HW2_Solutions

For this reaction δ h 92 38 kj mol 1 1 219 k 2 193 k

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00. For this reaction, Δ H = 92 . 38 kJ · mol 1 . 1. 219 K 2. 193 K 3. 466 K correct 4. 353 K 5. 492 K Explanation: 011 10.0 points A certain reaction has Δ H equal to 14.5 kJ/mol. This reaction is normally run at room temperature (25 C). At what new tem- perature should the reaction be run so that K is 3 times its value at 25 C? 1. 77 C 2. 31 C 3. 135 C 4. 94 C correct 5. 50 C 6. 65 C Explanation: Δ H = 14500 J/mol K 2 = 3 K 1 R = 8 . 314 J / mol · K ln K 2 K 1 = Δ H R parenleftbigg 1 T 1 - 1 T 2 parenrightbigg R Δ H ln K 2 K 1 = 1 T 1 - 1 T 2 T 2 = parenleftbigg 1 T 1 - R Δ H ln K 2 K 1 parenrightbigg 1 = parenleftbigg 1 298 - 8 . 314 14500 ln 3 parenrightbigg 1 = 367 K = 94 C 012 10.0 points The total barometric pressure on a certain day was 755.1 torr. The humidity was such that water vapor in the air contributed 20.4 torr. The other components of air that day contributed a pressure of 1. 760 torr. 2. Cannot be computed from the informa- tion given.

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estrada (gse77) – Homework 2 – Sutcliffe – (52410) 5 3. 734.7 torr. correct 4. 775.5 torr. Explanation: P bar = 755 . 1 torr P vapor = 20 . 4 torr Using Dalton’s Law of Partial Pressure, the sum of the partial pressures will be equal to the total pressure. In this instance, the sum of the partial pressure of the water vapor plus the partial pressures of all the other compo- nents of air will equal the total barometric pressure. P bar = P vapor + P everything else P everything else = P bar - P vapor = 755 . 1 torr - 20 . 4 torr = 734 . 7 torr 013 10.0 points Why does it take longer to hard boil an egg in boiling water at higher altitude than at or below sea level? 1. The ambient temperature is lower at higher altitude. 2. The boiling point increases. 3. The boiling point goes down. correct 4. It does not take longer. 5. There is an increase in atmospheric pres- sure. Explanation: The normal boiling point of a liquid will oc- cur when its vapor pressure reaches 760 torr (1 atm). By normal, we mean “normal” condi- tions; i.e. , 1 atm pressure. In general , liquids will boil when their vapor pressure reaches the applied (in this case, atmospheric) pressure. At higher altitudes, atomospheric pressures are less. Thus the observed boiling point of water in, say, Denver, will be less than the fa- miliar 100 C. Since the temperature at which water boils is less under these conditions, it will take longer to hard boil your egg. 014 10.0 points Estimate the enthalpy of vaporization of CCl 4 given that at 25 C and 58 C its vapor pressure is 107 and 405 torr, respectively. Assume that the enthalpy of vaporization is independent of the temperature. 1. 142 kJ · mol 1 2. 3.98 kJ · mol 1 3. 486 J · mol 1 4. 33.1 kJ · mol 1 correct 5. 48.6 kJ · mol 1 Explanation: T 1 = 25 C + 273 . 15 = 298 . 15 K P 1 = 107 torr T 2 = 58 C + 273 . 15 = 331 . 15 K P 2 = 405 torr Using the Clausius-Clapeyron equation, ln parenleftbigg P 2 P 1 parenrightbigg = Δ H vap R parenleftbigg 1 T 1 - 1 T 2 parenrightbigg Δ H 0 vap = R ln parenleftbigg P 2 P 1 parenrightbigg 1 T 1 + 1 T 2 = 8 . 314 J · mol 1 · K 1 298 . 15 K - 1 331 . 15 K × ln parenleftbigg 405 torr 107 torr parenrightbigg = 33109 . 5 J · mol 1 015 10.0 points The heat of vaporization of water is 40.7
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