1.1.9Chemical Substances, Reactions and Essential Resources
Percentage Yield
5 min read 6 parts Lesson 10 of 46 1 quiz
Section 1 of 6Types of yield
Theoretical yield
- Theoretical yield The maximum mass of product that could be made from the reactants, worked out from the balanced equation and the masses used.
- It assumes the reaction goes to completion and nothing is lost.
Actual yield
- Actual yield The mass of product you really collect when you carry out the reaction and weigh it.
- It is nearly always lower than the theoretical yield.
Percentage yield
- Percentage yield Compares the two: percentage yield = (actual yield ÷ theoretical yield) × 100.
- The answer cannot be above 100%. If it is, the fraction has been inverted or the product was impure or wet.
Section 2 of 6Why the yield is below 100%
Reasons for lost product
Reversible reactions
- Reversible reactions Products turn back into reactants, so the reaction never fully completes.
Side reactions
- Side reactions Some reactants react with gases in the air or with impurities and form unwanted substances.
Losses in handling
- Losses in handling Product is left on filter paper, stuck to glassware or spilt when transferring between containers.
Losses in separation
- Losses in separation Filtration, washing, distillation and crystallisation each lose a little of the product.
Unreacted material
- Unreacted material Some reactants stay behind in the apparatus without reacting.
Section 3 of 6Calculation method
1
Identify the actual yield, the mass collected in the experiment.
2
Identify the theoretical yield, given in the question or calculated from the equation.
3
Make sure both are in the same units, such as grams.
4
Divide actual by theoretical and multiply by 100.
5
Give the answer as a percentage and check it is between 0 and 100.
Section 4 of 6Worked example and importance
| Item | Detail |
|---|---|
| Question | A student makes 1.6 g of dry copper(II) sulfate crystals from copper(II) oxide and sulfuric acid. The theoretical yield is 2.0 g. |
| Working | (1.6 ÷ 2.0) × 100 |
| Answer | 80% |
| Why industry cares | A high percentage yield means less waste and lower cost, so more profit from the same raw materials |
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Yield | The amount of product obtained from a reaction. |
| Actual yield | The mass of product actually collected in an experiment. |
| Theoretical yield | The maximum mass of product predicted from the balanced equation. |
| Percentage yield | The actual yield as a percentage of the theoretical yield. |
| Reversible reaction | A reaction in which products can change back into reactants. |
Section 6 of 6Exam technique
1
Dividing theoretical yield by actual yield, which gives a value above 100%.
2
Claiming that the missing product 'disappears' instead of naming losses such as transfers, side reactions or incomplete reactions.
3
Mixing units, for example actual yield in milligrams and theoretical yield in grams.
4
Forgetting to multiply by 100 so that the answer is not a percentage.