number of each type of atom on both sides of the equation.
2 C8H18(l) + 25 O2(g)  CO2(g) + 18 H2O(g)
Reactants Products
16 C atoms  16 C atoms
36 H atoms  H atoms
50 O atoms  O atoms
The equation is balanced.
SKILLBUILDER 7.3
Write a balanced equation for the combustion of gaseous C4H10 in which it combines with gaseous oxygen to form gaseous carbon dioxide and gaseous water.
Answer: 2 C4H10(g) + 13 O2(g) → 8 CO2(g) + 10 H2O(g).
For More Practice Example 7.17; Problems 33, 34, 35, 36, 37, 38.

7. Example 7.4 Balancing Chemical Equations
Write a balanced equation for the reaction of solid aluminum with aqueous sulfuric acid to form aqueous aluminum
sulfate and hydrogen gas.
Use your knowledge of chemical nomenclature from Chapter 5 to write a skeletal equation containing formulas for each of the reactants and products. The formulas for each compound MUST BE CORRECT before you begin to balance the equation.
Since both aluminum and hydrogen occur as pure elements, balance those last. Sulfur and oxygen occur in only one compound on each side of the equation, so balance these first. Sulfur and oxygen are also part of a polyatomic ion that stays intact on both sides of the equation. Balance polyatomic ions such as these as a unit. There are 3 SO42– ions on the right side of the equation, so put a 3 in front of H2SO4.
Balance Al next. Since there are 2 Al atoms on the right side of the equation, place a 2 in front of Al on the left side of the equation.
Balance H next. Since there are 6 H atoms on the left side, place a 3 in front of H2(g) on the right side.
SOLUTION
Al(s) + H2SO4(aq)  Al2(SO4)3(aq) + H2(g)
Al(s) + 3 H2SO4(aq)  Al2(SO4)3(aq) + H2(g)
2 Al(s) + 3 H2SO4(aq)  Al2(SO4)3(aq) + H2(g)
2 Al(s) + 3 H2SO4(aq)  Al2(SO4)3(aq) + 3 H2(g)

8. Example 7.4 Balancing Chemical Equations
Continued
Finally, sum the number of atoms on each side to make sure that the equation is balanced.
2 Al(s) + 3 H2SO4(aq)  Al2(SO4)3(aq) + 3 H2(g)
Reactants Products
2 Al atoms  2 Al atoms
6 H atoms  6 H atoms
3 S atoms  3 S atoms
12 O atoms  O atoms
The equation is balanced.
SKILLBUILDER 7.4 Balancing Chemical Equations
Write a balanced equation for the reaction of aqueous lead(II) acetate with aqueous potassium iodide to form solid lead(II) iodide and aqueous potassium acetate.
Answer: Pb(C2H3O2)2(aq) + 2 KI(aq)  PbI2(s) + 2 KC2H3O2(aq)
For More Practice Problems 39, 40, 41, 42, 43, 44.

9. Example 7.5 Balancing Chemical Equations
Balance this chemical equation.
Fe(s) + HCl(aq)  FeCl3(aq) + H2(g)
Since Cl occurs in only one compound on each side of the equation, balance it first. There is 1 Cl atom on the left side of the equation and 3 Cl atoms on the right side. To balance Cl, place a 3 in front of HCl.
Since H and Fe occur as free elements, balance them last. There is 1 Fe atom on the left side of the equation and 1 Fe atom on the right, so Fe is balanced. There are 3 H atoms on the left and 2 H atoms on the right. Balance H by placing a in front of (That way you don’t alter other elements that are already balanced.)
Since the equation now contains a coefficient fraction, clear it by multiplying the entire equation (both sides) by 2.
SOLUTION
Fe(s) + 3 HCl(aq)  FeCl3(aq) + H2(g) 3 2

10. Example 7.5 Balancing Chemical Equations
Continued
Finally, sum the number of atoms on each side to check that the equation is balanced.
2 Fe(s) + 6 HCl(aq)  2 FeCl3(aq) + 3 H2(g)
Reactants Products
2 Fe atoms  2 Fe atoms
6 Cl atoms  6 Cl atoms
6 H atoms  6 H atoms
SKILLBUILDER 7.5 Balancing Chemical Equations
Balance this chemical equation.
HCl(g) + O2(g)  H2O(l) + Cl2(g)
Answer: 4 HCl(g) + O2(g)  2 H2O(l) + 2 Cl2(g)
For More Practice Problems 45, 46, 47, 48, 49, 50.

