Excess Reagent – IGCSE Chemistry Definition
IGCSE Chemistry definition of excess reagent: the reactant that remains after a reaction is complete because there is more of it than needed. Linked to limiting reagent.
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Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
The excess reagent is the reactant present in a greater amount than is needed for complete reaction. When a reaction finishes, the excess reagent is still present in the mixture because the other reactant (the limiting reagent) has been completely used up. The amount of product formed is determined by the limiting reagent, not the excess reagent.
How to identify the excess reagent
Compare the moles of each reactant with what the balanced equation requires. For example, in the reaction Mg + 2HCl → MgCl₂ + H₂, if you have 0.10 mol Mg and 0.30 mol HCl, the equation needs 2 mol HCl per mol Mg. For 0.10 mol Mg, you need 0.20 mol HCl, but you have 0.30 mol. HCl is in excess by 0.10 mol. Magnesium is the limiting reagent.
Practical importance
In salt preparation, excess reagent is deliberately used to ensure all of the acid reacts. For example, when making copper(II) sulfate from copper(II) oxide and sulfuric acid, excess copper(II) oxide (a black insoluble solid) is added. When the reaction is complete, the unreacted copper(II) oxide is removed by filtration. The use of excess ensures no acid remains in the final product.
Exam context
Paper 4 questions may give the masses of two reactants and ask candidates to identify which is in excess and calculate the mass of product formed. The key is to convert both masses to moles and compare them using the molar ratio from the balanced equation.
Worked exam question
3.0 g of magnesium is added to 100 cm³ of 1.0 mol/dm³ hydrochloric acid. Mg + 2HCl → MgCl₂ + H₂. Which reagent is in excess? (Ar: Mg = 24) (3 marks)
Moles of Mg = 3.0 / 24 = 0.125 mol [1]. Moles of HCl = 1.0 x 0.10 = 0.10 mol. The equation needs 2 mol HCl per mol Mg. For 0.125 mol Mg, 0.25 mol HCl is needed, but only 0.10 mol is available [1]. Magnesium is in excess (HCl is the limiting reagent) [1].
Common mistakes
Candidates often compare the raw masses or raw moles without considering the molar ratio. Having more moles does not automatically make a reagent the excess one --- you must check whether it has more than the equation requires. Another error is assuming the excess reagent determines the amount of product formed. It is always the limiting reagent that controls the yield.
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