Chemistry

Predict how the entropy of the substance is affected in the following processes:

a) O2 (g,200 kPa, 300k) --> O2 (g, 100, 300k)

b)Br2 (l, 1 bar, 25 degree C) --> Br2 (g, 1 bar, 25 degree C)

C)Fe (s, 1 bar, 250 degree C)--> Fe(s, 1 bar, 25 degree C)

will entropy either be increase, decrease, or remain the same?

I am quite confused on this because I know that pressure would cause positional disorder and temperature will cause thermal disorder. But I don't really understand how it correlates with entropy. I tried to do the problem and I got a) increase, b) remain the same, and c) decrease. But I don't know if that is correct or not. Please explain because I am just guessing I don't really have a reason behind my guess. Thanks

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  1. A. With increased pressure volume decreases so the molecules have less space to move around in. S decreases. T is same so that doesn't enter into the problem.

    B. T and P remains same. The only difference is liquid vs gas. As a gas there is MUCH more volume to move around and S increases.

    C. P and solid are same. Only change is T. Lower T always means less S. There is less kinetic motion with decreasing T.

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  2. Thank you. I understand it a lot better. And as for equation, I figure that I figure that I left out the kPa. so that for the first process, there is a decrease in pressure so more movement for the molecules therefore S increases, right?

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  3. Yes, for increase in P there is a decrease in S.

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