GCSE Combined Science (AQA)

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Limiting Reactants

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Quantitative chemistry Use of amount of substance in relation to masses of pure substances

Limiting Reactants

5:39 Limiting Reactant
Spec 4.3.2.4
Previous Reacting Mass Calculations Next Limiting Reactants Calculations
  • Reactions often involve two or more reactants.
  • Symbol equation represents a simple combustion reaction.
  • Carbon reacts with oxygen to produce carbon dioxide.
  • Excess of one reactant ensures the other is used up.
  • Amount of carbon dioxide depends on the amount of each reactant.
  • Oxygen gas is naturally found in the air.
  • Each mole of carbon reacts with one mole of oxygen to produce one mole of carbon dioxide.
  • Carbon is eventually used up, leaving oxygen in excess.
  • Limiting reactant is the reactant completely used up, limiting product formation.
  • Carbon is the limiting reactant in the combustion reaction.
  • To find the limiting reactant, compare the number of moles of reactants to the balanced equation ratios.
  • Example: Magnesium reacts with sulfuric acid to produce salt and hydrogen gas.
  • Balanced equation shows a 1:1:1 mole ratio.
  • If there are 5 moles of magnesium and 3 moles of sulfuric acid, sulfuric acid is the limiting reactant.
  • Sulfuric acid limits the amount of product made.
  • Magnesium is in excess, with some left over after the reaction.
  • In exams, identifying limiting reactants is necessary.
  • Example: N2 + 3H2 reacts to form 2NH3.
  • Given 0.50 moles of nitrogen and 1.75 moles of hydrogen.
  • Step 1: Identify the ratio between reactants (1:3).
  • Step 2: Pick the easiest reactant to work with (nitrogen).
  • Step 3: Calculate moles of the other reactant needed (0.5 moles of nitrogen needs 1.5 moles of hydrogen).
  • Step 4: Identify the limiting reactant (nitrogen is the limiting reactant, hydrogen is in excess).

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