The study of chemistry is the study of matter and the changes it undergoes. These changes, known as chemical reactions, involve substances transforming into new substances with different properties. To understand this process, it is helpful to identify the materials that begin the change, known as reactants. A reactant is the starting material that initiates a chemical transformation.
The Core Definition of a Reactant
A reactant is any substance consumed during a chemical reaction to make new compounds. For a transformation to occur, the particles of the reactants must first collide with sufficient energy and correct orientation. This energy input is necessary to break the existing chemical bonds that hold the atoms together within the reactant molecules.
As these original bonds break, the atoms are temporarily separated or rearranged into an unstable transition state. The atoms then recombine to form new arrangements. This rearrangement and the formation of new bonds result in the creation of a new substance.
The identity of the reactant is lost because its atoms are incorporated into a new molecular structure. For example, in the formation of water, hydrogen gas and oxygen gas are the reactants. A reaction cannot proceed unless the necessary reactants are present to supply the atoms and the potential energy locked within their bonds.
Reactants Versus Products: The Transformation
The relationship between a reactant and a product defines the direction and outcome of any chemical change. Reactants are the “before” state, and products are the “after” state, representing the chemical result. The process involves the reactants giving way to the products through a rearrangement of atoms.
This transformation requires energy to facilitate the change from starting materials to resulting substances. Energy is required to break the bonds within reactant molecules (an endothermic process). Conversely, energy is released when new bonds are formed to create product molecules (an exothermic process).
The overall energy flow of the reaction depends on the difference between the energy absorbed to break reactant bonds and the energy released by forming product bonds. If more energy is absorbed than released, the reaction requires a continuous energy input from the surroundings. If more energy is released, the reaction gives off heat or light. The properties of the products, such as color, state of matter, and reactivity, are distinct from the reactants.
Representing Reactants in a Chemical Equation
The symbolic language of chemistry uses a chemical equation to represent the transformation of reactants into products. In this notation, the reactants are always written on the left side of the equation. This placement visually signifies that they are the initial substances entering the reaction system.
When a reaction involves more than one starting material, a plus sign (+) is placed between the chemical formulas of the individual reactants. For instance, in the burning of methane, the equation shows methane and oxygen separated by a plus sign. This sign indicates they must be combined to react, meaning “reacts with.”
The reactants are separated from the products by a yield sign, typically a single arrow (\(\rightarrow\)). This arrow indicates the direction of the chemical change, pointing from the starting materials to the resulting materials. Numbers placed in front of the reactant formulas, known as stoichiometric coefficients, specify the relative number of molecules needed for the reaction to be balanced.