Thermodynamic Background

The law of mass action states that the rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reactants. This means that higher concentrations of reactants result in faster reaction rates. The more active mass or concentration of reactants present, the faster the reaction will proceed.

In a chemical reaction, the forward reaction converts reactants to products, while the reverse reaction converts products back to reactants. At equilibrium, the rates of these two reactions are equal, meaning there is no net change in reactant and product concentrations. Understanding reactions at equilibrium and the equilibrium constant is essential for predicting reaction behavior.

The equilibrium constant is expressed as the ratio of the forward reaction rate constant to the reverse reaction rate constant. This ratio quantifies the relative tendency of a reaction to proceed forward or reverse at equilibrium. A larger equilibrium constant indicates the forward reaction is favored.

Reaction Gibbs energy, ΔrG, quantifies the difference between chemical potentials of products and reactants. If ΔrG is less than zero, the forward reaction is spontaneous. If ΔrG is greater than zero, the reverse reaction is spontaneous. When ΔrG equals zero, the reaction reaches equilibrium with no net change.

The slope of the Gibbs energy plotted against the extent of reaction defines the reaction Gibbs energy, ΔrG. This graphical representation visualizes how the system's energy changes as the reaction progresses. The slope's sign and magnitude indicate the reaction's spontaneity and direction at any given composition.

A chemical reaction reaches equilibrium when the chemical potential of the reactants equals that of the products. At this point, ΔrG equals zero, and forward and reverse reactions occur at equal rates. The concentrations of reactants and products remain constant over time at equilibrium.

If the chemical potential of reactants exceeds that of products, ΔrG is negative and the forward reaction proceeds spontaneously. If product chemical potential exceeds reactant chemical potential, ΔrG is positive and the reverse reaction proceeds spontaneously. This relationship governs the direction of all chemical reactions.