I have these two homework problems, as well as solutions. What I do not understand is why the solution for one is not the solution for the other. First problem: A sample consisting of 1.00 mol Ar is expanded isothermally at 0 deg Celc from 22.4 dm3 to 44.8 dm3 reversibly. Calculate q, w, delta U, and delta H. (heat, work, internal energy, and enthalpy) Second problem: A sample of 1.00 mol H20(g) is condensed isothermally and reversibly to liquid water at 100 deg Celc. The standard enthalpy of vaporization of water at 100 deg C is 40.646 kJ mol-1. Find w, q delta U, and delta H. (work, heat internal energy, and enthalpy) Note: both problems refer to perfect gases For the first problem, the solutions manual says that delta U is 0 since the internal energy of a perfect gas depends only on temperature, and since temperature is constant internal energy must be 0. The second problem, however this is not the case. Why is the internal energy in the second problem not also zero if it is condensed reversibly and isothermally(which means constant T right?) ??