Assuming Constant Pressure?

Volume:
Temperature:

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Timothy_Yu_Dis3A
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Joined: Wed Sep 21, 2016 2:58 pm
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Assuming Constant Pressure?

Postby Timothy_Yu_Dis3A » Sat Jan 28, 2017 2:19 pm

For question 9.15, it asks you to calculate the entropy change for the freezing of 1.00 mol of H2O(l) at 0.00 degrees C. I get how to solve this by Delta S = Delta H/T but I understand that the only way Delta H = q is if there is constant pressure. This isn't explicitly mentioned in the problem so is this something we can assume for these type of questions that don't provide q?

Thanks!

Michael Lesgart 1H
Posts: 26
Joined: Wed Sep 21, 2016 2:57 pm

Re: Assuming Constant Pressure?

Postby Michael Lesgart 1H » Sat Jan 28, 2017 2:22 pm

I would think that if the problem doesn't mention pressure change, you can assume that it is under conditions of constant pressure.
I may be wrong though

stephanieyang_3F
Posts: 62
Joined: Wed Sep 21, 2016 2:55 pm

Re: Assuming Constant Pressure?

Postby stephanieyang_3F » Tue Jan 31, 2017 1:26 pm

Yes you can think of it like that at constant pressure. Well this makes sense because for phase changes (at constant temperature) the enthalpy of the reaction is basically delta H fusion, the amount of heat needed to convert 1 mol of solid to liquid. In this case delta H is negative since its the freezing of liquid at 0.0C.


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