A student reacts iron with sulfur to produce iron sulfide. The theoretical yield of iron sulfide is 44 g, but the student only obtains 33 g. Explain why the actual yield of a chemical reaction is often less than the theoretical yield, and describe what is meant by percentage yield.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Iron reacts with sulfur to produce iron sulfide: Fe + S → FeS
Model answer (4 marks)
Percentage yield is the actual yield expressed as a percentage of the theoretical yield.
One reason the actual yield is less than the theoretical yield is that the reaction may not go to completion; it can be reversible or limited by equilibrium.
A second reason is that some product may be lost during collection, purification or transfer.
The percentage yield in this reaction is 75 % (33 g ÷ 44 g × 100).
One reason the actual yield is less than the theoretical yield is that the reaction may not go to completion; it can be reversible or limited by equilibrium.
A second reason is that some product may be lost during collection, purification or transfer.
The percentage yield in this reaction is 75 % (33 g ÷ 44 g × 100).
Examiner tips
- Use the exact definition of percentage yield; include the calculation. Mention at least two common loss mechanisms. Show the percentage calculation with correct units and rounding.
Common mistakes
- Writing the definition incorrectly (e.g. ‘percentage yield is the theoretical yield of the actual yield’). Forgetting to calculate the percentage yield or giving the wrong value. Not providing two distinct reasons for loss of yield.
Mark scheme (4 marks)
- Percentage yield is the actual yield expressed as a percentage of the theoretical yield
- One valid reason why actual yield is less than theoretical yield — e.g. the reaction may be reversible / may not go to completion
- A second valid reason — e.g. some product may be lost during collection / purification / transfer
- The percentage yield in this reaction is 75% (33 ÷ 44 × 100)
Key terms in this question
actual yield · theoretical yield · percentage yield
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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