A student prepares calcium chloride by reacting calcium carbonate with hydrochloric acid. The theoretical yield of calcium chloride is 50 g, but the student only collects 35 g. Give two reasons why the actual yield is less than the theoretical yield, and explain what is meant by the term 'theoretical yield'.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Theoretical yield is the maximum amount of product that could be produced from the given reactants, assuming a complete reaction takes place.
1. Some of the calcium chloride is lost during the transfer, filtration, evaporation or collection steps, so the amount recovered is less than the amount that was formed.
2. The reaction may not have gone to completion; a portion of the calcium carbonate or hydrochloric acid remains unreacted, so less calcium chloride is produced than the theoretical maximum.
The theoretical yield is the amount of product that would be obtained if every mole of reactant were converted to product with no losses, so the actual yield of 35 g is less than the theoretical 50 g, giving a percentage yield of 70 % (35 g ÷ 50 g × 100).
1. Some of the calcium chloride is lost during the transfer, filtration, evaporation or collection steps, so the amount recovered is less than the amount that was formed.
2. The reaction may not have gone to completion; a portion of the calcium carbonate or hydrochloric acid remains unreacted, so less calcium chloride is produced than the theoretical maximum.
The theoretical yield is the amount of product that would be obtained if every mole of reactant were converted to product with no losses, so the actual yield of 35 g is less than the theoretical 50 g, giving a percentage yield of 70 % (35 g ÷ 50 g × 100).
Examiner tips
- Define theoretical yield first; then give two practical reasons for a lower actual yield; link the numbers to show the yield is <100%.
- Use the exact wording from the mark scheme and include the calculation of the percentage yield if asked.
Common mistakes
- Writing that the yield is less because the reaction is slow, rather than incomplete or practical losses; "slow" is not a reason for lower yield.
- Failing to define theoretical yield before giving reasons; or using vague terms like "some loss" without specifying transfer/filtration.
Mark scheme (4 marks)
- Theoretical yield is the maximum amount of product that could be produced from the given reactants, assuming a complete reaction takes place.
- First reason: product is lost during transfer/filtration/evaporation/collection (i.e. practical losses during the process).
- Second reason: the reaction may not have gone to completion / some reactant was not fully used up.
- Percentage yield is less than 100% because the actual yield (35 g) is less than the theoretical yield (50 g) — linking the two values to show the yield is not 100%.
Key terms in this question
actual yield · theoretical yield
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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