# What is the spontaneity of ΔH and ΔS?

0

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=+

• 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.

• A4

B2

C3

D1

That’s right

Source(s): just did this question

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