Understanding Limiting Reactants Revision Notes | GCSE AQA Higher Chemistry

1. The Limiting Factor Problem

Imagine you're making sandwiches for a party. You have 10 slices of bread but only 4 slices of cheese. How many cheese sandwiches can you make? Only 4 - because you run out of cheese first, even though you have bread left over.

Chemical reactions work exactly the same way. When two reactants combine, one usually runs out before the other. The one that runs out first is called the limiting reactant because it limits how much product you can make.

2. Why Use Excess Reactants?

In industry, chemists deliberately use an excess of one reactant. This ensures all of the more expensive or important reactant gets used up completely. It's like having extra bread when making sandwiches - you guarantee you can use all your cheese.

For example, In many industrial reactions, an excess of the cheaper reactant can be used so that the more expensive reactant is fully used up. In the Haber process, however, unreacted gases are recycled, so the key idea is recycling as well as choosing conditions that reduce waste.

3. Finding the Limiting Reactant

To identify the limiting reactant, you need to work out how much of each reactant is needed according to the balanced equation. The one that runs out first is your limiting reactant.

Worked example: Identifying the Limiting Reactant

2.3 g of sodium reacts with 3.2 g of chlorine gas. Find the limiting reactant. Equation: 2Na + Cl₂ → 2NaCl (Ar: Na = 23, Cl = 35.5)

1. Moles of Na = $\frac{2.3}{23} = 0.10$ mol Moles of Cl₂ = $\frac{3.2}{71} = 0.045$ mol — Use n = mass ÷ Mr. Mr of Cl₂ = 35.5 × 2 = 71
2. 2Na : 1Cl₂ Ratio is 2:1 — For every 2 mol Na, you need 1 mol Cl₂
3. For 0.10 mol Na, you need $\frac{0.10}{2} = 0.05$ mol Cl₂ But you only have 0.045 mol Cl₂ — Cl₂ is insufficient - you're 0.005 mol short
4. Cl₂ is the limiting reactant — Cl₂ runs out first, so it limits the reaction

Answer: Cl₂ is the limiting reactant

Notice how we didn't just compare the masses (2.3 g vs 3.2 g). We had to convert to moles and use the equation ratios. This is where students often go wrong.

4. Calculating Product Yield

Once you know the limiting reactant, you can calculate exactly how much product will form. The limiting reactant determines the maximum yield - you can't make more product than this reactant allows.

Worked example: Calculating Maximum Product Yield

Using the previous example (Cl₂ is limiting), calculate the maximum mass of NaCl that can be formed. 2Na + Cl₂ → 2NaCl (Mr: NaCl = 58.5)

1. Moles of Cl₂ = 0.045 mol (from previous calculation) — Always base calculations on the limiting reactant
2. From equation: 1 Cl₂ → 2 NaCl Moles of NaCl = 0.045 × 2 = 0.090 mol — Use the molar ratio from the balanced equation
3. Mass = moles × Mr Mass of NaCl = 0.090 × 58.5 = 5.27 g — This is the theoretical maximum yield
4. 5.27 g is reasonable because it is less than the total mass of reactants (2.3 g + 3.2 g = 5.5 g), which fits conservation of mass — Sanity check: product mass should be sensible

Answer: Maximum yield = 5.27 g of NaCl

5. Real-World Applications

Understanding limiting reactants is crucial in industry. Companies want to maximise profit by ensuring expensive reactants are completely used up. They'll use cheaper reactants in excess to guarantee this.

In the production of aspirin, salicylic acid (expensive) reacts with ethanoic anhydride (cheaper). Manufacturers use excess ethanoic anhydride to ensure all the salicylic acid reacts, maximising the yield of aspirin.

6. Exam Technique and Common Mistakes

Limiting reactant questions are mark-heavy in exams. Here's how to tackle them systematically and avoid the common traps that catch students out.

Command word focus: 'Calculate' questions need clear working with units. 'Explain' questions want you to state which reactant is limiting and why, using the equation ratios as evidence.

Exam precision note: In Understanding Limiting Reactants - Limiting reactants (HT only) - Use of amount of substance in relation to masses of pure substances, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

A limiting reactant is the reactant that is used up first and therefore stops the reaction from continuing. The excess reactant is left over at the end. When checking whether an answer is sensible, compare the product mass with the total starting mass rather than with just one reactant.