Question 17.11: Predicting the Effect of a Change in Concentration on the Eq......
Predicting the Effect of a Change in Concentration on the Equilibrium Position
Problem To improve air quality and obtain a useful product, chemists often remove sulfur from coal and natural gas by treating the contaminant hydrogen sulfide with O_2:
2H_2S(g) + O_2(g) \xrightleftharpoons[] 2S(s) + 2H_2O(g)What happens to
(a) [H_2O] if O_2 is added? (b) [H_2S] if O_2 is added?
(c) [O_2] if H_2S is removed? (d) [H_2S] if sulfur is added?
Plan We write the reaction quotient to see how Q_c is affected by each disturbance, relative to K_c. In each case, the effect tells us the direction in which the reaction proceeds for the system to reattain equilibrium and how each concentration changes.
Solution Writing the reaction quotient (solid sulfur is not included): Q_c = \frac{[H_2O]^2}{[H_2S]^2[O_2]}
(a) When O_2 is added, the denominator of Q_c increases, so Q_c < K_c. The reaction proceeds to the right until Q_c = K_c again, so [H_2O] increases.
(b) As in part (a), when O_2 is added, Q_c < K_c. Some H_2S reacts with the added O_2 as the reaction proceeds to the right, so [H_2S] decreases.
(c) When H_2S is removed, the denominator of Q_c decreases, so Q_c > K_c. As the reaction proceeds to the left to re-form H_2S, more O_2 forms as well, so [O_2] increases.
(d) The concentration of solid S does not change, so it does not appear in the reaction quotient. As long as some S is present, adding more S has no effect, so [H_2S] is unchanged (but see Comment 2 below).
Check Apply Le Châtelier’s principle to see that the reaction proceeds in the direction that lowers the increased concentration or raises the decreased concentration.
Comment 1. As you know, sulfur exists most commonly as S_8. How would this change in formula affect the answers? The balanced equation and Q_c would be
8H_2S(g) + 4O_2(g) \xrightleftharpoons[] S_8(s) + 8H_2O(g) Q_c = \frac{[H_2O]^8}{[H_2S]^8[O_2]^4}
The value of K_c is different for this equation, but the changes described in the problem have the same effects. Thus, shifts in equilibrium position predicted by Le Châtelier’s principle are not affected by a change in the balancing coefficients.
2. In part (d), you saw that adding a solid has no effect on the concentrations of other components: because the concentration of the solid cannot change, it does not appear in Q. But the amount of solid can change. Adding H_2S shifts the reaction to the right, so more S forms.