Winter 2013 Question 1B

[Kimberly Govea 2L]

Why is specific heat capacity the second entropy 3/2R? How do we know which ideal gas to use? I try to go by the "constant pressure" or "constant volume" but its not clear to me. The volume inside the balloon did change right? so is this second entropy referring to the volume of the surroundings?

[Chem_Mod]

Basically the way they solved the problem was that they calculated the entropy change in terms of the change in volume (step 1), then they calculated the entropy change in terms of the change in temperature (step 2). So when they calculated the entropy while the volume was changing, they assumed everything else was constant (i.e. no change in temperature). And then when they calculated the entropy for the change in temperature, they assumed that volume was held constant; and that's why they used C_v.

Even though volume was technically not held constant in this problem, just given the fact that you have information about volume and temperature, but no information about anything else, implies that you'll probably need to use C_v and not C_p