GCSE Chemistry (AQA)

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Percentage Yield

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Quantitative chemistry Yield and atom economy of chemical reactions

Percentage Yield

5:33 Efficiency of Chemical Reactions
Spec 4.3.3.1
Previous Factors Affecting Concentration Next Atom Economy
  • No atoms are gained or lost in a chemical reaction, so mass is always conserved, known as the law of conservation of mass.
  • The total mass of reactants equals the total mass of products in a reaction.
  • Balancing equations is important to track all atoms and mass involved.
  • In actual reactions, less of the desired product is often made than expected due to various reasons:
  • Reactants may react in unexpected ways, such as with oxygen or decomposing.
  • Reversible reactions may not go to completion, stopping at an equilibrium position.
  • Some product may be lost during separation from the reaction mixture.
  • Product can be lost by spillages if not careful during transfer.
  • The amount of product obtained is known as the yield.
  • Percentage yield is the actual yield compared to the maximum theoretical yield, expressed as a percentage.
  • Percentage yield is calculated using the formula: (mass of product actually made / maximum theoretical mass of product) x 100.
  • Example: If theoretical mass is 2.0 grams and actual mass is 1.4 grams, percentage yield is 70%.
  • For exams, be confident in calculating percentage yields using the formula.
  • Example problem: Magnesium reacted with hydrochloric acid to form magnesium chloride, with a theoretical yield of 2.0 grams and actual yield of 1.6 grams, resulting in an 80% yield.
  • Common mistake: Substituting masses incorrectly in the equation, leading to a yield above 100%, which is impossible.

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