- Reversible reaction
A reversible reaction is a
chemical reactionthat results in an equilibrium mixture of reactants and products. For a reaction involving two reactants and two products this can be expressed symbolically as :aA + bB ⇌ cC + dDA 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.
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". In turn the magnitude of the equilibrium constant depends on the Gibbs free energychange for the reaction. [at constant pressure.] 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 called an irreversible reaction. See also irreversibility.
The concept of a reversible reaction was introduced by
Bertholletin 1803, after he had observed the formation of sodium carbonatecrystals at the edge of a salt lake. [ [http://www.chem1.com/acad/webtext/chemeq/Eq-01.html#NAP Chemical Equilibrium ] ] :2NaCl + CaCO3 → Na2CO3 + CaCl2He recognized this as the reverse of the familiar reaction: Na2CO3 + CaCl2→ 2NaCl + CaCO3
chemical reactions were thought to always proceed in one direction. Berthollet reasoned that the excess of saltin 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.
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