The equilibrium condition of a chemical reaction, $\nu_{1}A_{1}+\nu_{2}A_{2}+.....\rightarrow 0$, in a closed system is: \begin{equation} \sum_{i}\nu_{i}\mu_{i}=0 \end{equation} During the course of a chemical reaction, the change in the number of moles of each substance is proportional to the stoichiometric coefficient $\nu$ , being the constant of proportionality the same for all species. This constant is called the degree of progress of the reaction $\xi$ and is defined as: \begin{equation} \xi =\frac{n_i -n_{i,0}}{\nu_i} \end{equation}