2013 Midterm Q6B  [ENDORSED]

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Ashley Curtis 2O
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2013 Midterm Q6B

Postby Ashley Curtis 2O » Sat Feb 11, 2017 3:14 pm

In the 2013 midterm Q6B, we are asked to determine when different quantities provide a criterion for assessing the spontaneity of a reaction. Why are we unable to tell whether the reaction is spontaneous or not using delta S but we can tell using delta S (total)?

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Re: 2013 Midterm Q6B  [ENDORSED]

Postby Chem_Mod » Sat Feb 11, 2017 6:22 pm

The second law of thermodynamics applies to the entropy of an isolated system, and effectively states that the entropy of an isolated system can only increase. Because the universe is the only true isolated system, the second law can be interpreted as anything that increases the entropy of the universe will be spontaneous.

The entropy of the universe can be thought of as the sum of the entropy of a system and the entropy of the surroundings:



The driving force of a process is therefore increasing the entropy of the universe. While the change in entropy of a system may be positive, the change in entropy of the system may be more negative leading to an overall decrease in the entropy of the universe. In this case, the system is not spontaneous despite the entropy of the system being positive.


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