## Entropy of Surroundings vs. System

$\Delta S = \frac{q_{rev}}{T}$

Angela 1K
Posts: 80
Joined: Fri Sep 29, 2017 7:05 am

### Entropy of Surroundings vs. System

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 $\Delta S=-\frac{\Delta H}{T}$?

Diego Zavala 2I
Posts: 65
Joined: Fri Sep 29, 2017 7:07 am
Been upvoted: 1 time

### Re: Entropy of Surroundings vs. System

Hi,
If a system is going from a solid state to liqud or liquid to vapor, then the entropy (disorder) of the system increases.
Similarly, if a system is going from vapor to liquid or liquid to solid, then the entropy of the system decreases. Futhermore, because the entropy of the universe must always increase, it can be concluded that the entropy of the surroundings increased by more than the decrease in entropy of the system.

melissa carey 1f
Posts: 53
Joined: Fri Sep 29, 2017 7:06 am

### Re: Entropy of Surroundings vs. System

When you ADD heat, ∆H is +. Ex - adding heat to boil water has +∆H, so ∆S = +∆H/T.