Assuming Constant Pressure?


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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?


Michael Lesgart 1H
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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

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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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