Is spontaneity determined by entropy or free energy?

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Harjas Sabharwal 1G
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Is spontaneity determined by entropy or free energy?

Postby Harjas Sabharwal 1G » Tue Feb 13, 2018 4:26 pm

I previously understood that a reaction was spontaneous when ΔG < 0?
But I was reading through the textbook and it stated that a reaction was spontaneous if ΔStot > 0. I though that a reaction did not have to be spontaneous if ΔS > 0?

Chem_Mod
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Re: Is spontaneity determined by entropy or free energy?

Postby Chem_Mod » Tue Feb 13, 2018 6:14 pm

Hi, Harjas,
The reaction doesn't have to be spontaneous if deltaS(sys) > 0 (for your reaction, for example). However, the second law of thermodynamics states, that if the total entropy is positive, the process will be spontaneous.

Yixin Angela Wang 2H
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Re: Is spontaneity determined by entropy or free energy?

Postby Yixin Angela Wang 2H » Thu Feb 15, 2018 3:02 am

Is it because the ΔS in the Gibbs free energy equation is only for the system, and not the total?

Manvir2K
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Re: Is spontaneity determined by entropy or free energy?

Postby Manvir2K » Thu Feb 15, 2018 12:38 pm

Yes. When the book only has deltaS, it is only talking about deltaS of the system. Spontaneity can be found if we have total S.

Irma Ramos 2I
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Re: Is spontaneity determined by entropy or free energy?

Postby Irma Ramos 2I » Thu Feb 15, 2018 1:04 pm

As mentioned above, one needs to pay attention to see if the entropy that one is talking about is for the system or the total entropy because whether or not the entropy can determine the spontaneity is based on whether it's the total or for the system.

Eva Zhao 4I
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Re: Is spontaneity determined by entropy or free energy?

Postby Eva Zhao 4I » Mon Feb 10, 2020 1:32 pm

As others have said, check for whether the question is asking for ∆S or total entropy. That being said, a useful chart to check for spontaneity with ∆S, provided ∆H is given, is (through the use of ∆G = ∆H - T∆S):
Image

Joanne Lee 1J
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Re: Is spontaneity determined by entropy or free energy?

Postby Joanne Lee 1J » Mon Feb 10, 2020 5:32 pm

Spontaneity is determined by Gibbs free energy and whether that value is positive or negative.

205389184
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Re: Is spontaneity determined by entropy or free energy?

Postby 205389184 » Sun Feb 16, 2020 10:23 pm

Even if your value of delta S of the surroundings is greater than zero, the reaction does not have to be spontaneous, but according to the 2nd law of thermodynamics, the process will be spontaneous if delta Stot is a positive value.

Hussain Chharawalla 1G
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Re: Is spontaneity determined by entropy or free energy?

Postby Hussain Chharawalla 1G » Sun Feb 16, 2020 10:40 pm

In that case, what is the difference between the reaction and the process?

Emil Velasco 1H
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Re: Is spontaneity determined by entropy or free energy?

Postby Emil Velasco 1H » Mon Feb 17, 2020 5:37 pm

Spotaneity is determined by both as presented in the chart above, but either could be used in order to see whether a reaction is spontaneous or not


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