11. Example 7.6 Determining Whether a Compound Is Soluble
Determine whether each compound is soluble or insoluble.
(a) AgBr (b) CaCl2 (c) Pb(NO3)2 (d) PbSO4
SOLUTION
(a) Insoluble; compounds containing Br– are normally soluble, but Ag+ is an exception.
(b) Soluble; compounds containing Cl– are normally soluble, and Ca2+ is not an exception.
(c) Soluble; compounds containing NO3– are always soluble.
(d) Insoluble; compounds containing SO42– are normally soluble, but Pb2+ is an exception.
SKILLBUILDER 7.6 Determining Whether a Compound Is Soluble
Determine whether each compound is soluble or insoluble.
(a) CuS (b) FeSO4 (c) PbCO3 (d) NH4Cl
Answers: (a) insoluble (b) soluble (c) insoluble (d) soluble.
For More Practice Example 7.18; Problems 57, 58, 59, 60, 61, 62.

12. Example 7.7 Writing Equations for Precipitation Reactions
Write an equation for the precipitation reaction that occurs (if any) when solutions of sodium carbonate and copper(II) chloride are mixed.
1. Write the formulas of the two compounds being mixed as reactants in a chemical equation.
2. Below the equation, write the formulas of the potentially insoluble products that could form from the reactants. Obtain these by combining the cation from one reactant with the anion from the other. Make sure to write correct (charge neutral) formulas for these ionic compounds as described in Section 5.5.
3. Use the solubility rules to determine whether any of the potential new products are indeed insoluble.
SOLUTION
Na2CO3(aq) + CuCl2(aq) →
Potentially Insoluble Products
NaCl CuCO3
NaCl is soluble (compounds containing Cl– are usually soluble, and Na+ is not an exception).
CuCO3 is insoluble (compounds containing CO32– are usually insoluble, and Cu2+ is not an exception).

13. Example 7.7 Writing Equations for Precipitation Reactions
Continued
4. If all of the potentially insoluble products are soluble, there will be no precipitate. Write NO REACTION next to the arrow.
5. If one or both of the potentially insoluble products are insoluble, write their formula(s) as the product(s) of the reaction, using (s) to indicate solid. Write any soluble products with (aq) to indicate aqueous.
6. Balance the equation. Remember to adjust only coefficients, not subscripts.
Since this example has an insoluble product, proceed to the next step.
Na2CO3(aq) + CuCl2(aq)  CuCO3(s) + NaCl(aq)
Na2CO3(aq) + CuCl2(aq)  CuCO3(s) + 2 NaCl(aq)
SKILLBUILDER 7.7
Write an equation for the precipitation reaction that occurs (if any) when solutions of potassium hydroxide and nickel(II) bromide are mixed.
Answer: 2 KOH(aq) + NiBr2(aq) → Ni(OH)2(s) + 2 KBr(aq)

14. Example 7.8 Writing Equations for Precipitation Reactions
Write an equation for the precipitation reaction that occurs (if any) when solutions of lithium nitrate and sodium sulfate are mixed.
1. Write the formulas of the two compounds being mixed as reactants in a chemical equation.
2. Below the equation, write the formulas of the potentially insoluble products that could form from the reactants. Obtain these by combining the cation from one reactant with the anion from the other. Make sure to write correct (charge neutral) formulas for these ionic compounds as described in Section 5.5.
3. Use the solubility rules to determine whether any of the potential new products are indeed insoluble.
SOLUTION
LiNO3(aq) + Na2SO4(aq) 
Potentially Insoluble Products
NaNO3 Li2SO4
NaNO3 is soluble (compounds containing NO3– are usually soluble, and Na+ is not an exception).
Li2SO4 is soluble (compounds containing SP42– are soluble and Li+ is not an exception).

