Percentage Yield
Chemistry: AQA A Level, topic 1.2.6
Percentage Yield
The percentage yield for any reaction is usually less than 100%. The following 5 factors cause deviations from 100%.
Incomplete reactions
- An incomplete reaction can happen because:
- The reaction is very slow and hasn't been given enough time to happen.
- The reaction is reversible and some of the products have turned back into reactants.
- This reduces the percentage yield.
Waste
- Some of the chemicals will remain stuck to the glassware and will be wasted.
- This reduces the percentage yield.
Extraction
- It might be difficult to fully separate the product from the reaction mixture.
- This reduces the percentage yield.
Side reactions
- Unexpected reactions (side reactions) can happen. This may mean that the intended product is not created.
- This reduces the percentage yield.
Hydration
- If a reaction is carried out in water, any extracted product may be 'wet'
- This means that there is water in the extracted product.
- Water is generally extremely difficult to fully remove from a product.
- If a percentage yield is calculated to be over 100%, it is likely that your product is full of water.
Calculating Percentage Yield
The percentage yield of a reaction is calculated as the real yield divided by the theoretical yield multiplied by 100.
Real yield
- The real yield is the mass of the desired product obtained from a reaction.
Theoretical yield
- The theoretical yield is the maximum mass of a product that could possibly be created from a reaction.
- This can be calculated from a balanced equation if we know the mass of the reactants.
Percentage yield
- A comparison of the real and theoretical yields of a reaction.
Test yourself on Percentage Yield
To understand percentage yield, it can be useful to imagine you are baking a cake...
What must be true of a reaction for it to reach a percentage yield of 100%?
What features of a reaction make it more likely to be incomplete?
What 2 figures must we know before we can calculate the percentage yield of a reaction?
If we divide the real yield by the theoretical yield and multiply by 100, what figure we get?