3 Step Entropy Change Calculations

[makenzie2K]

I was wondering if someone could explain to me how to calculate the overall change in entropy in equations like the one on Lyndon’s worksheet where you go from T1 to T2, have a delta H value, then go back to T1. I understand the rise and drop in temperature equations, I just don’t understand that middle step.

[chris_tsai_4H]

The middle step accounts for the change in volume while the other steps only account for change in temperature which is why you have to add all the changes in entropy together in order to find the total deltaS.

[Matthew Mar 1J]

There are also problems where it asks for the change in entropy for a phase change at a temperature other than its boiling/freezing point where you have to use the entropy of vaporisation or fusion at the boiling or freezing point as the middle step.