HW_wk6_solutions_06

# HW_wk6_solutions_06 - 2R03 HW for weeks 6_solutions 1 The...

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2R03 HW for weeks 6_solutions Oct 20 2006 1. The combustion of the fat tristearin: 2C 57 H 110 O 6 (aq) + 163 O 2 (g) à 110 H 2 O(l) + 114 CO 2 (g) is an exothermic reaction. Without performing a calculation, estimate whether: (A) ΔS system = ΔS reaction > 0 (B) ΔS system = ΔS reaction < 0 (C) ΔS system = ΔS reaction = 0 (D) We cannot say. Solution: Gas is more disordered than liquid and solid. The decrease of gas molecules from reactants to products will make the system less disordered, so the entropy will decrease, that is ΔS reaction < 0 à B 2. Assuming that the enthalpy and the entropy of reaction in the previous exercises do not depend on temperature, under which conditions is the reaction of the previous exercise spontaneous at constant T & P? (A) T < ΔH reaction / ΔS reaction (B) T > ΔH reaction / ΔS reaction (C) T = ΔH reaction / ΔS reaction (D) We cannot say Solution: The answer is (A). ΔG reaction = ΔH reaction - TΔS reaction For spontaneous process, the condition is ΔG reaction < 0 So, ΔG reaction = ΔH reaction - TΔS reaction < 0 ΔH reaction < TΔS reaction , dividing by ΔS reaction on both sides and xonsidering that ΔS reaction < 0: T < ΔH reaction / ΔS reaction 3. Which of the following statements is correct and which is not? Explain. (A) A very efficiently designed machine in the future is able to transfer all the heat produced by a hot object into work. (B) The entropy of 1 mol of O 2 will increase when it expands adiabatically, provided the gas behaves ideally. (C) A non-spontaneous reaction can never occur under any circumstances. (D) Similarly to the formation enthalpies, the molar entropies of the elements in the reference state are zero. Solution: (A) is wrong. Heat cannot be completely transferred into work by a machine because at least part of the heat must remain as such in order to ensure that the entropy of the universe still increases. (B) is correct. For adiabatic situations, q = 0, so q / T = 0 . (C) is wrong: A non-spontaneous reaction can still

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HW_wk6_solutions_06 - 2R03 HW for weeks 6_solutions 1 The...

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