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# 20 Problems: Thermodynamics and Le Chatelier's Principle

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1) Which statement describes characteristics of an endothermic reaction?

A. The sign of H is positive, and the products have less potential energy than the reactants.
B. The sign of H is positive, and the products have more potential energy than the reactants.
C. The sign of H is negative, and the products have less potential energy than the reactants.
D. The sign of H is negative, and the products have more potential energy than the reactants.
E. None of the above.

2) Which is a property of a reaction that has reached equilibrium?

A. The amount of products is greater than the amount of reactants.
B. The amount of products is equal to the amount of reactants.
C. The rate of the forward reaction is greater than the rate of the reverse reaction.
D. The rate of the forward reaction is equal to than the rate of the reverse reaction.
E. None of the above.

3) Given [N2] = 0.200 M, [H2] = 0.100 M and [NH3] = 1.18 x 10-1 M for the following reaction; N2(g) + 3 H2(g) à 2 NH3(g) at 350oC calculate the equilibrium constant.

A. 85.2
B. 76.4
C. 54.6
D. 69.6
E. 60.5

4) Given [CO] = 0.250 M, [H2] = 0.120 and [CH3OH] = 0.0378 for the following reaction; CO(g) + 2 H2(g) CH3OH(g) at 500ºK calculate the equilibrium constant.

A. 12.9
B. 9.6
C. 7.1
D. 10.5
E. 8.4

5)Given [N2O4] = 0.351 M and [NO2] = 4.94 x 10-2 M for the following reaction; N2O4(g) à 2 NO2(g) at 30ºC calculate the equilibrium constant.
A. 4.50 x 10-3
B. 7.92 x 10-3
C. 3.50 x 10-3
D. 6.95 x 10-3
E. 4.25 x 10-3

6) The equilibrium constant for the following reaction; 3 H2(g) + CO(g) à CH4(g) + H2O(g) is 3.93 at 1200ºK. A system at equilibrium has [CO] = 0.0613 M, [H2] = 0.1839 M and [CH4] = 0.0387 M. What is the [H2O]?

A. 0.0387
B. 0.0323
C. 0.0233
D. 0.0201
E. 0.0276

7) The equilibrium constant for the following reaction; N2O4(g) à 2 NO2(g) is 6.96 x 10-3 at 30oC. A system at equilibrium has [N2O4] = 0.351 M. What is the [NO2]?

A. 4.94 x 10-2
B. 3.20 x 10-2
C. 4.25 x 10-2
D. 3.80 x 10-2
E. 2.58 x 10-2

8) The Kc for the following reaction; CO(g) + 2 H2(g) à CH4(g) + H2O(g) at 1200oK is 3.93. Given the concentrations [CO] = 0.0450 M, [H2] = 0.132 M, [CH4] = 0.0620 M and [H2O] = 0.0550 M one can conclude that:

A. the system is not at equilibrium and the reaction will proceed to the right
B. the system is not at equilibrium and the reaction will proceed to the left
C. the system is at equilibrium and no net change will occur
D. Not enough information provided
E. None of the above

9) The Kc for the following reaction; N2O4(g) ó 2 NO2(g) at 35ºC is 9.37 x 10-3. Given the concentrations [N2O4] = 0.340 M, [NO2] = 5.33 x 10-2 M, one can conclude that:

A. the system is not at equilibrium and the reaction will proceed to the left
B. the system is not at equilibrium and the reaction will proceed to the right
C. the system is at equilibrium and no net change will occur
D. Not enough information provided
E. None of the above

10) The Kc for the following reaction; CO(g) + 2 H2(g) ó CH3OH(g) at 500ºK is 10.5. For a system with the concentrations [CO] = 0.120 M, [H2] = 0.250 and [CH3OH] = 0.0788 M one can conclude that:

A. the system is not at equilibrium and the reaction will proceed to the right
B. the system is at equilibrium and no net change will occur
C. the system is not at equilibrium and the reaction will proceed to the left
D. Not enough information provided
E. None of the above

11) Using atmospheres, what are the units of Kp?

CO(g) + 3 H2(g) ó CH4(g) + H2O(g)

A. none
B. atm
C. atm-2
D. atm2
E. atm-1

12) Consider the following reactions at equilibrium and determine which of the indicated changes will cause the reaction to proceed to the right.

1. 2 H2O(g) ó 2 H2(g) + O2(g) (remove water)
2. H2(g) + I2(g) ó 2 HI(g) (remove HI)
3. CaCO3(s) ó CaO(s) + CO2(g) (add CaCO3)
4. N2(g) + O2(g) ó 2 NO(g) (add O2)
5. BaO(s) + SO3(g) ó BaSO4(s) (add BaO)

A. 1, 3, 5
B. 2, 5
C. 2, 4
D. 1, 3
E. 3, 4

13) Consider the following reactions at equilibrium and determine which of the indicated changes will cause the reaction to proceed to the right.

