### LeChatelier's PrincipleFifteen Problems

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Problem #11: Old-fashioned “smelling salts” consist of ammonium carbonate, (NH4)2CO3. The reaction for the decomposition of ammonium carbonate,

(NH4)2CO3(s) ⇌ 2NH3(g) + CO2(g) + H2O(g)

is endothermic. Would the smell of ammonia increase or decrease as the temperature is increased?

Solution:

1) Write the chemical equation with heat included in an endothermic way:

heat + (NH4)2CO3(s) ⇌ 2NH3(g) + CO2(g) + H2O(g)

2) A temperature increase means more heat.

3) The systen reacts to this stress by attempting to consume heat.

4) The position of the equilibrium will shift back to the left. The amounts of each reactant will go down. The smell of ammonia would lessen.

Problem #12: Regarding the following equation:

4NH3(g) + 5O2(g) ⇌ 4NO(g) + 6H2O(g)

Increasing the pressure on the system by decreasing the volume would . . .

(a) drive the equilibrium to the right.
(b) drive the equilibrium to the left.
(c) have no effect on the equilibrium.

Solution:

Answer choice (b) is correct. The left-hand side has nine total molecules while the right-hand side has ten. Shifting the equilibrium to the left would decrease the total molecules in the system (for every ten molecules that react, nine would replace them), thereby lessening the pressure.

Problem #13: Will the equilibrium concentration of CO in this reaction:

Cl2(g) + CO(g) ⇌ COCl2(g)

increase, decrease or stay constant if we:

(a) add Cl2(g) at constant volume.
(b) add Ar (an inert gas) at constant volume.
(c) decrease the volume.
(d) increase the volume.

Solution:

(a) add Cl2(g) at constant volume.

CO decreases. The partial pressure of the Cl2 increases, pushing the reaction to the right. In so doing, some of the added Cl2 reacts with the CO to make COCl2. This decreases the amount of CO.

(b) add Ar (an inert gas) at constant volume.

CO remains the same. Ar is not part of the chemical reaction that is at equilibrium. The total pressure goes up, but the partial pressures of the three substances involved in the equilibrium do not change.

(c) decrease the volume.

CO decreases. With a decrease in volume, the position of the equilibrium shifts to the side with the least number of moles. That means a shift to the right. This means more COCl2 is produced and both Cl2 and CO decrease in amount.

(d) increase the volume.

CO increases. This is the reverse of (c). The shift occurs to the left in order to increase the pressure that was lost by the volume increase. Cl2 and CO increase while COCl2 decreases.

Problem #14: For the reaction below, which change would cause the equilibrium to shift to the right?

CH4(g) + 2H2S(g) ⇌ CS2(g) + 4H2(g)

(a) Decrease the concentration of hydrogen sulfide.
(b) Decrease the pressure on the system by increasing the volume.
(c) Increase the concentration of carbon disulfide.
(d) Decrease the concentration of methane.

Solution:

Answer choice (b) is the correct answer. The system will act to replace lost pressure. It will do so by shifting to the side that has the most molecules.The right-hand side has five total molecules whereas the left-hand side ha three. That's a shift to the right.

Problem #15: What would happen to the position of the equilibrium when the following changes are made to the equilibrium system below?

2SO3(g) ⇌ 2SO2(g) + O2(g)

(a) Sulfur dioxide is added to the system.
(b) Sulfur trioxide is removed from the system.
(c) Oxygen is added to the system.

Solution:

(a) shift left
(b) shift left
(c) shift left
(d) shift left Sorry! No change is the answer.

Problem #16: What would happen to the position of the equilibrium when the following changes are made to the reaction below?

2HgO(s) ⇌ Hg(ℓ) + O2(g)

(a) HgO(s) is added to the system.
(b) The pressure on the system increases.
(c) Hg(ℓ) is added to the system.

Solution:

(a) There will be no change in the position of the equilibrium. Solids are not part of the equilibrium expression.

(b) The position of the equilibrium will shift to the left. O2(g) will be used up as the system lowers the pressure.

(c) There will be no change in the position of the equilibrium. Liquids are not part of the equilibrium expression.

Problem #17: When the volume of the following mixture of gases at equilibrium is increased,

4HCl(g) + O2(g) ⇌ 2H2O(g) + 2Cl2(g)

what will be the effect on the equilibrium position?

Solution:

The equilibrium position will shift to the left. When the volume is increased, this causes the pressure to reduce. The system attempts to restore pressue by shifting to the side with more gas molecules.

Problem #18: Predict the effect of decreasing the volume of the container for each equilibrium.

