Limiting reactants

• Reactions don't go on forever-you need stuff in the reaction flask that can react
• If one reactant gets completely used up in a reaction before the rest, then the reaction will stop. That reactant's called the limiting reactant
• The reaction stops when all of one of the reactants is used up. Any other reactants are in excess
• Limiting reactant


  The reactant that's used up in a reaction (because it limits the amount of product that's formed)
• The amount of product formed is directly proportional to the amount of limiting reactant
• If you halve the amount of limiting reactant the amount of product formed will also halve. If you double the amount of limiting reactant the amount of product will double (as long as it is still the limiting reactant)
• If you add more reactant there will be more reactant particles to take part in the reaction, which means more product particles
• Calculating the mass of a product formed in a reaction


  1. Write out the balanced equation

  2. Work out relative formula masses (M) of the reactant and product

  3. Find out how many moles there are of the substance you know the mass of

  4. Use the balanced equation to work out how many moles there'll be of the other substance

  5. Use the number of moles to calculate the mass