AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Even though no atoms are gained or lost in a chemical reaction, it is not…
Full specification wording
Percentage yield: Even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the calculated amount of a product because: the reaction may not go to completion because it is reversible some of the product may be lost when it is separated from the reaction mixture some of the reactants may react in ways different to the expected reaction.
Theoretical yield assumes all reactants convert fully into the desired product.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Theoretical yield assumes all reactants convert fully into the desired product. In practice, actual yield is often lower. A reversible reaction may reach equilibrium before all reactants convert, so some remain unreacted. Product can be lost during separation, for example during filtration, evaporation or transfer between containers. Some reactants may undergo side reactions or react in unexpected ways, producing other substances instead of the desired product. Percentage yield compares actual yield with theoretical yield: percentage yield = (actual yield ÷ theoretical yield) × 100.
For example: If theoretical yield is 20 g but only 15 g is obtained, percentage yield is (15 ÷ 20) × 100 = 75%.
What earns the marks
- State that atoms are conserved but the calculated theoretical yield may not be achieved.
- Explain that reversible reactions may not go to completion, leaving reactants unconverted.
- Describe how product can be lost during separation or transfer, reducing actual yield.
- Explain that side reactions or unexpected reactant behaviour can form other products.
- Calculate percentage yield using percentage yield = (actual yield ÷ theoretical yield) × 100.
Common mistakes and how to put them right
Thinking atoms are lost when yield is low.
Put it right: That atoms are conserved but may form other products or remain unreacted.
Confusing actual yield with theoretical yield.
Put it right: Theoretical yield is the maximum calculated amount and actual yield is what is obtained.
Forgetting to multiply by 100 when calculating percentage yield.
Put it right: The corrected formula includes × 100.
Exam tips
- Link each reason for reduced yield to a specific practical situation, such as reversible reactions or filtration losses.
- Show the percentage yield calculation clearly, including the multiplication by 100.
- Use the terms theoretical yield and actual yield precisely in explanations.
Try the lesson
The start of the Percentage yield: Even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the calculated amount of a product because: the reaction may not go to completion because it is reversible some of the product may be lost when it is separated from the reaction mixture some of the reactants may react in ways different to the expected reaction. lesson, made from checked teaching notes for this specification point.
Have a go first
In a chemical reaction, the mass of product actually obtained is often less than the mass calculated from the equation. Which statement explains why this can happen?
Listen
Theoretical yield assumes all reactants convert fully into the desired product. In practice, actual yield is often lower. A reversible reaction may reach equilibrium before all reactants convert, so some remain unreacted. Product can be lost during separation, for example during filtration, evaporation or transfer between containers. Some reactants may undergo side reactions or react in unexpected ways, producing other substances instead of the desired product. Percentage yield compares actual yield with theoretical yield: percentage yield = (actual yield ÷ theoretical yield) × 100. For example, if theoretical yield is 20 g but only 15 g is obtained, percentage yield is (15 ÷ 20) × 100 = 75%. A strong answer earns marks for these points. State that atoms are conserved but the calculated theoretical yield may not be achieved. Explain that reversible reactions may not go to completion, leaving reactants unconverted. Describe how product can be lost during separation or transfer, reducing actual yield. Explain that side reactions or unexpected reactant behaviour can form other products. A common slip: Thinking atoms are lost when yield is low. Instead, that atoms are conserved but may form other products or remain unreacted. Exam tip: Link each reason for reduced yield to a specific practical situation, such as reversible reactions or filtration losses.
Key words
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Theoretical yield
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Actual yield
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Percentage yield
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Reversible reaction
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See every definition
- Theoretical yield
- The maximum calculated amount of product, assuming all reactants convert fully into the desired product.
- Actual yield
- The amount of product that is obtained in practice, which is often lower than the theoretical yield.
- Percentage yield
- A comparison of actual yield with theoretical yield, calculated as (actual yield ÷ theoretical yield) × 100.
- Reversible reaction
- A reaction that may not go to completion because it can reach equilibrium before all reactants convert, so some remain unreacted.
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