Classify the possible combinations of signs for a reaction’s ΔH and ΔS values by the resulting spontaneity.
A) Spontaneous as written at all temps.
B) Spontaneous in reverse at all temps.
C) Spontaneous as written above a certain temp.
D) Spontaneous as written below a certain temp.
Match the letters with the numbers?
1) ΔH=- ΔS=-
2) ΔH=+ ΔS=-
3) ΔH=+ ΔS=+
4) ΔH=- ΔS=+
2 Answers
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Remember that ΔG = ΔH – TΔS. For a reaction to be spontaneous, ΔG must be negative. So substitute in signs for ΔH and ΔS and see which work to give a negative ΔG.
Let’s start with #1 above. If you have both ΔH and ΔS negative, then the TΔS term will be positive. At 0 K, ΔG will be negative (from the negative ΔH). As T increases, this term will get larger. Eventually, it will get larger than ΔH, so above a certain T, ΔG will be positive for this combination. Therefore, this reaction will be spontaneous below a given temperature, matching #1 up with D.
Try the rest in a similar way.
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A4
B2
C3
D1
That’s right
Source(s): just did this question
