Chapter 15 – Chemical Equilibrium

1. For each of the following equilibria, write the equilibrium constant expression for Kc:

a) PCl5(g) ó PCl3(g) + Cl2(g)

b) 2NO(g) + Cl2(g) ó 2NOCl(g)

c) CH4(g) + 2H2S(g) ó CS2(g) + 4H2(g)

d) Fe2O3(s) + 3CO(g) ó 2Fe(l) + 3CO2(g)

e) 4Fe(s) + 3O2(g) ó 2Fe2O3(s)

f) BaSO4(s) ó BaO(s) + SO3(g)

g) BaSO4(s) ó Ba+2(aq) + SO-2(aq)

2. Diethyl ether is synthesized by heating ethanol with concentrated sulfuric acid. Write the equilibrium constant expression for Kc.

2C2H5OH(l) ó C2H5OC2H5(l) + H2O(l)

3. An equilibrium mixture of PCl5, PCl3, and Cl2, at a certain temperature contains 8.3 x 10-3M PCl5, 1.5 x 10-2M PCl3, and 3.2 x 10-2M Cl2. Calculate the equilibrium constant, Kc, for the following reaction: PCl5(g) ó PCl3(g) + Cl2(g)

4. Vinegar contains acetic acid, a weak acid that is partially dissociated in aqueous solution:

CH3COOH(aq) ó H+(aq) + CH3CO2-(aq)

a) Write the equilibrium constant expression for Kc.

b) What is the value of Kc if the extent of dissociation in 1.0M CH3COOH is 0.42%?

5. Ethyl alcohol is produced by the reaction of acetic acid with ethanol:

CH3COOH(l) + C2H5OH(l) ó CH3COOC2H5(l) + H2O(l)

a) Write the equilibrium constant expression for Kc.

b) A solution prepared by mixing 1.00 mol of acetic acid and 1.00 mol of ethanol contains 0.65 mol of ethyl acetate at equilibrium. Calculate the value of Kc. Explain why you can calculate Kc without knowing the volume of the solution.

6. When the following reactions come to equilibrium, does the equilibrium mixture contain mostly reactants or mostly products?

a) 2SO2(g) + O2(g) ó 2SO3(g) Kc = 1.2 x 109

b) 2HCl(g) ó H2(g) + Cl2(g) Kc = 2.0 x 10-17

7. Which of the following reactions goes almost all the way to completion and which proceeds hardly at all toward completion?

a) N2(g) + O2(g) ó 2NO(g) Kc = 2.7 x 10-18

b) 2NO(g) + O2(g) ó 2NO2(g) Kc = 6.0 x 1013

8. For which of the following reactions will the equilibrium mixture contain an appreciable concentration of both reactants and products?

a) Cl2(g) ó 2Cl(g) Kc = 6.4 x 10-39

b) Cl2(g) + 2NO(g) ó 2NOCl(g) Kc = 3.7 x 108

c) Cl2(g) + 2NO2(g) ó 2NO2Cl(g) Kc = 1.8

9. At 1400K, Kc = 2.5 x 10-3 for the reaction CH4(g) + 2H2S(g) ó CS2(g) + 4H2(g). A 10L reaction vessel at 1400K contains 2.0 mol of CH4, 3.0 mol of CS2, 3.0 mol of H2, and 4.0 mol of H2S. Is this reaction at equilibrium? If not, in which direction does the reaction proceed to reach equilibrium?

10. The first step in the industrial synthesis of hydrogen is the reaction of steam and methane to give synthesis gas, a mixture of carbon monoxide and hydrogen:

H2O(g) + CH4(g) ó CO(g) + 3H2(g)

The equilibrium constant Kc is 4.7 at 1400K. A mixture of reactants and products at 1400K contains 0.035M H2O, 0.050M CH4, 0.15M CO, and 0.20M H2. In which direction does the reaction proceed to reach equilibrium?

11. An equilibrium mixture of N2, H2, and NH3 at 500K contains 0.020M N2 and 0.18M H2. At this temperature, Kc for the reaction N2(g) + 3H2(g) ó 2NH3(g) is 1.7 x 102. What is the concentration of NH3?

12. The air pollutant NO is produced in automobile engines because of the high-temperature reaction, N2(g) + O2(g) ó 2NO(g) (Kc = 1.7 x 10-3 at 2300K). If the initial concentrations of N2 and O2 at 2300K are both 1.40M, what are the concentrations of NO, N2, and O2 when the reaction mixture reaches equilibrium?

13. At a certain temperature, the reaction PCl5(g) ó PCl3(g) + Cl2(g) has an equilibrium constant

Kc = 5.8 x 10-2. Calculate the equilibrium concentrations of PCl5, PCl3, and Cl2 if only PCl5 is present initially at a concentration of 0.160M.

14. Consider the following equilibrium: Ag+1(aq) + Cl-1(aq) ó AgCl(s). Use Le Chatelier’s Principle to predict how the amount of solid silver chloride will change when the equilibrium is disturbed by the following:

a) adding NaCl

b) adding AgNO3

c) removing Cl-1

15. Will the concentration of NO2 increase, decrease, or remain the same when the equilibrium CINO2(g) + NO(g) ó CINO(g) + NO2(g) is disturbed by the following:

a) adding CINO2

b) adding CINO

c) adding NO

d) for (a), account for the change in terms of the reaction quotient Qc

16. When each of the following is disturbed by increasing the pressure as a result of decreasing the volume, does the number of moles of reaction products increase, decrease, or remain the same?

a) 2CO2(g) ó 2CO(g) + O2(g)

b) N2(g) + O2(g) ó 2NO(g)

c) Si(s) + 2Cl2(g) ó SiCl4(g)

17. For each of the following equilibria, use Le Chatelier’s Principle to predict the direction of the reaction when the volume is increased.

a) C(s) + H2O(g) ó CO(g) + H2(g)

b) 2H2(g) + O2(g) ó 2H2O(g)

c) 2Fe(s) + 3H2O(g) ó Fe2O3(s) + 3H2(g)

18. For the gas shift reaction CO(g) + H2O(g) ó CO2(g) + H2(g), ΔH° = -41.2kJ. Does the amount of H2 in an equilibrium mixture increase or decrease when the temperature is increased? How does Kc change when the temperature is decreased? Justify your answers using Le Chatelier’s Principle.

19. The value of ΔH° for the reaction 3O2(g) ó 2O2(g) is 285kJ. Does the equilibrium constant for this reaction increase or decrease when the temperature increases? Justify your answer by using Le-Chatelier’s Principle.

20. For the reaction H2(g) + I2(g) ó 2HI(g), ΔH° = -9.4kJ. Will the equilibrium concentration of HI increase or decrease when the temperature increases?

21. Consider the following reaction:

CO(g) + 2H2(g) ó CH3OH(g) ΔH° = -91kJ

Does the amount of methanol increase, decrease, or remain the same when an equilibrium mixture of reactants and products is subjected to the following changes:

a) the temperature is increased

b) the volume is decreased

c) helium is added

22. Consider the following reaction:

(CH3)2CHOH(g) ó (CH3)2CO(g) + H2(g) ΔH° = +57.3kJ

Does the amount of acetone ((CH3)2CO) increase, decrease, or remain the same when an equilibrium mixture of reactants and products is subjected to the following changes?

a) temperature is increased

b) volume is increased

c) H2 is added

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