15. Example 7.8 Writing Equations for Precipitation Reactions
Continued
4. If all of the potentially insoluble products are soluble, there will be no precipitate. Write NO REACTION next to the arrow.
5. If one or both of the potentially insoluble products are insoluble, write their formula(s) as the product(s) of the reaction, using (s) to indicate solid. Write any soluble products with (aq) to indicate aqueous.
6. Balance the equation. Remember to adjust only coefficients, not subscripts.
LiNO3(aq) + Na2SO4(aq)  NO REACTION
SKILLBUILDER 7.8
Write an equation for the precipitation reaction that occurs (if any) when solutions of ammonium chloride and iron(III) nitrate are mixed.
Answer: NH4Cl(aq) + Fe(NO3)3(aq)  NO REACTION
For More Practice Example 7.19; Problems 63, 64, 65, 66.

16. Example 7.9 Predicting and Writing Equations for Precipitation Reactions
Write an equation for the precipitation reaction (if any) that occurs when solutions of lead(II) acetate and sodium sulfate are mixed. If no reaction occurs, write NO REACTION.
1. Write the formulas of the two compounds being mixed as reactants in a chemical equation.
2. Below the equation, write the formulas of the potentially insoluble products that could form from the reactants. These are obtained by combining the cation from one reactant with the anion from the other. Make sure to adjust the subscripts so that all formulas are charge-neutral.
3. Use the solubility rules to determine whether any of the potentially insoluble products are indeed insoluble.
4. If all of the potentially insoluble products are soluble, there will be no precipitate. Write NO REACTION next to the arrow.
SOLUTION
Pb(C2H3O2)2(aq) + Na2SO4(aq) 
Potentially Insoluble Products
NaC2H3O2 PbSO4
NaC2H3O2 is soluble (compounds containing Na+ are usually soluble). PbSO4 is insoluble (compounds containing SO42– are normally
soluble, but Pb2+ is an exception).
Since this reaction has an insoluble product, proceed to the next step.

17. Example 7.9 Predicting and Writing Equations for Precipitation Reactions
Continued
5. If one or both of the potentially insoluble products are indeed insoluble, write their formula(s) as the product(s) of the reaction, using (s) to indicate solid. Write any soluble products with (aq) to indicate aqueous.
6. Balance the equation.
Pb(C2H3O2)2(aq) + Na2SO4(aq)  PbSO4(s) + NaC2H3O2(aq)
Pb(C2H3O2)2(aq) + Na2SO4(aq)  PbSO4(s) + 2 NaC2H3O2(aq)
SKILLBUILDER 7.9 Predicting and Writing Equations for Precipitation Reactions
Write an equation for the precipitation reaction (if any) that occurs when solutions of potassium sulfate and strontium nitrate are mixed. If no reaction occurs, write NO REACTION.
Answers: K2SO4(aq) + Sr(NO3)2(aq)  SrSO4(s) + 2 KNO3(aq)
For More Practice Problems 67, 68.

18. Example 7.10 Writing Complete Ionic and Net Ionic Equations
Consider this precipitation reaction occurring in aqueous solution.
Pb(NO3)2(aq) + 2 LiCl(aq)  PbCl2(s) + 2 LiNO3(aq)
Write a complete ionic equation and net ionic equation for the reaction.
SOLUTION
Complete ionic equation
Pb2+(aq) + 2 NO3–(aq) + 2 Li+(aq) + 2 Cl–(aq)  PbCl2(s) + 2 Li+(aq) + 2 NO3– (aq)
Net ionic equation
Pb2+(aq) + 2 Cl–(aq)  PbCl2(s)
Write the complete ionic equation by separating aqueous ionic compounds into their constituent ions. The remains as one unit.
Write the net ionic equation by eliminating the spectator ions, those that do not change during the reaction.
SKILLBUILDER 7.10 Writing Complete Ionic and Net Ionic Equations
Consider this reaction occurring in aqueous solution.
2 HBr(aq) + Ca(OH)2(aq)  2 H2O(l) + CaBr2(aq)
Write a complete ionic equation and net ionic equation for the reaction.
Answer: Complete ionic equation:
2 H+(aq) + 2 Br–(aq) + Ca2+(aq) + 2 OH–(aq)  2 H2O(l) + Ca2+(aq) + 2 Br–(aq)
Net ionic equation:
2 H+(aq) + 2 OH–(aq)  2 H2O(l), or simply H+(aq) + OH–(aq)  H2O(l)
For More Practice Example 7.20; Problems 69, 70, 71, 72.