(a) SiO2(s) + 4HF(g) ⇌ SiF4(g) + 2H2O(g)
(b) CO(g) + H2(g) ⇌ C(s) + H2O(g)
(c) 2H2O(g) + N2(g) ⇌ 2H2(g) + 2NO(g)

Solution:

Decreasing the volume causes the pressure to increase. Each system will shift so as to reduce the number of gas molecules. Those directions are:

 (a) right (b) right (c) left

Problem #19: Predict the effect of decreasing the temperature on the position of the following equilibria.

 (a) H2(g) + Cl2(g) ⇌ 2HCl(g) + 49.7 kJ (b) 2NH3(g) ⇌ N2(g) + 3H2(g) ΔH = 37.2 kJ (c) CO(g) + H2O(g) ⇌ CO2(g) + H2(g) ΔH = −27.6 kJ (d) 27.6 kJ + CO2(g) + H2(g) ⇌ CO(g) + H2O(g)

Solution:

(a) Exothermic reaction. Heat is removed when temperature decreases. System will attempt to restore the lost heat. The position of the equilibrium shifts to the right.

(b) Endothermic reaction. Heat is removed when temperature decreases. System will attept to restore heat by shifting in the exothermic directon. The position of the equilibrium shifts to the left.

(c) Exothermic reaction. Heat is removed when temperature decreases. System will attempt to restore the lost heat by shifting in the exothermic direction. The position of the equilibrium shifts to the right.

(d) Endothermic reaction. Heat is removed when temperature decreases. System will attept to restore heat by shifting in the exothermic directon. The position of the equilibrium shifts to the left.

Problem #20: According to Le Chatelier's Principle, adding more product will . . . [mark all as Yes or No]

(a) cause equilibrium to shift to the left.
(b) cause equilibrium to shift to the right.
(c) cause more reactants to form.
(d) cause the equilibrium to adjust in order to relieve the stress.
(e) do nothing if temperature is not changed.

Solution:

 (a) Yes (b) No (c) Yes (d) Yes (e) No

Problem #21: According to Le Chatlelier's Principle,

(a) stress on a system causes the system to stop working.
(b) adding heat causes the equilibrium to shift to the left.
(c) endothermic reactions will cause an increase in solubility.
(d) a system at equilibrium will adjust to minimize the effect of an external stress placed on it.

Solution:

(a) nonsense

(b) only if the system is exothermic

(c) not likely, consider this, AB(s) + heat ⇌ A + B. The solubility of AB increases only if heat is added

(d) Yes! This is a correct statement of Le Chatelier's principle

Problem #22: If the temperature of the following exothermic reaction is decreased, then the equilibrium constant for this reaction equation will increase. (a) True or False and (b) Explain.

N2(g) + 3H2(g) ⇌ 2NH3(g)

Solution:

In an exothermic reaction, the heat term can be considered to be on the product side:
N2(g) + 3H2(g) ⇌ 2NH3(g) + heat

Decreasing the temperature causes the reaction to shift to the right, thereby increasing the amount of ammonia and decreasing the amounts of nitrogen and hydrogen.

Considering the equilibrium expression to be this:

 [NH3]2 Kc = ––––––––– [N2] [H2]3

We see that an increase in the amount of NH3 as well as a decrease in N2 and H2 amounts will cause the Kc to increase Thus, (a) is found to be True and the explanation appears just above.

Problem #23:Consider the following chemical reaction at equilibrium and then answer the following five questions:

N2(g) + 3H2(g) ⇌ 2NH3(g)

(a) When extra NH3 is added to the system referenced above, the equilibrium point will shift in which direction of the equation (to the reactant side or to the product side)?

(b) When N2 is removed from the system referenced above, the equilibrium point will shift in which direction of the equation (to the reactant side or to the product side)?

(c) When H2 is added to the system referenced above, the equilibrium point will shift in which direction of the equation (to the reactant side or to the product side)?

(d) When the pressure is increased in the system referenced above, the equilibrium point will shift in which direction of the equation (to the reactant side or to the product side)?

(e) When the pressure is decreased in the system referenced above, the equilibrium point will shift in which direction of the equation (to the reactant side or to the product side)?

Solution:

Problem #24: Consider the following generic chemical reaction at equilibrium:

A(g) + B(g) ⇌ AB(g) + heat

(a) When the temperature is increased in the system referenced above, the equilibrium point will shift in which direction of the equation (to the reactant side or to the product side)? Explain.

(b) When the temperature is decreased in the system referenced above, the equilibrium point will shift in which direction of the equation (to the reactant side or to the product side)? Explain.

Solution:

Problem #25: An iron(III) chloride is added to the solution below at equilibrium. Will the solution will become colorless, more yellow or darker red. Explain the reasoning behind your choice.

Fe3+ (yellow) + SCN¯ (colorless) ⇌ [FeSCN]2+ (dark red)

Solution:

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