General formula for phase changes for enthalpy.

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Andrew Uesugi 3I
Posts: 38
Joined: Wed Sep 21, 2016 2:56 pm

General formula for phase changes for enthalpy.

Postby Andrew Uesugi 3I » Sat Mar 18, 2017 9:34 am

When you calculate the enthalpy of state change, do you generally do this:

Enthalpy of fus/sub of given substance x mass of substance

Is that the correct way to do it?

Lucy Nakashima 1E
Posts: 21
Joined: Sat Jul 09, 2016 3:00 am

Re: General formula for phase changes for enthalpy.

Postby Lucy Nakashima 1E » Sat Mar 18, 2017 10:21 am

When you're asked to find the required heat for some substance to undergo a phase change and you're given a mass, you usually convert it to moles first because enthalpies are generally given in kJ/mol so you'd have to find the moles of the sample to make the units cancel. So convert grams->moles then multiply moles x enthalpy (of fusion, vaporization, etc).

Kira_Maszewski_1B
Posts: 17
Joined: Wed Sep 21, 2016 2:57 pm

Re: General formula for phase changes for enthalpy.

Postby Kira_Maszewski_1B » Sun Mar 19, 2017 7:54 pm

For something with multiple phase changes and temperature changes, we use both the q=m*C*delta_T formula and the q=n*delta_H formulas. For example: If we we have a change in temperature of water from -5 degrees centigrade to 125 degrees centigrade, then we will need to use the q=m*C*delta_T for the heat transfer from -5 degrees to 0 degrees centigrade, the q=n*delta_H formula for the phase change(ice to liquid), and q=m*C*delta_T for the heat transfer from 0 degrees to 100 degrees centigrade, and the the q=n*delta_H formula for the phase change (liquid to vapor), and finally q=m*C*delta_T for the heat transfer from 100 degrees to 125 degrees centigrade. We add all of the four calculated q values to find the total heat. Also, not that we commonly had to solve for the moles of substance if its given to you in grams and you only have the standard enthalpy values in kj/mol.


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