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Topic: Calculating entropy change  (Read 19944 times)

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Offline retracell

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Calculating entropy change
« on: November 21, 2009, 05:03:16 PM »
The enthalpy of vaporization of methanol is 35.3 kJ/mol at the boiling point of 64.2 degrees Celsius. Calculate the entropy change for methanol going from a liquid to a vapor.

I understand that the entropy change must be a positive value since vapor is more disordered but I do not know how to go about calculating this without the change in free energy.

Offline renge ishyo

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Re: Calculating entropy change
« Reply #1 on: November 21, 2009, 05:11:27 PM »
In the case of a phase transition, the entropy is related to the enthalpy in a simple way:

 :delta: Svap =  :delta: Hvap/T

Be sure to convert the temperature into Kelvin units before performing the calculation, and I also advise converting the given enthalpy into units of J/mol.

Offline retracell

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Re: Calculating entropy change
« Reply #2 on: November 21, 2009, 05:20:51 PM »
Thanks a lot for your quick and easy to understand response.

Is that relation derived from
 
 :delta: G =  :delta: H -T :delta: S

where the change in free energy is zero?

And if it is, is the change in free energy always zero in the case of phase transition?

Offline gregdwulet

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Re: Calculating entropy change
« Reply #3 on: November 23, 2009, 01:44:36 AM »
Yes and yes.

 :delta: G = 0 for any phase change.

So:

 :delta: G =  :delta: H - T :delta: S
0= :delta: H -T :delta: S
- :delta: H = -T :delta: S
  - :delta: H / T =  :delta: S

I think I did that right... Either way the principle is the same.

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