Gibbs Free Energy Decreasing

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Linh Vo 2J
Posts: 61
Joined: Sat Apr 28, 2018 3:00 am

Gibbs Free Energy Decreasing

Postby Linh Vo 2J » Thu Feb 21, 2019 7:34 pm

Why does Gibbs free energy of a substance decrease (become less positive) as T increases at constant pressure?

Mona Lee 4L
Posts: 60
Joined: Fri Sep 28, 2018 12:16 am

Re: Gibbs Free Energy Decreasing

Postby Mona Lee 4L » Thu Feb 21, 2019 8:41 pm

I believe that that depends on deltaH and deltaS. Because deltaG = deltaH - TdeltaS, the change in deltaG in response to temperature will depend on the signs of deltaH and deltaS. If deltaS is negative, deltaG will become more positive as T increases. If deltaS is positive, deltaG will become less positive as T increases.

Jennifer Su 2L
Posts: 47
Joined: Wed Nov 21, 2018 12:20 am

Re: Gibbs Free Energy Decreasing

Postby Jennifer Su 2L » Thu Feb 21, 2019 10:04 pm

You can take a look at problem 9.65 (6th edition). The solution manual states:
"To understand what happens to deltaG as temperature is raised, we use the relationship deltaG=deltaH-T*deltaS. From this it is clear that the free energy of the reaction becomes less favorable (more positive) as temperature increases, only if deltaS is a negative number. Therefore, we need only to find out whether the standard entropy of formation of the compound is a negative number [in order to determine if the compound is less stable at higher temperatures]."

When deltaS is negative and temperature is increasing, then the term "-T*deltaS" will become positive, and so deltaG will also become more positive.

When deltaS is positive and temperature is increasing, then the term "-T*deltaS" will remain negative, and deltaG will become more negative.

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