19. Example 7.11 Writing Equations for Acid–Base Reactions
Write a molecular and net ionic equation for the reaction between aqueous HNO3 and aqueous Ca(OH)2.
You must recognize these substances as an acid and a base. Write the skeletal reaction following the general pattern of acid plus base goes to water plus salt.
Next, balance the equation.
Write the net ionic equation by eliminating those ions that remain the same on both sides of the equation.
SOLUTION
HNO3(aq) + Ca(OH)2(aq)  H2O(l) + Ca(NO3)2(aq)
acid base water salt
2 HNO3(aq) + Ca(OH)2(aq)  H2O(l) + Ca(NO3)2(aq)
2 H+(aq) + 2 OH–(aq)  2 H2O(l)
or simply
H+(aq) + OH–(aq)  H2O(l)
SKILLBUILDER 7.11 Writing Equations for Acid–Base Reactions
Write a molecular and net ionic equation for the reaction that occurs between aqueous H2SO4 and aqueous KOH.
Answer: Molecular equation:
H2SO4(aq) + 2 KOH(aq)  2 H2O(l) + K2SO4(aq)
Net ionic equation:
2 H+(aq) + 2 OH–(aq)  2 H2O(l)
For More Practice Example 7.21; Problems 77, 78, 79, 80.

20. Example 7.12 Writing Equations for Gas Evolution Reactions
Write a molecular equation for the gas evolution reaction that occurs when you mix aqueous nitric acid and aqueous
sodium carbonate.
SOLUTION
Begin by writing a skeletal equation that includes the reactants and products that form when the cation of each reactant combines with the anion of the other.
You must recognize that H2CO3(aq) decomposes into H2O(l) and CO2 and write the corresponding equation.
Finally, balance (g) the equation.
HNO3(aq) + Na2CO3(aq)  H2O(l) + CO2(g) + NaNO3(aq)
2 HNO3(aq) + Na2CO3(aq)  H2O(l) + CO2(g) + 2 NaNO3(aq)
SKILLBUILDER 7.12 Writing Equations for Gas Evolution Reactions
Write a molecular equation for the gas evolution reaction that occurs when you mix aqueous hydrobromic acid and aqueous potassium sulfite.
Answer: 2 HBr(aq) + K2SO3(aq)  H2O(l) + SO2(g) + 2 KBr(aq)

21. Example 7.12 Writing Equations for Gas Evolution Reactions
Continued
SKILLBUILDER PLUS
Write a net ionic equation for the previous reaction.
Answer: 2 H+(aq) + SO32– (aq)  H2O(l) + SO2(g)
For More Practice Example 7.22; Problems 81, 82.

22. Example 7.13 Identifying Redox Reactions
Which of these are redox reactions?
(a) 2 Mg(s) + O2(g)  2 MgO(s)
(b) 2 HBr(aq) + Ca(OH)2(aq)  2 H2O(l) + CaBr2(aq)
(c) Ca(s) + Cl2(g)  CaCl2(s)
(d) Zn(s) + Fe2+(aq)  Zn2+(aq) + Fe(s)
SOLUTION
(a) Redox reaction; Mg reacts with elemental oxygen.
(b) Not a redox reaction; it is an acid–base reaction.
(c) Redox reaction; a metal reacts with a nonmetal.
(d) Redox reaction; Zn transfers two electrons to Fe2+.
SKILLBUILDER 7.13 Identifying Redox Reactions
Which of these are redox reactions?
(a) 2 Li(s) + Cl2(g)  2 LiCl(s)
(b) 2 Al(s) + 3 Sn2+(aq)  2 Al3+(aq) + 3 Sn(s)
(c) Pb(NO3)2(aq) + 2 LiCl(aq)  PbCl2(s) + 2 LiNO3(aq)
(d) C(s) + O2(g)  CO2(g)
Answer: (a), (b), and (d) are all redox reactions; (c) is a precipitation reaction.
For More Practice Example 7.23; Problems 83, 84.

