An excess reactant is the substance in a chemical reaction that is not completely used up when the reaction finishes. After the limiting reactant is consumed, the reaction stops, but some amount of the excess reactant remains unreacted.
In other words:
- The limiting reactant controls how much product can form.
- The excess reactant is left over because there is more than enough of it compared to the amount required by the balanced chemical equation.
Example:
Reaction: 2H₂ + O₂ → 2H₂O
Suppose we have 5 moles of H₂ and 2 moles of O₂.
- From the ratio (2:1), 5 moles of H₂ would need 2.5 moles of O₂.
- But only 2 moles of O₂ are available, so O₂ is the limiting reactant.
- Hydrogen (H₂) is present in excess, and 0.5 moles of H₂ will remain unreacted.