Limiting and Excess Reactants

Example

Two moles of nitrogen (\(\ce{N2}\)) react with four moles of hydrogen (\(\ce{H2}\)) to form ammonia (\(\ce{NH3}\)). The reaction is: \[\ce{N2} + 3\ce{H2} \rightarrow 2\ce{NH3}\] The ratio of reactant amount to their stoichiometric coefficients: \[\ce{N2} = \frac{2}{1} = 2 \qquad \ce{H2} = \frac{4}{3} = 1.33\] The ratio for \(\ce{H2}\) is the least; therefore, \(\ce{H2}\) is the limiting reactant.

Stoichiometric \(\ce{N2}\) requirement: 3 mole of \(\ce{H2}\) demands 1 mole of \(\ce{N2}\). Therefore, for 4 mole of \(\ce{H2}\), the requirement of \(\ce{N2}\) = 4/3 mole. \[\% \text{excess of } \ce{N2} = \frac{2-\frac{4}{3}}{\frac{4}{3}} \times 100 = 50\%\]