Question

Classify the possible combinations of signs for a reaction's ΔH and ΔS values by the resulting spontaneity

Answer 1

Answer :

(A) ΔH is positive and ΔS is negative  → (2) Spontaneous in reverse at all temperatures

(B) ΔH is positive and ΔS is positive  → (3) Spontaneous as written above a certain temperature

(C) ΔH is negative and ΔS is positive  → (1) Spontaneous as written at all temperatures

(D) ΔH negative and ΔS is negative → (4) Spontaneous as written below a certain temperature

Explanation :

According to Gibb's equation:

$\Delta G=\Delta H-T\Delta S$

$\Delta G$ = Gibbs free energy  

$\Delta H$ = enthalpy change

$\Delta S$ = entropy change  

T = temperature in Kelvin

As we know that:

$\Delta G$= +ve, reaction is non spontaneous

$\Delta G$= -ve, reaction is spontaneous

$\Delta G$= 0, reaction is in equilibrium

(A) ΔH is positive and ΔS is negative.

$\Delta G=\Delta H-T\Delta S$

$\Delta G=(+ve)-T(-ve)$

$\Delta G=(+ve)$

The reaction is non-spontaneous at all temperatures or spontaneous in reverse at all temperatures.

(B) ΔH is positive and ΔS is positive.

$\Delta G=\Delta H-T\Delta S$

$\Delta G=(+ve)-T(+ve)$

$\Delta G=(+ve)$    (at low temperature)   (non-spontaneous)

$\Delta G=(-ve)$    (at high temperature)   (spontaneous)

The reaction is spontaneous as written above a certain temperature.

(C) ΔH is negative and ΔS is positive.

$\Delta G=\Delta H-T\Delta S$

$\Delta G=(-ve)-T(+ve)$

$\Delta G=(-ve)$

The reaction is spontaneous as written at all temperatures

(D) ΔH is negative and ΔS is negative.

$\Delta G=\Delta H-T\Delta S$

$\Delta G=(-ve)-T(-ve)$

$\Delta G=(+ve)$    (at high temperature)   (non-spontaneous)

$\Delta G=(-ve)$    (at low temperature)   (spontaneous)

The reaction is spontaneous as written below a certain temperature.