PS3_Key - Chemistry 2080 Spring 2009 Problem Set#3 due date...

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Chemistry 2080 Spring 2009 Problem Set #3 - due date: Friday February 13th at 2 pm Name:____( KEY)_______ Lab TA Name:__________________ Lab Day/Time:______________________ Practice problems (not graded). Chapter 7: 39, 45, 56, 59, 71, 97. 1) Given the following thermochemical data (all at 25°C): (1) . . . . . . .NH 4 Cl(s) + HCl(g) –––––––> H 2 (g) + Cl 2 (g) + NH 3 (l) Δ rxn = +336.5 kJ (2) . . . . . . .NH 3 (g) + HCl(g) ––––––––> NH 4 Cl(s) Δ rxn = – 176.2 kJ The standard enthalpy of vaporization of NH 3 is + 23.4 kJ/mol. The standard enthalpy of formation of NH 3 (g) is – 46.1 kJ/mol Calculate Δ rxn for the reaction: N 2 (g) + 4H 2 (g) + Cl 2 (g) –––––––> 2NH 4 Cl(s) ******************************************* Let’s call the two chemical equations (above) equations (1) and (2). Also, writing out the equations for the standard enthalpy of vaporization and the standard enthalpy of formation of ammonia (and calling them equations (3) and (4), we get: (3) . . . . NH 3 (l) ––––> NH 3 (g) Δ vap = +23.4 kJ (4) . . . . N 2 + 3H 2 –––––> 2NH 3 (g) Δ f = 2x(– 46.1 kJ/mol) = –92.2 kJ If we now add the equations in the following way: (2) + (–1) + (4) + (– 3) we get: NH 3 (g) + HCl(g) +H 2 (g) + Cl 2 (g) + NH 3 (l) + N 2 +3H 2 +NH 3 (g) –––––––> NH 4 Cl(s) + NH 4 Cl(s) + HCl(g) + 2NH 3 (g) + NH 3 (l) which reduces to the equation that we want, namely: N 2 (g) + 4H 2 (g) + Cl 2 (g) –––––––> 2NH 4 Cl(s) So we need to add the standard enthalpies in the same way [i.e. (2) + (–1) + (4) + (– 3)]: i.e. (– 176.2 kJ) + (– 336.5 kJ) + (– 92.2 kJ) + (– 23.4 kJ) = – 628.3 kJ which is Δ rxn for the reaction as written
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2) As we saw in lecture, the complete combustion of benzene(l) (C 6 H 6 ) in oxygen in a bomb calorimeter resulted in the liberation of 3,268 kJ/mol of heat. The sole products were H 2 O(l) and CO 2 (g). Thus, for the combustion of benzene at 25°C, q v = – 3,268 kJ/mol of benzene. Write a balanced equation for the complete combustion of benzene, then use all this information, and relevant values of Δ f given in Table 7.2 in Petrucci, to
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