A chemical reaction is a process where the atoms of one or more substances are rearranged to form entirely new substances with different properties. This transformation is represented by a chemical equation, which details the process. The substances that you start with, the “ingredients” that undergo this change, are known as the reactants. Reactants are the initial components that transform into something else as bonds are broken and new bonds are formed.
Reactants: The Starting Materials
Reactants are the chemical substances present at the beginning of any chemical reaction, and they are defined by the fact that they are consumed during the process itself. In a standard chemical equation, these materials are always written on the left side of the reaction arrow, which signifies the direction of the change. For example, in the formation of water, hydrogen gas and oxygen gas are the reactants that combine to form the final molecule. These starting substances can be simple elements, such as pure iron, or complex compounds, like glucose in the process of cellular respiration.
The defining characteristic of a reactant is that its chemical identity is changed as the reaction proceeds. The bonds holding the atoms together in the reactant molecules are broken, allowing the constituent atoms to recombine in a new configuration. In the laboratory or in biological systems, the amount of each reactant available influences the final outcome.
When chemists consider a reaction, they often identify a “limiting reactant,” which is the substance that will be completely used up first, thereby stopping the reaction and determining the maximum amount of product that can be formed. Any reactant present in an amount greater than what is needed to react with the limiting reactant is referred to as an “excess reactant.” Identifying limiting and excess reactants helps maximize the desired reaction yield.
Products: The End Result
The substances that result from the transformation of the reactants are called the products. These new substances are found on the right side of the reaction arrow in a chemical equation, signifying that they are the outcome of the process. The products possess chemical and physical properties that are distinct from the original starting materials. For instance, in a combustion reaction, the reactants are typically a fuel and oxygen, but the products are entirely different, often consisting of carbon dioxide and water vapor.
A core principle governing this transformation is the Law of Conservation of Mass, which dictates that atoms are neither created nor destroyed in a chemical reaction. Instead, the atoms present in the reactants are merely rearranged to form the product molecules. This means the total number of atoms of each element remains constant throughout the process.
Catalysts and Inhibitors: Modifiers of the Reaction
While reactants are the materials that are transformed, other substances can significantly influence how quickly and easily a reaction takes place without being consumed themselves. A catalyst is one such substance, which works by providing an alternate reaction pathway that requires less energy to start the reaction. This threshold, known as the activation energy, must be overcome for the reaction to proceed, and a catalyst effectively lowers this barrier, thereby speeding up the rate of product formation.
Catalysts are not considered reactants because they are recovered chemically unchanged at the end of the process, meaning they can be used over and over again. In biological systems, protein molecules called enzymes function as highly selective catalysts. For example, digestive enzymes accelerate the chemical breakdown of food molecules in the body by many orders of magnitude, making reactions that would otherwise take days occur in seconds.
Conversely, a substance known as an inhibitor works to decrease the rate of a chemical reaction, or in some cases, stop it entirely. Inhibitors often function by interfering with a catalyst, such as an enzyme, binding to it and blocking its ability to lower the activation energy. Understanding these modifiers is important in many fields, from industrial chemistry, where catalysts are used to make manufacturing processes more efficient, to medicine, where many drugs function as inhibitors to slow down unwanted biological reactions.