Reversible reaction

A reversible reaction is a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously.^[1]

${\displaystyle {\ce {{\mathit {a}}A{}+{\mathit {b}}B<=>{\mathit {c}}C{}+{\mathit {d}}D}}}$

A and B can react to form C and D or, in the reverse reaction, C and D can react to form A and B. This is distinct from a reversible process in thermodynamics.

Weak acids and bases undergo reversible reactions. For example, carbonic acid:

H₂CO_{3 (l)} + H₂O_(l) ⇌ HCO₃⁻_(aq) + H₃O⁺_(aq).

The concentrations of reactants and products in an equilibrium mixture are determined by the analytical concentrations of the reagents (A and B or C and D) and the equilibrium constant, K. The magnitude of the equilibrium constant depends on the Gibbs free energy change for the reaction.^[2] So, when the free energy change is large (more than about 30 kJ mol⁻¹), the equilibrium constant is large (log K > 3) and the concentrations of the reactants at equilibrium are very small. Such a reaction is sometimes considered to be an irreversible reaction, although small amounts of the reactants are still expected to be present in the reacting system. A truly irreversible chemical reaction is usually achieved when one of the products exits the reacting system, for example, as does carbon dioxide (volatile) in the reaction

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