2.1.3—Limiting reactant

The limiting reactant is used up first and determines the maximum, or theoretical, yield. An excess reactant remains after the reaction is complete.

Convert each reactant to moles, divide by its balanced-equation coefficient, and identify the smallest normalized amount as limiting. Use that reactant's ratio to calculate product.

For 2H₂ + O₂ → 2H₂O with 3.0 mol H₂ and 2.0 mol O₂, compare n/coefficient: 1.5 for H₂ and 2.0 for O₂, so H₂ limits and forms 3.0 mol H₂O. A smaller starting mass is not necessarily limiting; the decision depends on moles relative to coefficients.

Representative question

[Maximum number: 2]

Deduce which reactant is limiting. Use sections 1, 4 and 7 of the data booklet.

$\begin{array}{ll} n\left(\mathrm{CaCO}_{3}\right) & <=\frac{3.162 \mathrm{~g}}{100.09 \mathrm{~g} \mathrm{~mol}^{-1}}>= \text { AND } & 0.0316<\mathrm{mol}> n(\mathrm{HCl})<=4.00 \mathrm{~mol} \mathrm{dm} \mathrm{~m}^{-3} \times 20.0 \times 10^{-3} \mathrm{dm}^{3}> =0.0800<\mathrm{mol}> \end{array}$ $\mathrm{CaCO}_{3}$ is limiting

Do not award M2 without working/answer seen for M1

Retrieve the route: balance the equation, convert through the coefficient mole ratio, identify the limiting reactant, calculate theoretical and percentage yield, then assess atom economy.

Check formula subscripts, state symbols, ratio direction, units, the smallest normalized reactant amount, the theoretical-yield denominator, and the total reactant mass used for atom economy.

2.1.3—Limiting reactant

Limiting and Excess Reactants

Finding the Limiting Reactant

Assessment in practice

Amount of Chemical Change Summary

Question 1