23. Example 7.14 Writing Combustion Reactions
Write a balanced equation for the combustion of liquid methyl alcohol (CH3OH).
SOLUTION
CH3OH(l) + O2(g)  CO2(g) + H2O(g)
2 CH3OH(l) + 3 O2(g)  2 CO2(g) + 4 H2O(g)
Begin by writing a skeletal equation showing the reaction of CH3OH with O2 to form CO2 and H2O.
Balance the skeletal equation using the rules in Section 7.4.
SKILLBUILDER 7.14 Writing Combustion Reactions
Write a balanced equation for the combustion of liquid pentane (C5H12), a component of gasoline.
Answer: C5H12(l) + 8 O2(g)  5 CO2(g) + 6 H2O(g)
SKILLBUILDER PLUS
Write a balanced equation for the combustion of liquid propanol (C3H7OH).
Answer: 2 C3H7OH(l) + 9 O2(g)  6 CO2(g) + 8 H2O(g)
For More Practice Example 7.24; Problems 85, 86.

24. Example 7.15 Classifying Chemical Reactions According to What Atoms Do
Classify each reaction as a synthesis, decomposition, single-displacement, or double-displacement reaction.
(a) Na2O(s) + H2O(l)  2 NaOH(aq)
(b) Ba(NO3)2(aq) + K2SO4(aq)  BaSO4(s) + 2 KNO3(aq)
(c) 2 Al(s) + Fe2O3(s)  Al2O3(s) + 2 Fe(l)
(d) 2 H2O2(aq)  2 H2O(l) + O2(g)
(e) Ca(s) + Cl2(g)  CaCl2(s)
SOLUTION
(a) Synthesis; a more complex substance forms from two simpler ones.
(b) Double-displacment; Ba and K switch places to form two new compounds.
(c) Single-displacement; Al displaces Fe in Fe2O3.
(d) Decomposition; a complex substance decomposes into simpler ones.
(e) Synthesis; a more complex substance forms from two simpler ones.

25. Example 7.15 Classifying Chemical Reactions According to What Atoms Do
Continued
SKILLBUILDER 7.15 Classifying Chemical Reactions According to What Atoms Do
Classify each reaction as a synthesis, decomposition, single-displacement, or double-displacement reaction.
(a) 2 Al(s) + 2 H3PO4(aq)  2 AlPO4(aq) + 3 H2(g)
(b) CuSO4(aq) + 2 KOH(aq)  Cu(OH)2(s) + K2SO4(aq)
(c) 2 K(s) + Br2(l)  2 KBr(s)
(d) CuCl2(aq)  Cu(s) + Cl2(g)
electrical current
Answers: (a) single-displacement (b) double-displacement (c) synthesis (d) decomposition.
For More Practice Example 7.25; Problems 89, 90, 91, 92.

26. Example 7.16 Identifying a Chemical Reaction
Which of these are chemical reactions?
(a) Copper turns green on exposure to air.
(b) When sodium bicarbonate is combined with hydrochloric acid, bubbling is observed.
(c) Liquid water freezes to form solid ice.
(d) A pure copper penny forms bubbles of a dark brown gas when dropped into nitric acid. The nitric acid solution turns blue.
SOLUTION
(a) Chemical reaction, as evidenced by the color change.
(b) Chemical reaction, as evidenced by the evolution of a gas.
(c) Not a chemical reaction; solid ice is still water.
(d) Chemical reaction, as evidenced by the evolution of a gas and by a color change.

27. Example 7.17 Writing Balanced Chemical Equations
Write a balanced chemical equation for the reaction of solid vanadium(V) oxide with hydrogen gas to form solid vanadium(III) oxide and liquid water.
V2O5(s) + H2(g)  V2O3(s) + H2O(l)
SOLUTION
Skeletal equation is given. Proceed to Step 2.
Vanadium occurs in only one compound on both sides of the equation. However, it is balanced, so you can proceed and balance oxygen by placing a 2 in front of H2O on the right side.
V2O5(s) + H2(g)  V2O3(s) + 2 H2O(l)
Hydrogen occurs as a free element, so balance it last by placing a 2 in front of H2 on the left side.
V2O5(s) + 2 H2(g)  V2O3(s) + 2 H2O(l)
Equation does not contain coefficient fractions. Proceed to Step 5.
Check the equation.
Reactants Products
2 V atoms  2 V atoms
5 O atoms  5 O atoms
4 H atoms  4 H atoms

28. Example 7.18 Determining Whether a Compound Is Soluble
Determine whether each compound is soluble.
(a) CuCO3
(b) BaSO4
(c) Fe(NO3)3
SOLUTION
(a) Insoluble; compounds containing CO32– are insoluble, and Cu2+ is not an exception.
(b) Insoluble; compounds containing SO42– are usually soluble, but Ba2+ is an exception.
(c) Soluble; all compounds NO3– containing are soluble.

