Explain the term 'excess reactant'.

An excess reactant is the substance that remains after all the other reactants have been used up in a chemical reaction.

In a chemical reaction, the reactants are the substances that interact with each other to produce a new substance or substances, known as the products. The amount of product that can be formed is determined by the amount of reactants available. If one reactant is used up before the others, it is known as the limiting reactant, because its quantity limits the amount of product that can be formed. The other reactants, which are present in quantities greater than needed to react with the limiting reactant, are known as excess reactants.

The concept of excess reactants is important in practical applications of chemistry, such as in industrial processes. For example, in the manufacture of ammonia by the Haber process, nitrogen and hydrogen are reacted together. To ensure that all the nitrogen is used up and none is wasted, an excess of hydrogen is used. This ensures that the reaction goes to completion, with all the nitrogen being converted into ammonia.

In the laboratory, chemists often use an excess of one reactant to ensure that the reaction goes to completion. This can make it easier to calculate the yield of the reaction, as it is known that all of the limiting reactant has been used up. However, it also means that the excess reactant needs to be separated from the product, which can add to the complexity of the process.

In summary, an excess reactant is a substance that is present in a greater quantity than is needed to react with the limiting reactant in a chemical reaction. It is left over after the reaction has gone to completion. Understanding the concept of excess reactants is crucial for both theoretical and practical aspects of chemistry.