## Change in Entropy Equations

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

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

### Change in Entropy Equations

I'm having difficulty differentiating between when to use the equation $\Delta S=\frac{qREV}{T}$ and $\Delta S=-\frac{\Delta H}{T}$.

If someone could please explain, that would be appreciated.

Thanks!

Curtis Wong 2D
Posts: 62
Joined: Sat Jul 22, 2017 3:00 am

### Re: Change in Entropy Equations

Alright, so the first equation is typically the base for all the equations. It is useful for all scenarios because you can modify it. It's based on a constant temperature too.

The only instance where I found the second equation is when it talked about phase change. So, when there's a phase change, you use the second equation.

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

### Re: Change in Entropy Equations

Then how come, for example, in question 9.15, which asks to calculate the entropy change for (a) the freezing of 1.00 mol H2O(l) at 0.00 C and the vaporization of 50.0g of ethanol at 531.5K, the solutions manual uses the equation $\Delta S=\frac{q}{T}=\frac{\Delta H}{T}$? Why isn't it negative?

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