Extent of reaction
In physical chemistry, the extent of reaction is a quantity that measures the extent in which the reaction proceeds. It is usually denoted by the Greek letter ξ. The extent of a reaction has units of amount (moles).
Consider the reaction
- A ⇌ B
Suppose an infinitesimal amount dξ of the reactant A that changes into B. The change of the amount of A can be represented by the equation dnA=-dξ and the change of B is dnB=dξ. The extent of reaction is then defined as
where denotes the amount of the i-th reactant and is the stoichiometric number of the i-th reactant. In other words, it is the amount of substance that is being changed in an equilibrium reaction. Considering finite changes instead of infinitesimal changes, one can write the equation for the extent of a reaction as
The extent of a reaction is defined as zero at the beginning of the reaction. Thus the change of ξ is the extent itself.
The relation between the change in Gibbs reaction energy and Gibbs energy can be defined as the slope of the Gibbs energy plotted against the extent of reaction at constant pressure and temperature.
The extent of reaction is a useful quantity in computations with equilibrium reactions. Let us consider the reaction
- 2A ⇌ B + 3 C
where the initial amounts are , and the equilibrium amount of A is 0.5 . We can calculate the extent of reaction from its definition
Do not forget that the stoichiometric number of reactants is negative. Now when we know the extent, we can rearrange the equation and calculate the equilibrium amounts of B and C.
- Atkins, Peter; de Paula, Julio (2006). Physical chemistry (8 ed.). p. 201. ISBN 0-7167-8759-8.
- Lisý, Ján Mikuláš; Valko, Ladislav (1979). Príklady a úlohy z fyzikálnej chémie. p. 593.
- Ulický, Ladislav (1983). Chemický náučný slovník. p. 313.
- Lisý, Ján; Valko, Ladislav (1979). Príklady a úlohy z fyzikálnej chémie. p. 593. Unknown parameter