Understanding Percentage Yield and Why Reactions Don't Give 100% Yield Revision Notes | GCSE AQA Higher Chemistry

1. Why Don't Reactions Give 100% Yield?

Picture this: you follow a recipe perfectly, measure everything exactly, but somehow end up with less cake than expected. Chemical reactions work the same way. Even when we balance equations perfectly and calculate exactly how much product we should get, we never actually get that amount.

This isn't because atoms disappear - they don't. The law of conservation of mass still applies. The problem is that real-world chemistry is messier than our neat equations suggest.

2. The Three Reasons for Lost Yield

There are exactly three reasons why reactions don't give 100% yield. Each one represents a different type of 'loss' that happens in real laboratories and industrial processes.

Reason 1: The reaction doesn't go to completion because it's reversible. Many reactions can go both ways. As products form, some convert back to reactants. You reach an equilibrium where both forward and backward reactions happen at the same rate.

Reason 2: Some product is lost during separation. After the reaction finishes, you need to separate your product from the reaction mixture. Some always gets left behind in filters, stuck to glassware, or lost during transfers.

Reason 3: Side reactions occur. Your reactants don't just make the product you want. They also react in unexpected ways, forming different products that you can't use.

3. Calculating Percentage Yield

Worked example: Calculating Percentage Yield

A student reacts 10.0g of magnesium with excess hydrochloric acid. The equation is: Mg + 2HCl → MgCl₂ + H₂. They collect 0.75g of hydrogen gas. Calculate the percentage yield. [4 marks]

1. Moles of Mg = $\frac{10.0}{24} = 0.417$ mol — Mr of Mg = 24. Always start with what you're given [M1]
2. From equation: 1 mol Mg produces 1 mol H₂ So 0.417 mol Mg produces 0.417 mol H₂ — The ratio is 1:1 from the balanced equation [M1]
3. Theoretical yield = $0.417 \times 2 = 0.834$ g H₂ — Mr of H₂ = 2. This is the maximum possible [A1]
4. Percentage yield = $\frac{0.75}{0.834} \times 100\% = 90.0\%$ — Always give percentage yield to 3 significant figures [A1]

Answer: 90.0%

Notice how we needed the balanced equation to work out the theoretical yield first. You can't calculate percentage yield without knowing what the maximum possible yield should be.

4. Real-World Examples of Yield Loss

Let's see these three causes of yield loss in action with specific examples that happen in real laboratories.

Incomplete reactions: When making ammonia (N₂ + 3H₂ ⇌ 2NH₃), the reaction is reversible. As ammonia forms, some breaks down back to nitrogen and hydrogen. Industrial plants only achieve about 15% conversion per pass through the reactor.

Product loss during separation: When making aspirin, you filter the crystals and wash them. Some aspirin always dissolves in the wash water and gets lost. More gets left behind on the filter paper. Typical loss: 10-20%.

Side reactions: When burning methane, you want CH₄ + 2O₂ → CO₂ + 2H₂O. But if oxygen runs low, you also get 2CH₄ + 3O₂ → 2CO + 4H₂O. The carbon monoxide is a waste product that reduces your yield of carbon dioxide.

5. Exam Technique and Common Mistakes

Percentage yield questions are mark-heavy in exams because they test multiple skills: balanced equations, mole calculations, and the percentage yield formula. Here's how to get full marks every time.

The biggest trap: Students often confuse actual yield with percentage yield. If a question asks 'What is the yield?', check whether it wants the mass (actual yield) or the percentage. The units tell you - grams means actual yield, % means percentage yield.

Exam precision note: In Understanding Percentage Yield and Why Reactions Don't Give 100% Yield - Percentage yield - Yield and atom economy of chemical reactions (chemistry only), 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 percentage yield below 100% does not mean conservation of mass has failed. It usually means that some product was lost during separation, some reactants formed unwanted side products, or the reaction was reversible so not all of the reactants turned into the desired product.