An excess reactant is the substance in a chemical reaction that is not completely used up when the reaction takes place. It is present in a larger amount than needed to react with the other substance.
Key points:
- In any reaction, one reactant is used up first, called the limiting reactant. This is the substance that decides how much product can be formed.
- The other reactant, which remains after the reaction is complete, is called the excess reactant.
- The excess reactant is left over because there is more of it than required to combine with the limiting reactant.
- Identifying the excess reactant is important in laboratory and industrial processes so that resources are not wasted.
Example in daily life: If you are making sandwiches and you have ten slices of bread and only three slices of cheese, the cheese is the limiting ingredient (limiting reactant) because it will run out first. Bread is the excess ingredient (excess reactant) because some of it will be left over.