The spontaneity of a standard reaction, ?Go, depends on both ?Ho and ?So. Given

Question

The spontaneity of a standard reaction, ?Go, depends on both ?Ho and ?So. Given the following reaction and data table, decide which of the statements shown below are true. Assume that ?Ho and ?So are independent of Temperature. 2 H2S (g) + SO2 (g) ? 3 S (s) + 2 H2O (l) ?Horxn -233 kJ ?Sorxn -424 J/K Select all that are True. This reaction is endothermic This reaction is exothermic This reaction is endergonic (?Go > 0) at 298 K This reaction is exergonic (?Go < 0) at 298 K This standard reaction will only be spontaneous at high temperatures (T > 550 K) This standard reaction will only be spontaneous at low temperatures (T < 550 K) This standard reaction will be spontaneous at all temperatures This standard reaction will not be spontaneous at any temperature

Explanation / Answer

a) Endothermic means ?H > 0. So, it is false. b) True - exothermic means ?H < 0. Now, ?G = ?H - T?S, so ?G = -233 - 298(-0.424) = -233+126 = -107 kJ/mol rxn. c) False - ?G > 0. d) True. e) The contribution of entropy increases as the temperature rises. Since the entropy change is negative (unfavorable), increasing the temperature will NOT help the reaction proceed. f) True. The enthalpy change is negative (favorable); thus, when the contribution of entropy is small (temperature low), the reaction is energetically favored. g) False, since ?H and ?S have the same sign. A reaction spontaneous at all temperatures has ?H < 0 and ?S > 0 h) False yet again. A reaction not spontaneous at any temperature has ?H > 0 and ?S < 0.

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