Chapter 18, Problem 18.81SP

Interpretation Introduction

(a)

Interpretation:

Whether the reaction endothermic or exothermic should be determined.

Concept introduction:

The relationship between $\Delta H$ and $\Delta S$ is as follows:

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

Where,

• $\Delta G$ is free energy change.
• $\Delta H$ is enthalpy change.
• $T$ is temperature.
• $\Delta S$ is change in entropy.

The entropy criteria for spontaneous change at constant $T$ and $P$ can also be stated with respect to free energy change as follows:

(1) When $\Delta G<0$, reaction is spontaneous.

(2) When $\Delta G>0$, reaction is nonspontaneous.

(3) When $\Delta G=0$, reaction is at equilibrium.

To follow these criteria, there are signs of $\Delta H$ and $\Delta S$ that must be followed. These signs can be described as follows:

(1) The value of $\Delta G$ is negative at all temperature if $\Delta H$ is negative and $\Delta S$ is positive. Hence reaction is always spontaneous.

(2) The value of $\Delta G$ is positive at all temperature if $\Delta H$ is positive and $\Delta S$ is negative. Hence reaction is always nonspontaneous.

(3) If both $\Delta H$ and $\Delta S$ are negative then reaction would be spontaneous only if value of $\Delta H$ is larger then $T\Delta S$ or at low temperature.

(4) If both $\Delta H$ and $\Delta S$ are positive then reaction would be spontaneous only if value of $T\Delta S$ is larger then $\Delta H$ or at high temperature

Interpretation Introduction

(b)

Interpretation:

The minimum value of $\Delta H$ for reaction is to be calculated.

Concept introduction:

The relationship between $\Delta H$ and $\Delta S$ is as follows:

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

Where,

• $\Delta G$ is free energy change.
• $\Delta H$ is enthalpy change.
• $T$ is temperature.
• $\Delta S$ is change in entropy.

The entropy criteria for spontaneous change at constant $T$ and $P$ can also be stated with respect to free energy change as follows:

(1) When $\Delta G<0$, reaction is spontaneous.

(2) When $\Delta G>0$, reaction is nonspontaneous.

(3) When $\Delta G=0$, reaction is at equilibrium.

To follow these criteria, there are signs of $\Delta H$ and $\Delta S$ that must be followed. These signs can be described as follows:

(1) The value of $\Delta G$ is negative at all temperature if $\Delta H$ is negative and $\Delta S$ is positive. Hence reaction is always spontaneous.

(2) The value of $\Delta G$ is positive at all temperature if $\Delta H$ is positive and $\Delta S$ is negative. Hence reaction is always nonspontaneous.

(3) If both $\Delta H$ and $\Delta S$ are negative then reaction would be spontaneous only if value of $\Delta H$ is larger then $T\Delta S$ or at low temperature.

(4) If both $\Delta H$ and $\Delta S$ are positive then reaction would be spontaneous only if value of $T\Delta S$ is larger then $\Delta H$ or at high temperature