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19 Chemical Thermodynamics Solutions to Exercises 19.114 (a) = H₂O(g) - SO₂(g) - H₂S(g) = 3(0) + 2(-228.57) - (-300.4) 2(-33.01) = -90.72 = -90.7 kJ In K = RT = = 36.6; K = -(-90.72kJ) = (b) The reaction is highly spontaneous at 298 K and feasible in principle. However, use of H₂S(g) produces a severe safety hazard for workers and the surrounding community. (c) 0.033 K = 7.99 10¹⁵ = ; = 7.99 10¹⁵ (d) = H₂O(g) - - H₂S(g) = 3(0) + 2(-241.82) - (-296.9) 2(-20.17) = -146.4 = 3(31.88) + 2(188.83) - 2(205.6) = The reaction is exothermic so the value of will decrease with increasing temperature. The negative value means that the reaction will become nonspontaneous at some higher temperature. The process will be less effective at elevated temperatures. 19.115 (a) When the rubber band is stretched, the molecules become more ordered, so the entropy of the system decreases, is negative. (b) = Since is negative, qrev is negative and heat is emitted by the system. (c) The unstretched rubber band feels cooler. This confirms our answer to (b). If heat is emitted by the system when it is stretched, the surroundings feel warmer. Upon return to the initial state, heat is absorbed by the system (the rubber band) and the surroundings (your lip) feel cooler. 610