AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
In a chemical reaction involving two reactants, it is common to use an…
Full specification wording
Limiting reactants (HT only): In a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other reactant is used.
When two reactants could react, adding extra of one ensures the other is fully consumed.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When two reactants could react, adding extra of one ensures the other is fully consumed. The reactant in excess is left over at the end; the other is the limiting reactant. For example, in burning magnesium in air, oxygen is in excess so all the magnesium reacts. In a school preparation, an acid may be added in excess to dissolve all of a metal carbonate, ensuring complete reaction. Using excess is common because it drives the reaction to completion, maximises yield based on the valuable reactant, and avoids unreacted starting material. In calculations, you identify which reactant is limiting by comparing mole ratios, then use that reactant to find the amount of product formed.
What earns the marks
- Explain that excess means more of a reactant is present than is needed to react with the other reactant.
- State that the purpose of excess is to ensure the other reactant is completely used up.
- Identify that the reactant not in excess is the limiting reactant and determines the amount of product.
- Recognise that the excess reactant remains after the reaction and may need to be separated or recovered.
- Apply the idea to a named reaction, such as an acid in excess reacting with a metal or carbonate.
Common mistakes and how to put them right
Assuming the reactant with the larger mass is always in excess.
Put it right by comparing moles and the equation ratio, not masses alone.
Thinking excess reactant is used up first.
Put it right by stating that the limiting reactant is consumed completely while excess remains.
Believing excess guarantees a higher percentage yield of product from the limiting reactant.
Put it right by explaining it ensures complete reaction of the limiting reactant, not extra product beyond the stoichiometric amount.
Exam tips
- Define excess and limiting reactant clearly before applying them to a calculation.
- Use the balanced equation to compare the mole ratio of the two reactants when deciding which is limiting.
- Link the purpose of excess to a practical context, such as ensuring complete reaction or maximising use of an expensive reactant.
Try the lesson
The start of the Limiting reactants (HT only): In a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other reactant is used. lesson, made from checked teaching notes for this specification point.
Have a go first
In the reaction between magnesium and hydrochloric acid, why is it common to use an excess of hydrochloric acid?
Listen
When two reactants could react, adding extra of one ensures the other is fully consumed. The reactant in excess is left over at the end; the other is the limiting reactant. For example, in burning magnesium in air, oxygen is in excess so all the magnesium reacts. In a school preparation, an acid may be added in excess to dissolve all of a metal carbonate, ensuring complete reaction. Using excess is common because it drives the reaction to completion, maximises yield based on the valuable reactant, and avoids unreacted starting material. In calculations, you identify which reactant is limiting by comparing mole ratios, then use that reactant to find the amount of product formed. A strong answer earns marks for these points. Explain that excess means more of a reactant is present than is needed to react with the other reactant. State that the purpose of excess is to ensure the other reactant is completely used up. Identify that the reactant not in excess is the limiting reactant and determines the amount of product. Recognise that the excess reactant remains after the reaction and may need to be separated or recovered. A common slip: Assuming the reactant with the larger mass is always in excess. Instead, comparing moles and the equation ratio, not masses alone. Exam tip: Define excess and limiting reactant clearly before applying them to a calculation.
See it
Key words
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Excess
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Limiting reactant
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Purpose of excess
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Excess reactant after reaction
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See every definition
- Excess
- More of a reactant is present than is needed to react with the other reactant.
- Limiting reactant
- The reactant not in excess, which is completely used up and determines the amount of product.
- Purpose of excess
- To ensure that all of the other reactant is used and the reaction is driven to completion.
- Excess reactant after reaction
- The reactant in excess remains after the reaction and may need to be separated or recovered.
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