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Negative delta s value

Posted: Thu Feb 01, 2018 6:16 pm
by Ryan Neis 2L
Why does a negative delta s value correspond to a spontaneous process?

Re: Negative delta s value

Posted: Thu Feb 01, 2018 6:30 pm
by Aditya Pimplaskar 1J
A negative ΔS value will correspond to a spontaneous process when temperature is low and ΔH is negative. This is because of the relationship ΔG = ΔH - TΔS. We know that ΔG needs to be negative in order for the reaction to be spontaneous as that means that the entropy in the system is increasing (By the relationship -ΔG/T = ΔS total).

At a low temperature for negative ΔH and negative ΔS, ΔG will remain negative, making the reaction spontaneous. If ΔH is positive and ΔS is negative, the reaction will not be spontaneous, regardless of temperature.

Re: Negative delta s value

Posted: Thu Feb 01, 2018 6:33 pm
by Cooper1C
A negative delta S corresponds to a spontaneous process when the magnitude of T * delta S is less than delta H (which must be negative).

This is because delta G must be negative, and

delta G = delta H - (T * delta S).

A negative delta S would mean that the products have a lower entropy than the reactants, which is not spontaneous by itself.

Re: Negative delta s value

Posted: Thu Feb 01, 2018 7:00 pm
by Harrison Wang 1H
Negative delta s does not by itself correspond to a spontaneous process, but rather the product of T and delta S should be less than delta H, which has to be negative. Negative delta S actually means that either temperature has decreased, pressure has increased, or volume has decreased.

Re: Negative delta s value

Posted: Thu Feb 01, 2018 8:19 pm
by Leah Savage 2F
If you think of the idea of a spontaneous process, it would make sense that it would often correspond to a positive entropy. The universe always wants to be more disordered and be going to less energy, so this would make a positive delta S and a negative delta H.