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A '''reversible reaction''' is a [[chemical reaction]] that results in an [[chemical equilibrium|equilibrium]] mixture of [[reactant]]s and [[Product (chemistry)|products]]. For a reaction involving two reactants and two products this can be expressed symbolically as | |||
::<math> aA + bB \rightleftharpoons cC + dD</math> | |||
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 (thermodynamics)|reversible process]] in [[thermodynamics]]. | |||
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>−1</sup>), 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<sub>3</sub> + 2HCl → CaCl<sub>2</sub> + H<sub>2</sub>O + CO<sub>2</sub>↑ | |||
== History == | |||
The concept of a reversible reaction was introduced by [[Claude Louis Berthollet|Berthollet]] in 1803, after he had observed the formation of [[sodium carbonate]] crystals at the edge of a [[salt lake]]<ref>[http://www.chem1.com/acad/webtext/chemeq/Eq-01.html#NAP How did Napoleon Bonaparte help discover reversible reactions?]. Chem<sub>1</sub> General Chemistry Virtual Textbook: Chemical Equilibrium Introduction: reactions that go both ways.</ref> (one of the [[natron]] lakes in Egypt, in [[limestone]]): | |||
:2NaCl + CaCO<sub>3</sub> → Na<sub>2</sub>CO<sub>3</sub> + CaCl<sub>2</sub> | |||
He recognized this as the reverse of the familiar reaction | |||
: Na<sub>2</sub>CO<sub>3</sub> + CaCl<sub>2</sub>→ 2NaCl + CaCO<sub>3</sub> | |||
Until then, [[chemical reaction]]s were thought to always 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.<ref>Claude-Louis Berthollet,"Essai de statique chimique", Paris, 1803. [http://books.google.com/books?id=cKU5AAAAcAAJ&dq=berthollet+essai&printsec=frontcover&source=bl&ots=uUX0IKnoS0&sig=C0kakNKSnQBEkofFjkL9GyCAtBc&hl=en&ei=6DRGS-ftDYvgsQOL7OD1Dw&sa=X&oi=book_result&ct=result&resnum=14&ved=0CDUQ6AEwDQ#v=onepage&q&f=false (Google books)]</ref> | |||
In 1864, [[Peter Waage|Waage]] and [[Cato Maximilian Guldberg|Guldberg]] formulated their [[law of mass action]] which quantified Berthollet's observation. Between 1884 and 1888, [[Henry Louis Le Chatelier|Le Chatelier]] and [[Karl Ferdinand Braun|Braun]] formulated [[Le Chatelier's principle]], which extended the same idea to a more general statement on the effects of factors other than concentration on the position of the equilibrium. | |||
==See also== | |||
* [[Dynamic equilibrium]] | |||
* [[Irreversibility]] | |||
* [[Microscopic reversibility]] | |||
== References == | |||
{{reflist}} | |||
[[Category:Equilibrium chemistry]] | |||
[[Category:Physical chemistry]] | |||
Latest revision as of 12:53, 18 August 2013
A reversible reaction is a chemical reaction that results in an equilibrium mixture of reactants and products. For a reaction involving two reactants and two products this can be expressed symbolically as
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.
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.[1] So, when the free energy change is large (more than about 30 kJ mol−1), 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 CO3 + 2HCl → CaCl2 + H2O + CO2↑
History
The concept of a reversible reaction was introduced by Berthollet in 1803, after he had observed the formation of sodium carbonate crystals at the edge of a salt lake[2] (one of the natron lakes in Egypt, in limestone):
- 2NaCl + CaCO3 → Na2CO3 + CaCl2
He recognized this as the reverse of the familiar reaction
- Na2CO3 + CaCl2→ 2NaCl + CaCO3
Until then, chemical reactions were thought to always 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.[3]
In 1864, Waage and Guldberg formulated their law of mass action which quantified Berthollet's observation. Between 1884 and 1888, Le Chatelier and Braun formulated Le Chatelier's principle, which extended the same idea to a more general statement on the effects of factors other than concentration on the position of the equilibrium.
See also
References
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- ↑ at constant pressure.
- ↑ How did Napoleon Bonaparte help discover reversible reactions?. Chem1 General Chemistry Virtual Textbook: Chemical Equilibrium Introduction: reactions that go both ways.
- ↑ Claude-Louis Berthollet,"Essai de statique chimique", Paris, 1803. (Google books)