phase change entropy


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phase change entropy

Postby 005391550 » Sat Feb 08, 2020 8:55 pm

for the entropy change of a phase change, we use the fact that q(reversible)/T is the same as (change in enthalpy)/T. Why can we assume that q(reversible) = change in enthalpy of phase change? I thought that q at constant P = change in enthalpy but reversible reactions do not have a constant pressure. Doesn't these contradict each other

Alice Chang 2H
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Joined: Fri Aug 30, 2019 12:18 am

Re: phase change entropy

Postby Alice Chang 2H » Sun Feb 09, 2020 6:16 pm

I'm probably wrong about this, but I think reversible reactions can have constant pressure. But according to Le Chatelier's, a change in pressure will cause the reaction to want to stabilize to accommodate for the new pressure.

I read somewhere that the q(rev) is usually substituted as the amount of heat put into the system, so substituting q(rev) as change in enthalpy makes sense in this context. Not sure if this is right though.

JonathanS 1H
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Joined: Thu Jul 11, 2019 12:17 am

Re: phase change entropy

Postby JonathanS 1H » Sun Feb 09, 2020 6:21 pm

Im pretty sure in this case that qrev = qp because we can consider phase changes to be both reversible and at constant pressure. An easier way to think of this is the transition from ice to water and vice versa under constant pressure because the system in this case is only having heat added to it.

Alex Chen 2L
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Joined: Wed Sep 18, 2019 12:21 am

Re: phase change entropy

Postby Alex Chen 2L » Sun Feb 09, 2020 10:16 pm

I could be wrong, but I don't think having constant pressure really applies in this case since a phase change involves a phase that is not a gas. Also, no expansion work is being done.

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