Question

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

Answer 1

The spontaneity of a chemical reaction is determined by the signs of its ΔH and ΔS. A positive ΔH and a negative ΔS means the reaction is spontaneous in reverse at all temperatures. Other combinations of ΔH and ΔS affect spontaneity differently depending on the temperature.

Step-by-step explanation:

The spontaneity of a chemical reaction can be analyzed by considering the signs of the reaction's change in enthalpy (ΔH) and change in entropy (ΔS), as these factors influence the change in Gibbs free energy (ΔG) which predicts spontaneity. The sign of ΔG must be negative for a reaction to be spontaneous. Here's how the four combinations determine spontaneity:

• A. ΔH is positive and ΔS is negative - 2. Spontaneous in reverse at all temperatures. Since both driving forces work against product formation, ΔG will always be positive, indicating that the reaction is non-spontaneous in the forward direction at all temperatures.
• B. ΔH is positive and ΔS is positive - 3. Spontaneous as written above a certain temperature. A positive ΔS can drive the reaction to spontaneity at sufficiently high temperatures where the TΔS term outweighs the positive ΔH.
• C. ΔH is negative and ΔS is positive - 1. Spontaneous as written at all temperatures. Both driving forces favor product formation, ensuring ΔG is always negative and the reaction is spontaneously forward.
• D. ΔH negative and ΔS is negative - 4. Spontaneous as written below a certain temperature. Here, the negative TΔS term may dominate at low temperatures, making ΔG negative and the reaction spontaneous.

Answer 2

Step-by-step explanation:

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