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### Entropy of Surroundings vs. System

Posted: Sat Mar 17, 2018 1:25 am
When can you tell if the entropy of the surroundings/system is supposed to be positive/negative?

Like in problem 9.45, it asks to find $\Delta S$ of the systems and surroundings for the following situations:
(a) vaporization of 1.00 mol CH4(l) at its normal BP
(b) melting of 1.00 mol C2H5OH(s) at its normal MP
(c) freezing of 1.00 mol C2H5OH(l) at its normal FP

How can you tell when to use $\Delta S=\frac{\Delta H}{T}$ or $\Delta S=-\frac{\Delta H}{T}$?