Classify the possible combinations of signs for a reaction's Î H and Î S values by the resulting spontaneity.
A. ÎH is positive and ÎS is negativeB. ÎH is positive and ÎS is positiveC. ÎH is negative and ÎS is positiveD. ÎH negative and ÎS is negative
For A, B, C and D find out which of following they are:1. Spontaneous as written at all temperatures2. Spontaneous in reverse at all temperatures3. Spontaneous as written above a certain temperature4. Spontaneous as written below a certain temperature

The basis to solve this question is the Gibbs Free Energy relation between the change in enthalpy , ΔH, and the change in entropy ΔS, through the relation

ΔG = ΔH - TΔS ( T is temperature )

For a given reaction ΔG must be negative for the reaction to be positive.

So to solve our question we have to determine the sign for the change in free energy studying whether the enthalpy is positive or positive, and doing the same with the term TΔS.

A. ΔH positive, ΔS negative ⇒ ΔG : always positive non spontaneous.

B.

ΔH positive, ΔS positive ⇒ If TΔS > ΔH, ΔG is negative spontaneous

If TΔS < ΔH, ΔG is positive nonspontaneous

C. ΔH negative, ΔS positive ⇒ ΔG: always is negative spontaneous

D.

ΔH negative, ΔS negative ⇒ If TΔS < ΔH, ΔG is negative spontaneous

If TΔS > ΔH, ΔG is positive nonspontaneous

Then,

1. Spontaneous as written at all temperatures: C

2. Spontaneous in reverse at all temperatures: A

3. Spontaneous as written above a certain temperature : B

4. Spontaneous as written below a certain temperature : D