Reversible reaction

The concept of a reversible reaction was introduced by Berthollet (1803) after he had observed the formation of sodium carbonate crystals at the edge of a salt lake.^[1]

2NaCl + CaCO₃ → Na₂CO₃ + CaCl₂

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He recognized this as the reverse of the familiar reaction

Na₂CO₃ + CaCl₂→ 2NaCl + CaCO₃

Up to that time chemical reactions were thought always to proceed in one direction. Berthollet reasoned that the excess of salt in the lake helped push the "reverse" reaction towards the formation of sodium carbonate. Le Chatelier later extended this idea to a more general statement of the effects on equilibrium of various factors.

In modern terminology a reversible reaction is one which results in the formation of an equilibrium mixture.

Note that this usage of "reversible" is distinct from reversible process as used in thermodynamics. See also irreversibility.

An irreversible reaction is one in which the equilibrium lies almost exclusively on the side of either the reactants or the products.

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