A substance that is used up during a chemical reaction is called a reactant. Chemical reactions are processes of transformation where starting materials change into entirely new substances. Reactants undergo a chemical change when they interact. In a chemical equation, reactants are traditionally shown on the left side of an arrow pointing toward the resulting products.
The Role of Reactants in Chemical Change
The transformation of reactants occurs through the rearrangement of atoms at the molecular level. For a reaction to proceed, the chemical bonds holding the atoms together in the reactant molecules must be broken, which is typically an energy-requiring process. Once these bonds are broken, the atoms rearrange themselves to form new, stable chemical bonds, creating the product molecules. This process explains why a reactant is consumed, as its original molecular structure is altered. Reactants must be present in specific proportions for the reaction to proceed as written in a balanced chemical equation, a concept known as stoichiometry.
If the initial amounts of reactants are not in this exact ratio, one substance will be entirely used up before the others, known as the limiting reactant. The limiting reactant determines the maximum amount of product that can be formed because once it is consumed, the reaction stops. Reactants that are not fully consumed are referred to as excess reactants. Understanding the limiting substance is crucial for chemists to predict the yield and efficiency of a reaction.
Distinguishing Reactants from Products
The difference between a reactant and a product lies in their role during the chemical change. Reactants are the substances present at the start of the process, while products are the new substances formed as a result of the transformation. For example, hydrogen gas and oxygen gas are reactants, and the resulting water is the product. The arrow in a chemical equation visually separates the reactants on the left from the products on the right.
Although the substances change, the total amount of matter remains constant, a principle known as the Law of Conservation of Mass. This means that atoms are neither created nor destroyed during the reaction; they are simply rearranged. The total mass of all reactants must equal the total mass of all products. This conservation rule requires chemical equations to be balanced, ensuring the same number of each type of atom appears on both sides of the reaction arrow.
Substances That Are Not Consumed
Not every substance present during a chemical process is a reactant that gets used up. The most notable example of a non-consumed participant is a catalyst. A catalyst increases the rate of a chemical reaction without undergoing a permanent chemical change. It works by providing an alternate reaction pathway that requires less activation energy.
Because the catalyst is not chemically altered, it can be recovered unchanged at the end of the reaction. Similarly, solvents, which are often used as the medium for reactions, are also not chemically consumed. These non-consumed substances help define reactants as those substances whose chemical identity is irreversibly lost during the transformation.