RI 2024 Energetics Part 2 Tut 5b_Self-checked Ans

Answers to Self-Check Questions 1. ∆S = –284.6 J mol−1 K−1 ∆Hr = (−314.4) – [(–46.2) + (–92.3)] = – 175.9 kJ mol−1 ∆G = ∆H – T∆S = (–175.9) – 298 (–284.6/1000) = –91.1 kJ mol−1 Since ∆G < 0, the reaction is spontaneous. [Note: The reaction is spontaneous despite the decrease in entropy because it is enthalpy−driven as strong ionic bonds are formed.] 2. ∆G = ∆H – T∆S (a) X(s) → Y(s) + Z(g) ∆H = +250 kJ mol−1 ∆S > 0 ⇒ –T∆S < 0 In this case, ∆G < 0 when the negative –T∆S outweighs the positive ∆H at high temperatures. Hence the reaction is thermodynamically feasible at high T where T > ∆Ho ∆So such that ∆G < 0. (b) 2X(g) + Y(g) → 2Z(g) ∆H = –100 kJ mol−1 ∆S < 0 ⇒ –T∆S > 0 In this case, ∆G < 0 when the negative ∆H outweighs the positive –T∆S at low temperatures. Hence the reaction is thermodynamically feasible at low T where T < ∆Ho ∆So such that ∆G < 0. (c) X(l) → Y(l) + Z(g) ∆H = –200 kJ mol−1 ∆S > 0 ⇒ –T∆S < 0 In this case, ∆G < 0 at all temperatures because both ∆H and –T∆S are negative. The reaction is thermodynamically feasible at all T. (d) 2X(s) + 2Y(g) → Z(g) ∆H = +50 kJ mol−1 ∆S < 0 ⇒ –T∆S > 0 In this case, ∆G > 0 at all temperatures because both ∆H and –T∆S are positive. The reaction is not thermodynamically feasible at all T. 3. Ans: A According to the equation, there is decrease in number of moles of gas in the forward reaction, thus ∆S < 0. Since H-bonds are formed between CO2 and H2O, ∆H < 0. 4. Ans: B SOCl2 + Ba(OH)2 → BaSO4 + 2HCl Since a reaction occurs vigorously, ∆G < 0 Since temperature decreases, the reaction is endothermic ⇒ ∆H > 0 ∆G = ∆H − T∆S Since ∆G < 0 and ∆H > 0, −T∆S is negative. Thus, ∆S is positive. 5. Ans: D White tin → Grey tin ∆Hf = –2.09 – 0 = –2.09 kJ mol–1 = –2090 J mol–1 ∆Sf = S(products) – S(reactants) = 44.1 – 51.4 = –7.3 J K–1 mol–1 ∆G = –2090 – (273+12)(–7.3) = –2090 – 285(–7.3) HCI 2008 Prelim/II/3 Note: It is stated in learning outcome (d) that the calculation of ΔS using S, is not required but it is needed here.