The Reaction Gibbs Energy

Based on the reaction Gibbs energy, Δ_rG, reactions or processes can be categorized into two types: exergonic and endergonic.

Exergonic reactions are characterized by a negative Δ_rG value, indicating that they are spontaneous in the forward direction and can be used to make other reactions easier.

For example, in biological cells, the oxidation of carbohydrates into simpler molecules provides energy, which is used for various biological activities.

Reactions with positive Δ_rG values are called endergonic reactions. A common example is the conversion of glucose to glucose-6-phosphate.

Now, consider a reversible reaction where ‘a’ moles of A and ‘b’ moles of B react to form 'c' moles of C and 'd ' moles of D.

For this reaction, the standard reaction Gibbs energy, Δ_rG°, is the difference between the sum of the standard molar Gibbs energies of the products and that of the reactants, each multiplied by their respective stoichiometric coefficients.

As Δ_rG = 0 at equilibrium, Δ_rG° can be measured using its relationship with the reaction quotient, Q, where Q depends on the activities of the reactants and products.