Limiting reactants (HT only) — AQA GCSE Chemistry
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Limiting reactants (HT only) explained
When reactants are not in the exact ratio of the balanced equation, the reactant that runs out first is the limiting reactant; it fixes the maximum amount of product.
Read the full explanation
The other reactant is in excess and some remains unreacted. Use the balanced equation to convert the limiting amount into product amount. For example, 2 mol H₂ reacts with 1 mol O₂ to give 2 mol H₂O. If only 1 mol O₂ is available with 3 mol H₂, oxygen is limiting, so at most 2 mol H₂O forms and 1 mol H₂ is left over. The same logic works with masses: convert masses to moles using n = m ÷ Mᵣ, apply the mole ratio, then convert back to mass if required.
Your focus
- Identify the limiting reactant from given amounts and a balanced equation.
- Calculate the maximum moles or mass of product from the limiting reactant.
- Explain why the excess reactant remains after reaction and does not increase product yield.
Limiting reactants (HT only) exam tips
Marking Points
- Identify the limiting reactant by comparing the amounts of each reactant with the ratio required by the balanced equation.
- State that the limiting reactant determines the maximum amount of product that can be formed.
- Use the balanced equation to convert the amount of limiting reactant into the amount of product in moles.
- Convert between mass and moles using n = m ÷ Mᵣ when the question gives masses in grams.
- Explain that the reactant in excess is not fully used up and remains after the reaction.
Examiner Tips
- 💡Write the balanced equation and label the mole ratio before doing any calculation.
- 💡Show clearly which reactant is limiting and state that it controls the product amount.
- 💡Check units and significant figures at the end, and include the unit with any mass answer.
Common Mistakes
- Assuming the reactant with the smaller mass is always limiting; the correct approach is to compare mole amounts with the equation ratio.
- Using the amount of the excess reactant to calculate product; the correct method uses the limiting reactant only.
- Forgetting to convert grams to moles before applying the ratio; the corrected method converts mass to moles first, then uses the ratio.