Limiting reactant

Chemical reactions are commonly carried out with the objective of preparing a desirable compound from a precious reactant. In order to maximise conversion of the one reactant to the desirable product, It is commonplace to use an excess of other reactants. The reactant present in limited amount limits the amount of reaction.

Thus in order to predict the maximum amount of a particular product that will be formed from a mixture of reactants of given masses or concentrations and volumes, it is necessary to decide which reactant limits the amount of reaction that occurs. This can be done by predicting the amount in moles of reaction based on each of the reactants.

For a reaction aA + bB

cC + dD
A,B,C and D are substances
a, b, c and d are the stoichiometric coefficients

n(reaction) =	n(A)	=	n(B)	=	n(C)	=	n(D)
	a		b		c		d

The limiting reactant predicts the smallest amount in moles of reaction.
Any other reactants are said to be present in excess.
The amount in moles of other substances that are consumed or produced can be calculated from the known amount in moles of reaction.

If the smallest n(reaction) is predicted using n(B); B is the limiting reactant AND
n(A) consumed = a × n(reaction)
n(C) produced = c × n(reaction)
n(D) produced = d × n(reaction)

For a reactant present in excess of the amount required for complete reaction (such as A in the example above), the amount that remains after reaction is complete is the difference between the amount that was present initially and the amount that was consumed.

For the reaction of iodate ion with iodide ion it is common to use excess I^–.
IO₃^– + 5I^– + 6H⁺

I₂ + 3H₂O
If, in a particular experiment involving 1 mol IO₃^–and therefore 1 mol of reaction, 5 mol I^– would be consumed.
If 8 mol I^– were used, 3 mol I^– would remain unreacted when reaction is complete.