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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: retracell on November 21, 2009, 05:03:16 PM

Title: Calculating entropy change
Post by: retracell 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.

Title: Re: Calculating entropy change
Post by: renge ishyo 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: S_vap =  :delta: H_vap/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.

Title: Re: Calculating entropy change
Post by: retracell 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?

Title: Re: Calculating entropy change
Post by: gregdwulet 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.