Editing Reversible reaction (section)

{{Short description|Chemical reaction whose products can react together to produce the reactants again}}
A '''reversible reaction''' is a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously.<ref>{{Cite web|url=https://courses.lumenlearning.com/cheminter/chapter/reversible-reaction/ |title=Reversible Reaction |website=lumenlearning.com |access-date=2021-01-08}}</ref>

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

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 (thermodynamics)|reversible process]] in [[thermodynamics]].

Weak [[acids]] and [[Base (chemistry)|bases]] undergo reversible reactions. For example, [[carbonic acid]]: 

: H<sub>2</sub>CO<sub>3 (l)</sub> + H<sub>2</sub>O<sub>(l)</sub> ⇌ HCO<sub>3</sub><sup>−</sup><sub>(aq)</sub> + H<sub>3</sub>O<sup>+</sup><sub>(aq)</sub>.

The [[concentration]]s of reactants and products in an equilibrium mixture are determined by the [[analytical concentration]]s 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.<ref>at constant pressure.</ref> So, when the free energy change is large (more than about 30 kJ mol<sup>&minus;1</sup>), 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
: CaCO<sub>3</sub> + 2HCl → CaCl<sub>2</sub> + H<sub>2</sub>O + CO<sub>2</sub>↑