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3.3.1a Percentage yield

Real reactions rarely give all the product they could

Definition

Product

A product is a substance formed during a chemical reaction.

  1. The yield is the mass of product you actually collect from a reaction.
  2. The yield is almost always less than the maximum the equation predicts, even when the method is careful.
Key Idea

Percentage yield compares what you actually got with the most you could ever get, so it can never be more than 100%.

Three reasons a yield falls short of the maximum

Definition

Reversible reaction

A reaction in which the products can react to form the original reactants.

  1. The reaction may not finish, which is common in a reversible reaction that reaches a balance before all reactants are used.
  2. Some product is lost when it is separated, for example left behind on filter paper or spilled during transfer.
  3. Some reactants take part in unwanted side reactions, making different products instead of the one you want.
Example

When a salt is made by filtering and drying crystals, a little always stays stuck to the filter paper, so the yield drops below the maximum.

Percentage yield compares actual mass with the maximum

Definition

Percentage yield

The mass of product actually made, written as a percentage of the maximum theoretical mass that could form.

  1. The formula is percentage yield =mass of product mademaximum theoretical mass×100= \dfrac{\text{mass of product made}}{\text{maximum theoretical mass}} \times 100=maximum theoretical massmass of product made​×100.
  2. Both masses must be in the same unit so that the ratio is correct.
Exam technique
  • A percentage yield above 100% means a mistake, usually a product that was still damp when weighed.
  • Put the actual mass on top and the theoretical mass on the bottom, as swapping them is the most common error.
Example
  • A reaction could make a maximum of 8 g8\ \text{g}8 g of product, but only 6 g6\ \text{g}6 g is collected.
    • The percentage yield is 68×100=75%\dfrac{6}{8} \times 100 = 75\%86​×100=75%.
Self review
  • Give three reasons a percentage yield is less than 100%.
  • Write down the formula for percentage yield.
  • A reaction makes 9 g9\ \text{g}9 g out of a possible 12 g12\ \text{g}12 g; what is the percentage yield?
  • Why would a damp product give a percentage yield above 100%?
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A product is a substance formed during a chemical reaction. The yield is the mass of product actually collected, but real reactions rarely produce the maximum mass predicted by the balanced equation.

The theoretical mass is the maximum mass that could form if all of the limiting reactant reacted and no product was lost. Percentage yield compares the actual mass with this theoretical maximum.

percentage yield=mass of product mademaximum theoretical mass×100 \text{percentage yield} = \frac{\text{mass of product made}}{\text{maximum theoretical mass}} \times 100 percentage yield=maximum theoretical massmass of product made​×100

Both masses must be in the same unit.

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What is the yield of a reaction?

3.3.1a Percentage yield Revision Guide

  1. GCSE
  2. /Chemistry
  3. /3.3.1a Percentage yield

Revision notes for AQA GCSE Chemistry 3.3.1a Percentage yield: explanations and worked examples.

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