Chapter 18, Problem 18.97SP

Interpretation Introduction

Interpretation:

The value of $\Delta G°$ for following reaction at $25°\text{C}$ is to be calculated.

${\text{N}}_2\left(g\right)+2{\text{H}}_2\left(g\right)\to {\text{N}}_{\text{2}}{\text{H}}_{\text{4}}\left(l\right)$

Concept introduction:

Standard free-energy of formation use to define the free-energy change for formation of $1\text{}\text{mol}$ of substance in its standard state. The reactants must be in their 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 1 mole ammonia is equal to $-16.5\text{ kJ}$.