29. Example 7.19 Predicting Precipitation Reactions
Write an equation for the precipitation reaction that occurs, if any, when solutions of sodium phosphate and cobalt(II) chloride are mixed.
SOLUTION
Na3PO4(aq) + CoCl2(aq) 
Potentially Insoluble Products:
NaCl Co3(PO4)2
NaCl is soluble.
Co3(PO4)2 is insoluble.
Reaction contains an insoluble product; proceed to Step 5.
Na3PO4(aq) + CoCl2(aq)  Co3(PO4)2(s) + NaCl(aq)
2 Na3PO4(aq) + 3 CoCl2(aq)  CO3(PO4)2(s) + 6 NaCl(aq)

30. 2 Cl–(aq) + Hg22+(aq)  Hg2Cl2(s)
Example 7.20 Writing Complete Ionic and Net Ionic Equations
Write a complete ionic and net ionic equation for the reaction.
2 NH4Cl(aq) + Hg2(NO3)2(aq)  Hg2Cl2(s) + 2 NH4NO3(aq)
SOLUTION
Complete ionic equation:
2 NH4+(aq) + 2 Cl–(aq) + Hg22+(aq) + 2 NO3–(aq)  Hg2Cl2(s) + 2 NH4+(aq) + 2 NO3–(aq)
Net ionic equation:
2 Cl–(aq) + Hg22+(aq)  Hg2Cl2(s)

31. Example 7.21 Writing Equations for Acid–Base Reactions
Write an equation for the reaction that occurs when aqueous hydroiodic acid is mixed with aqueous barium hydroxide.
SOLUTION
2 HI(aq) + Ba(OH)2(aq)  2 H2O(l) + BaI2(aq)
acid base water salt

32. Example 7.22 Writing Equations for Gas Evolution Reactions
Write an equation for the reaction that occurs when aqueous hydrobromic acid is mixed with aqueous potassium bisulfite.
SOLUTION
HBr(aq) + KHSO3(aq)  H2SO3(aq) + KBr(aq)  H2O(l) + SO2(g) + KBr(aq)

33. Example 7.23 Identifying Redox Reactions
Which of these reactions is a redox reaction?
(a) 4 Fe(s) + 3 O2(g)  2 Fe2O3(s)
(b) CaO(s) + CO2(g)  CaCO3(s)
(c) AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq)
SOLUTION
Only (a) is a redox reaction.

34. Example 7.24 Writing Equations for Combustion Reactions
Write a balanced equation for the combustion of gaseous ethane (C2H6), a minority component of natural gas.
SOLUTION
The skeletal equation is:
C2H6(g) + O2(g)  CO2(g) + H2O(g)
The balanced equation is:
2 C2H6(g) + 7 O2(g)  4 CO2(g) + 6 H2O(g)

35. Example 7.25 Classifying Chemical Reactions
Classify each chemical reaction as a synthesis, decomposition, single-displacement, or double-displacement reaction.
(a) 2 K(s) + Br2(g)  2 KBr(s)
(b) Fe(s) + 2 AgNO3(aq)  Fe(NO3)2(aq) + 2 Ag(s)
(c) CaSO3(s)  CaO(s) + SO2(g)
(d) CaCl2(aq) + Li2SO4(aq)  CaSO4(s) + 2 LiCl(aq)
SOLUTION
(a) Synthesis; KBr, a more complex substance, is formed from simpler substances.
(b) Single-displacement; Fe displaces Ag in AgNO3.
(c) Decomposition; CaSO3 decomposes into simpler substances.
(d) Double-displacement; Ca and Li switch places to form new compounds.