Calculating enthalpy change with phase changes

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Adam Kramer 1A
Posts: 103
Joined: Sat Aug 24, 2019 12:15 am

Calculating enthalpy change with phase changes

Postby Adam Kramer 1A » Sun Jan 26, 2020 3:07 pm

Can someone remind me what Lavelle was saying about having to add enthalpy if the phase changes? How do we calculate the change of enthalpy this way?

Kaylee Sepulveda 4G
Posts: 100
Joined: Thu Jul 11, 2019 12:16 am

Re: Calculating enthalpy change with phase changes

Postby Kaylee Sepulveda 4G » Sun Jan 26, 2020 3:36 pm

ΔHsub=ΔHfus+ΔHvap

If we know the enthalpies of certain phase changes, we can calculate an unknown value using what is given to us.

Some helpful diagrams:
Annotation 2020-01-26 153503.png


Annotation 2020-01-26 153446.png

Ritu Gaikwad 2E
Posts: 52
Joined: Thu Jul 11, 2019 12:16 am

Re: Calculating enthalpy change with phase changes

Postby Ritu Gaikwad 2E » Sun Jan 26, 2020 4:16 pm

When Dr. Lavelle was talking about phase changes, he was saying that in the standard state all substances should be a gaseous phase so if some of your reactants are in a liquid or solid phase you have to factor in an extra heat of vaporization or an extra enthalpy value in order to account for the phase change because that would change the value of your reaction.

Althea Zhao 1B
Posts: 101
Joined: Mon Jun 17, 2019 7:24 am

Re: Calculating enthalpy change with phase changes

Postby Althea Zhao 1B » Sun Jan 26, 2020 10:59 pm

You need to calculate a reaction's enthalpy when all reactants and products are in the gas phase. If there are solids and/or liquids present, you need to add in the enthalpy it takes to convert them to gases.


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