Chapter 18, Problem 18.103SP

Interpretation Introduction

Interpretation:

Value of free energy for standard state $\Delta G°$ for following reaction is to be calculated. The reaction is spontaneous at $25°\text{C}$ should be shown. Also, the reason and temperature at which reaction becomes nonspontaneous should be determined.

$2{\text{H}}_{\text{2}}\text{S}\left(g\right)+{\text{SO}}_2\left(g\right)\to 2\text{S}\left(s\right)+2{\text{H}}_{\text{2}}\text{O}\left(g\right)$

Concept introduction:

Standard free-energy of substance formation use to define free-energy change to form $1\text{}\text{mol}$ of substance in standard state. The reactant must be in its maximum stable form of constituent elements. It is denoted by $\Delta G{°}_{\text{f}}$. The general expression for $\Delta G{°}_{\text{f}}$ is as follows:

$\Delta G°=\Delta G{°}_{\text{f}}\left(\text{product}\right)-\Delta G{°}_{\text{f}}\left(\text{reactant}\right)$

For example, formation reaction of ammonia is as follows:

${\text{N}}_2\left(g\right)+3{\text{H}}_2\left(g\right)\to 2{\text{NH}}_3\left(g\right)$

Standard free-energy change of $\text{2 mol}$ of ammonia is $-33.0\text{ kJ}$, thus free-energy change of formation of $\text{1 mol}$

${\text{NH}}_3$ is equal to $-16.5\text{ kJ}$.