The fractional extent of a reaction

The fractional extent of a reaction is a veiy practical notion because it defines a dimensionless quantity. The definition of the notion of fractional extent leads us to consider three cases. 

Closed reactors with respect to the principal components of the reaction 

Such a reactor does not exchange any substance with its surroundings. At the initial moment it contains a certain amount of reactants and the stoichiometrically least abundant reactant in reaction [1.R3] is the constituent denoted i. 

The fractional extent (a) of the reaction is the ratio of the actual extent to the initial amount of the stoichiometrically less abundant reactant  

For example, let us reconsider reaction [1.R1] with 4 moles of nitric oxide and 6 moles of hydrogen at the start. The stoichiometrically least abundant compound is the nitric oxide, as 4/2 is less than 6/2. According to [1.11], the fractional extent will be  

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We note from equation [1.16] that the fractional extent represents the portion of the reference reactant that has reacted if its stoichiometric coefficient is one (which can often be chosen). Hence the name fractional conversion is sometimes used to refer to the fractional extent of a reaction. 

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