What is "reversible reaction"

A reversible reaction is a reaction where the reactants form products, which react together to give the reactants back.

{\mathit aA} + \mathit bB <=> {\mathit cC} + \mathit dD

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 reversible process in thermodynamics.

Weak acids and bases undertake reversible reactions. For example, carbonic acid: HCO + HO ⇌ HCO + HO.

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. So, when the free energy change is large (more than about 30 kJ mol), then 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 in reality 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

Ca CO + 2HCl → CaCl + HO + CO↑