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### Phase changes and entropy

Posted: Sun Mar 18, 2018 3:44 am
When we calculate phase changes, why is q in S = q/T replaced with the enthalpy H of the phase change? Is it just easier or a connection between q and H

### Re: Phase changes and entropy

Posted: Sun Mar 18, 2018 3:48 am
Since phase changes are typically given in delta H Fusion, etc. it's better to make that connection between delta H and q.

### Re: Phase changes and entropy

Posted: Sun Mar 18, 2018 2:07 pm
Delta H is equal to q at a constant pressure.

### Re: Phase changes and entropy

Posted: Sun Mar 18, 2018 3:02 pm
Like Lavelle said, Enthalpy is just heat at constant pressure [q][/p].

### Re: Phase changes and entropy

Posted: Sun Mar 18, 2018 3:14 pm
You can replace q with delta H when the system is at constant pressure. We often use the formula deltaS=deltaH/T when we are calculating the entropy change at boiling point or other phase changes. Just make sure when you do this, you use the correct T (the boiling point temperature).

### Re: Phase changes and entropy

Posted: Sun Mar 18, 2018 4:59 pm
dH=qp