This challenging Cambridge exam question would ruin most
Singapore H2 Chem students if it came out for Singapore A
levels.
To solve the 1st part of the Cambridge question, use the
formula
ΔS surroundings = - ΔH / T
(Cambridge may, in a challenging Singapore A level H2 Chem
exam question, require you to qualitatively explain this formula,
ie.
1. an exothermic reaction (ie. negative ΔH) generates heat which
results in an increase in the disorderliness and hence entropy of
the surroundings
2. the more negative the value of ΔH, ie. the more exothermic the
reaction, the more positive the entropy increase of the
surroundings
3. the same amount of energy released into the surroundings will
make a bigger difference at a lower ambient temperature, hence the
division by T)
To solve the 2nd part of the Cambridge question, use
ΔS total = ΔS system + ΔS surroundings
To solve the 3rd part of the Cambridge question, note that :
ΔS total = ΔS system + ΔS surroundings, hence
ΔS total = ΔS system + (- ΔH / T ), hence
Multiplying through by -T, we get :
- T x ΔS total = ΔH - T x ΔS system
ie. Δ Gibbs free energy = ΔH - T x ΔS system
ie. Hence, when ΔS total = 0 (as specified by the exam
question), which means Δ Gibbs free energy = 0, then
0 = ΔH - T x ΔS system, hence
ΔH = T x ΔS system, hence
T = ΔH / ΔS system, which will enable you to solve the
3rd part of the Cambridge question.
To solve the 4th part of the Cambridge question, you need to
understand that the reaction will only proceed if the total entropy
change of the universe is zero or positive, ie. Gibbs free energy
change = zero or negative. Hence to ensure the reaction proceeds,
ie. for the reaction to be thermodynamically feasible, you need the
temperature to be above the value calculated when you used
(as instructed by the question) total entropy change = zero.