3.2.4Quantitative Chemistry
Limiting Reactants — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 9 of 10
Section 1 of 6Limiting and excess (HT only)
Limiting reactant
- Limiting reactant The reactant that is completely used up first, so the reaction stops when it has gone.
- It decides the maximum mass of product that can form.
Excess reactant
- Excess reactant The reactant left over once the reaction has stopped.
- Only a small surplus is needed for a reactant to count as being in excess.
Proportionality
- Proportionality With excess of everything else, the amount of product is directly proportional to the amount of limiting reactant.
- Halve the limiting reactant and you halve the product; double it and you double the product.
Section 2 of 6Finding the limiting reactant (HT only)
1
Write the balanced symbol equation for the reaction.
2
Convert the mass of each reactant to moles: moles = mass ÷ Ar (or Mr).
3
Read the mole ratio from the balanced equation.
4
Choose one reactant and work out how many moles of the other would be needed to react completely with it.
5
Compare that requirement with what is actually available: if there is less than needed, that reactant is limiting.
6
Use the moles of the limiting reactant, with the equation ratio, for every later calculation of product.
Section 3 of 6Worked example: sodium and sulfur
| Stage | Working |
|---|---|
| Equation | 2Na + S → Na2S, so the ratio Na : S is 2 : 1 |
| Moles of Na | 9.2 g ÷ 23 g/mol = 0.40 mol |
| Moles of S | 8.0 g ÷ 32 g/mol = 0.25 mol |
| Sulfur needed for 0.40 mol Na | 0.40 ÷ 2 = 0.20 mol |
| Comparison | 0.25 mol S is available, which is more than 0.20 mol |
| Conclusion | Sulfur is in excess and sodium is the limiting reactant |
Section 4 of 6Why reactions stop
1Reaction starts
- Reaction starts Particles of both reactants collide and form product.
2Limiting reactant falls
- Limiting reactant falls Its particles are used up as product forms.
3Collisions run out
- Collisions run out With none of the limiting reactant left, no more successful reactions can happen.
4Reaction ends
- Reaction ends Excess reactant remains unreacted, and the amount of product has reached its maximum.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Limiting reactant | The reactant that is used up first and so limits the amount of product. |
| Excess reactant | A reactant present in a greater amount than is needed, so some is left over. |
| Mole ratio | The ratio of moles of substances in a balanced equation. |
| Directly proportional | Two quantities that increase or decrease in the same ratio. |
| Relative atomic mass (Ar) | The average mass of an atom of an element compared with one-twelfth of a carbon-12 atom. |
Section 6 of 6Exam technique
1
Comparing masses rather than moles — the reactant with the smaller mass is not automatically limiting.
2
Ignoring the equation ratio0.40 mol Na does not need 0.40 mol S when the ratio is 2 : 1.
3
Using the excess reactant to calculate the product mass; always base the calculation on the limiting reactant.
4
Saying the excess reactant is used up last — some of it is never used at all.
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