Reaction yields (AQA GCSE Chemistry): Revision Notes
Reaction yields
What is a yield?
When we carry out a chemical reaction, the yield is simply the amount of product we get at the end. Think of it like baking - if you follow a recipe for 12 biscuits, the yield would be how many biscuits you actually end up with.
This baking analogy is useful because it shows that even when following a recipe (chemical equation) exactly, you might not always get the expected amount due to various factors during the process.
Maximum theoretical yield
Every chemical reaction has a maximum theoretical yield. This is the biggest amount of product you could possibly make from your starting materials (called reactants).
The theoretical yield is based on two important rules about atom conservation:
Fundamental Rules of Chemical Reactions:
- No atoms are gained in a reaction
- No atoms are lost in a reaction
These rules are the foundation for all yield calculations and explain why we can predict maximum possible yields.
This means we can work out the maximum yield by using the balanced equation for the reaction and calculating with the masses involved.
For example, if you start with 10g of reactants, there's only a certain maximum amount of product you can possibly make from those 10g.
Actual yield
In real life, the actual yield is what you actually get when you do the experiment. The actual yield is nearly always less than the theoretical yield.
Why don't we get the theoretical yield?
There are several reasons why reactions don't give us the maximum amount:
Common Reasons for Lower Actual Yields:
- The reaction might be reversible - some of the product changes back into reactants
- The reaction might not finish completely - some reactants are left unreacted
- Side reactions happen - the reactants make other unwanted products instead
- We lose some product when separating it from the reaction mixture
Understanding these factors helps chemists optimise their reactions and improve yields.
Percentage yield
We can compare the actual yield to the theoretical yield using percentage yield.
Formula:
What the percentages mean:
Interpreting Percentage Yields:
- 0% = No expected product was made
- 100% = All the expected product was made (perfect result!)
- Less than 100% = Some expected product was missing (most common scenario)
Industrial processes typically aim for yields above 70-80% to be economically viable.
Worked example
Worked Example: Copper Carbonate Decomposition
Let's look at this reaction: When copper carbonate is heated, it breaks down to make copper oxide and carbon dioxide.
Given information:
- Theoretical yield: 1.0g
- Actual yield: 0.8g
Calculation:
Result: This means we got 80% of what we expected to get.
Practice calculation
Practice Example: Making Magnesium Oxide
For the reaction:
Given data:
- Expected yield: 1.2g
- Actual yield: 0.9g
Solution:
This represents a good yield for this type of reaction.
Key Points to Remember:
- Yield is the amount of product you get from a reaction
- Theoretical yield is the maximum possible amount (calculated from balanced equations)
- Actual yield is what you really get (usually less than theoretical)
- Percentage yield = (Actual ÷ Theoretical) × 100
- Reactions rarely give 100% yield due to incomplete reactions, reversible reactions, side reactions, and product loss