Question 17.13: Predicting the Effect of a Change in Temperature on the Equi......
Predicting the Effect of a Change in Temperature on the Equilibrium Position
Problem How does an increase in temperature affect the equilibrium concentration of the underlined substance and K for each of the following reactions?
(a) CaO(s) + H_2O(l) \xrightleftharpoons[] \underline{Ca(OH)_2}(aq) ΔH^\circ = −82 kJ
(b) CaCO_3(s) \xrightleftharpoons[] CaO(s) + \underline{CO_2}(g) ΔH^\circ = 178 k
(c) \underline{SO_2}(g) \xrightleftharpoons[] S(s) + O_2(g) ΔH^\circ = 297 kJ
Plan We write each equation to show heat as a reactant or product. The temperature increases when we add heat, so the system shifts to absorb the heat; that is, the endothermic reaction occurs. Thus, K will increase if the forward reaction is endothermic and decrease if it is exothermic.
Solution (a) CaO(s) + H_2O(l) \xrightleftharpoons[] Ca(OH)_2(aq) + heat
Adding heat shifts the system to the left: [Ca(OH)_2] and K will decrease.
(b) CaCO_3(s) + heat \xrightleftharpoons[] CaO(s) + CO_2(g)
Adding heat shifts the system to the right: [CO_2] and K will increase.
(c) SO_2(g) + heat \xrightleftharpoons[] S(s) + O_2(g)
Adding heat shifts the system to the right: [SO_2] will decrease and K will increase.
Check Check your answers by reasoning through a decrease in temperature: heat is removed and the exothermic direction is favored. All the answers should be opposite.
Comment Note that, as part (a) shows, these ideas hold for solutions as well.