1. CO(g) + 3 H2(g) ó CH4(g) + H2O(g) (remove CO)
2. CO2(g) + C(s) ó 2 CO(g) (add CO2)
3. N2(g) + 3 H2(g) ó 2 NH3(g) (add nitrogen)
4. 2 CO2(g) ó 2 CO(g) + O2(g) (add CO)
5. CO(g) + 2H2(g) ó CH3OH(g) (remove CH3OH)

A. 3, 4
B. 1, 4
C. 2, 3, 5
D. 2, 3
E. 2, 4

14) Consider the reactions given below. In which cases will the reaction proceed more to the right by decreasing the pressure?

1. 4 HCl(g) + O2(g) ó 2 Cl2(g) + 2 H2O(g)
2. 2 HCl(g) + 1/2 O2(g) ó Cl2(g) + H2O(g
3. Cl2(g) + H2O(g) ó 2 HCl(g) + 1/2 O2(g)
4. CO2(g) + 4 H2(g) ó CH4(g) + 2 H2O(g)
5. N2(g) + O2(g) ó 2 NO(g)

A. 3 and 4
B. 3
C. 4 and 5
D. 4 and 5
E. 1, 2 and 4

15) Consider the reactions given below. In which case(s) will the reaction proceed more to the right by decreasing the pressure?

1. 2 CO(g) + O2(g) ó 2 CO2(g)
2. 2 NO(g) ó N2(g) + O2(g
3. N2O4(g) ó 2 NO2(g)
4. Ni(s) + 4 CO(g) ó Ni(CO)4(g)
5. N2(g) + 3 H2(g) ó 2 NH3(g)

A. 1 and 3
B. 3
C. 1, 2 and 3
D. 1, 4 and 5
E. 2, 3 and 4

16) Consider the following reactions:

1. CO(g) + 3 H2(g) ó CH4(g) + H2O(g) DHº = -206.2 kJ
2. CO2(g) + C(s) ó 2 CO(g) DHº = + 172.5 kJ
3. H2(g) + I2(g) ó 2 HI(g) DHº = -9.4 kJ
4. H2(g) + F2(g) ó 2 HF(g) DHº = -541 kJ
5. N2(g) + 2 O2(g) ó 2 NO2(g) DHº = 66.4 kJ

In which cases is the product formation unaffected by pressure?

A. 1, 2
B. 3, 4
C. 2
D. 2, 5
E. 1, 5

17) Consider the following reactions:

1. N2(g) + 3 H2(g) ó 2 NH3(g) DHº = -91.8 kJ
2. 2 CO2(g) ó 2 CO(g) + O2(g) DHº = 566 k
3. CO(g) + 2H2(g) ó CH3OH(g) DHº = -21.7 kJ
4. N2(g) + O2(g) ó 2 NO(g) DHº = 181 kJ
5. H2(g) + F2(g) ó 2 HF(g) DHº = -541 kJ

In which cases is product formation favored by increased temperature?
A. 3, 5
B. 1, 3
C. 1, 3, 5
D. 2, 4
E. 5

18) Consider the reactions below:

1. CO(g) + 3 H2(g) ó CH4(g) + H2O(g) DHº = -206.2 kJ
2. CO2(g) + C(s) ó 2 CO(g) DHº = 172.5 kJ
3. H2(g) + I2(g) ó 2 HI(g) DHº = -9.4 kJ
4. 3 O2(g) ó 2 O3(g) DHº = 285 kJ
5. 2 H2O(g) ó 2 H2(g) + O2(g) DHº = 484.6 kJ

In which cases is product formation favored by decreased temperature?

A. 3, 5
B. 1, 4
C. 2, 4, 5
D. 1, 3
E. 1, 2 and 5

19) If the pressure on a reaction flask for the following system; 2 CO(g) ó C(s) + CO2(g) is increased by decreasing the volume, what will happen to the concentrations of CO and CO2?

A. both the [CO] and [CO2] will decrease

B. neither the [CO] nor the [CO2] will change

C. both the [CO] and [CO2] will increase

D. the [CO] will decrease and the [CO2] will increase

E. the [CO] will increase and the [CO2] will decrease

20) The equilibrium constant Kc for the following reaction; CO(g) +2 H2(g) CH3OH(g) at 227ºC is 10.5 x 10-2. What is Kp at this temperature?

A. 10.5
B. 2.56 x 10-1
C. 177
D. 6.23 x 10-5
E. 1.76 x 104

keywords: lechatelier's principle, lechateliers, le chateliers