'A' Level Chemistry Problem Analysis: Effect of Temperature Changes on ΔG° and Kc |
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QN: The reversible reaction A⇌2B has ΔH° > 0 and ΔS° > 0. It is observed that the yield of B increases as temperature increases. Which of these is a possible explanation? (A) Since ΔH° > 0, as temperature increases the system works to counteract this change by Le Chatelier's Principle by favouring the forward endothermic reaction. Hence equilibrium shifts to the right and yield of B increases. (B) Since ΔS° > 0 and ΔG° = ΔH° - TΔS°, ΔG° is much less than 0 at high temperatures, meaning that the reaction is more thermodynamically feasible which increases the yield of B. Or are (A) and (B) both possible explanations? Thought process: Shifting of position of equilibrium owing to a change in temperature, means K changes. (In contrast, shifting of position of equilibrium owing to changes in molarities, partial pressures of either reactants or products, means Q changes while K remains constant). If (and only if) temperature is kept constant, ΔG° shall always become more negative (ie. forward reaction becomes more thermodynamically feasible) as K becomes larger (ie. position of equilibrium shifts to the RHS), and of course, vice-versa. This can be easily proven mathematically using ΔG° = -RT ln K. However, when temperature changes, it is ΔH° that determines whether K increases or decreases, not ΔS°. It is true that going by the formula ΔG° = ΔH° - TΔS° (and assuming ΔH° and ΔS° are constants regardless of temperature, which is erroneous but a simplification acceptable at the 'A' Levels), whether ΔS° is positive or negative does indeed determine whether ΔG° becomes more positive or more negative as temperature changes. However, and here we come to the erroneous assumption or misconception all too commonly suffered at the 'A' Levels, is that ΔG° always has a direct relationship with K, eg. as K increases, ΔG° becomes more negative. This is true only at constant temperature, but no longer necessarily so as temperature changes. The effect of temperature changes on K is determined only by ΔH° and not ΔS°. And the effect of temperature changes on ΔG° is determined only by ΔS° and not ΔH°. All in all, the only correct answer accepted by the Cambridge Examinations Board is (A), not (B). ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The above content is contributed by Mr Heng, owner and 'A' Level Chemistry tutor at Bedok Funland JC. He also goes by the handle UltimaOnline on various online popular homework forums. |
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