In an irreversible process taking place at constant $\mathrm{T}$ and $\mathrm{P}$ and in which only the pressure-volume work is being done, the change in Gibbs free energy ($\mathrm{dG}$) and the change in entropy ($\mathrm{dS}$) satisfy the criteria

The factor of $\mathrm{\Delta G}$ values is important in metallurgy. The $\mathrm{\Delta G}$ values for the following reactions at $800°\mathrm{C}$ are given as:

${\mathrm{S}}_2 \left(\mathrm{s}\right)+2{\mathrm{O}}_2 \left(\mathrm{g}\right)⟶2{\mathrm{SO}}_2 \left(\mathrm{g}\right); \mathrm{\Delta G}=-544 \mathrm{kJ}$
$2\mathrm{Zn} \left(\mathrm{s}\right)+{\mathrm{S}}_2 \left(\mathrm{s}\right)⟶2\mathrm{ZnS} \left(\mathrm{s}\right); \mathrm{\Delta G}=-293 \mathrm{kJ}$
$2\mathrm{Zn} \left(\mathrm{s}\right)+{\mathrm{O}}_2 \left(\mathrm{g}\right)⟶2\mathrm{ZnO} \left(\mathrm{s}\right); \mathrm{\Delta G}=-480 \mathrm{kJ}$

$\mathrm{\Delta G}$ for the below reaction will be

$2\mathrm{ZnS} \left(\mathrm{s}\right)+3{\mathrm{O}}_2 \left(\mathrm{g}\right)⟶2\mathrm{ZnO} \left(\mathrm{s}\right)+2{\mathrm{SO}}_2 \left(\mathrm{g}\right)$

$$\begin{gathered} {\mathrm{PbO}}_2\to \mathrm{PbO}; {\mathrm{\Delta G}}_{298 \mathrm{K}}<0 \\ {\mathrm{SnO}}_2\to \mathrm{SnO}; {\mathrm{\Delta G}}_{298 \mathrm{K}}>0 \end{gathered}$$

The most probable oxidation state of $\mathrm{Pb}$ and $\mathrm{Sn}$ will be