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problem set 2

# problem set 2 - Page 1 of 5 CHEMISTRY 123 Problem Set#2...

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CHEMISTRY 123 Problem Set #2 Thermodynamics 1. Given the enthalpy changes at a certain temperature for reactions (a) and (b), calculate the enthalpy change for reaction (c). Is reaction (c) endothermic or exothermic at this temperature? (a) 3H 2 (g) + N 2 (g) 2NH 3 (g) Δ H = 92.4 kJ mol 1 (b) 2H 2 (g) + O 2 (g) 2H 2 O(g) Δ H = 483 kJ mol 1 (c) 4NH 3 (g) + 3O 2 (g) 2N 2 (g) + 6H 2 O 2. Given the following data, evaluate the standard enthalpy of formation of ICl(g) (in kJ mol 1 ). Cl 2 (g) 2Cl(g) Δ H° = 242.0 kJ mol 1 I 2 (g) 2I(g) Δ H° = 151.0 kJ mol 1 ICl(g) I(g) + Cl(g) Δ H° = 211.2 kJ mol 1 Δ sublimation of I 2 (s) Δ sub = 63.7 kJ mol 1 (1) 244.5 (2) 244.5 (3) 211.2 (4) 17.1 (5) 17.1 3. The standard enthalpies of formation of CO 2 (g) and H 2 O(l) are 393.5 kJ mol 1 and 285.8 kJ mol 1 at 298 K, repectively. The heat of combustion of liquid pentane, C 5 H 12 (l), according to the following equation is C 5 H 12 (l) + 8O 2 (g) 5CO 2 (g) + 6H 2 O(l) Δ H° = 3536 kJ What is the standard enthalpy of formation of C 5 H 12 (l) (in kJ mol 1 )? (1) 254.0 (2) 146.3 (3) +146.3 (4) +254.0 4. Carbon disulphide is made industrially by passing sulphur vapour slowly through red-hot carbon. In a calorimeter, this is not an easy reaction to carry out, and the determination of the standard enthalpy of formation of CS 2 is best made indirectly. Given that the standard enthalpies of formation of gaseous CO 2 and SO 2 at 25 °C are 393.13 kJ mol 1 and 296.53 kJ mol 1 , respectively, and that the standard enthalpy of combustion (at 25 °C) of gaseous CS 2 to CO 2 and SO 2 is 1103.4 kJ mol 1 , calculate the standard enthalpy of formation of gaseous CS 2 at 25 °C. 5. Which of the following statements is correct? (1) The standard enthalpy of formation of an element in its normal state is negative at 298 K. (2) When a system does work on the surroundings, the internal energy of the system is increased. (3) q and w are in general independent of path. (4) S is not a state function (property of the system), since it is always increasing. (5) A given system in a given state has a definite, fixed value for S. 6 9. When 44.0 g of solid carbon dioxide at 1.00 atm pressure are reversibly converted to vapour at 1 atm pressure and 78 °C, 25.33 kJ of heat are absorbed by the system. The density of solid CO 2 is 1.56 g cm 3 .

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problem set 2 - Page 1 of 5 CHEMISTRY 123 Problem Set#2...

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