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Reversible reaction

 

 
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Reversible reaction
 
 
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A reversible reaction is a chemical reaction
Chemical reaction

A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
 that results in an equilibrium
Chemical equilibrium

In a chemical process, chemical equilibrium is the state in which the Activity or concentrations of the reactants and products have no net change over time....
 mixture of reactants and products
Product (chemistry)

A product is a substance that forms as a result of a biological- or chemical reaction. While the end product of some chemical reactions may be the result of a relatively rapid reaction, nanoseconds to seconds, chemical equilibrium in complex systems may require years or even centuries to be established....
. For a reaction involving two reactants and two products this can be expressed symbolically as
aA + bB ? cC + 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
Reversible process (thermodynamics)

In thermodynamics, a reversible process, or reversible cycle if the process is cyclic, is a process that can be "reversed" by means of infinitesimal changes in some property of the system without loss or dissipation of energy....
 in thermodynamics
Thermodynamics

In physics, thermodynamics is the study of the conversion of heat energy into different forms of energy ; different energy conversions into heat energy; and its relation to macroscopic variables such as temperature, pressure, and volume....
.

The concentration
Concentration

In chemistry, concentration is the measure of how much of a given chemical substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in the solvent....
s 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
Equilibrium constant

For a general chemical equilibriumthe equilibrium constant can be defined bywhere is the activity of the chemical species A etc . It is conventional to put the activities of the products in the numerator and those of the reactants in the denominator....
, K.






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Encyclopedia


A reversible reaction is a chemical reaction
Chemical reaction

A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
 that results in an equilibrium
Chemical equilibrium

In a chemical process, chemical equilibrium is the state in which the Activity or concentrations of the reactants and products have no net change over time....
 mixture of reactants and products
Product (chemistry)

A product is a substance that forms as a result of a biological- or chemical reaction. While the end product of some chemical reactions may be the result of a relatively rapid reaction, nanoseconds to seconds, chemical equilibrium in complex systems may require years or even centuries to be established....
. For a reaction involving two reactants and two products this can be expressed symbolically as
aA + bB ? cC + 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
Reversible process (thermodynamics)

In thermodynamics, a reversible process, or reversible cycle if the process is cyclic, is a process that can be "reversed" by means of infinitesimal changes in some property of the system without loss or dissipation of energy....
 in thermodynamics
Thermodynamics

In physics, thermodynamics is the study of the conversion of heat energy into different forms of energy ; different energy conversions into heat energy; and its relation to macroscopic variables such as temperature, pressure, and volume....
.

The concentration
Concentration

In chemistry, concentration is the measure of how much of a given chemical substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in the solvent....
s 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
Equilibrium constant

For a general chemical equilibriumthe equilibrium constant can be defined bywhere is the activity of the chemical species A etc . It is conventional to put the activities of the products in the numerator and those of the reactants in the denominator....
, K. The magnitude of the equilibrium constant depends on the Gibbs free energy
Gibbs free energy

In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating Work obtainable from an isothermal, Isobaric process thermodynamic system....
 change for the reaction. 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:
CaCO3 + 2HCl ? CaCl2 + H2O + CO2?


History


The concept of a reversible reaction was introduced by Berthollet
Claude Louis Berthollet

Claude Louis Berthollet was a Duchy of Savoyard-French chemist who became vice president of the French Senate in 1804....
 in 1803, after he had observed the formation of sodium carbonate
Sodium carbonate

Sodium carbonate , , is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily efflorescence to form a white powder, the monohydrate....
 crystals at the edge of a salt lake
Salt Lake

For a lake containing a high concentration of salt, see salt lake .More specifically, Salt Lake may refer to:...
 .
2NaCl + CaCO3 ? Na2CO3 + CaCl2
He recognized this as the reverse of the familiar reaction
Na2CO3 + CaCl2? 2NaCl + CaCO3


Until then, chemical reaction
Chemical reaction

A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
s were thought to always proceed in one direction. Berthollet reasoned that the excess of salt
Salt

A salt, in chemistry, is defined as the product formed from the neutralisation reaction of acids and base . Salts are ionic compounds composed of cations and anions so that the product is electrically electric charge ....
 in the lake helped push the "reverse" reaction towards the formation of sodium carbonate. In 1864, Waage
Peter Waage

Peter Waage was a significant Norway chemist and professor at the University of Oslo. Along with Cato Guldberg, he co-discovered and developed the law of mass action between 1864 and 1879....
 and Guldberg
Cato Maximilian Guldberg

Cato Maximilian Guldberg was a Norway mathematician and chemist....
 formulated their law of mass action which quantified Berthollet's observation. Between 1884 and 1888, Le Chatelier and Braun
Karl Ferdinand Braun

Karl Ferdinand Braun was a German inventor, physicist and Nobel Prize in Physics . Braun contributed significantly to the development of the radio and TV technology....
 formulated Le Chatelier's principle
Le Châtelier's principle

In chemistry, Le Chatelier's Principle, also called the Le Chatelier-Braun principle, can be used to predict the effect of a change in conditions on a chemical equilibrium....
, 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
    Dynamic equilibrium

    A system in dynamic equilibrium is a particular example of a system in a steady state. In a steady state the rate of inputs is equal to the rate of outputs so that the composition of the system is unchanging in time....
  • Irreversibility
    Irreversibility

    In science, a process that is not reversible is called irreversible. This concept arises most frequently in thermodynamics, as applied to thermodynamic processes....
  • Microscopic reversibility
    Microscopic reversibility

    The principle of Microscopic reversibility in chemistry states that in a reversible reaction the mechanism in one direction is exactly the reverse of the mechanism in the other direction....


External links

  • on reversible reactions