2nd Law of Thermodynamics question

[kejs]

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Which of the following is a statement of the 2nd Law of Thermodynamics?

A) Energy can’t be created or destroyed
B) Expanding gases cool
C) For a spontaneous process, ΔSuniverse > 0
D) The entropy of a system increases for a spontaneous process


The correct answer is C but why is D not correct?

 

[bharat008]

Which of the following is a statement of the 2nd Law of Thermodynamics?

A) Energy can’t be created or destroyed
B) Expanding gases cool
C) For a spontaneous process, ΔSuniverse > 0
D) The entropy of a system increases for a spontaneous process


The correct answer is C but why is D not correct?

Universe = system + surroundings
Entropy of the system can be positive or negative but the overall ENTROPY of the Universe is positive. That's why it can't be D..

 

[kejs]

Universe = system + surroundings
Entropy of the system can be positive or negative but the overall ENTROPY of the Universe is positive. That's why it can't be D..

But since it says the process is spontaneous wouldn't the entropy of the system be increasing as well? Isn't that a true statement?

 

[wenjie1988]

But since it says the process is spontaneous wouldn't the entropy of the system be increasing as well? Isn't that a true statement?

Not necessarily. As bharat pointed out, the process can still be spontaneous even if the entropy of the system is decreasing, as long as the entropy of the surrounding is increasing and outweighs the system.

Ex: -3(system) + 7(surrounding) = +4 (universe). So the entropy of the system doesn't have to be positive for the process to be spontaneous.

 

[mh0000]

An ice cube will freeze spontaneously at any temperature below water's freezing point. In this situation, the systems entropy decreases; therefore, statement D is not correct in all instances. What is correct is what bharat said, the entropy of the universe will increase in spontaneous reactions.

 

[kejs]

Ok, I get it now. Thanks everyone.