A chemical reaction is a fundamental process in which substances change their identity, resulting in the formation of entirely new materials. This transformation happens constantly, from living cells converting sugar into energy to the combustion of fuel in an engine. Understanding these changes requires identifying the substances that participate in the process. The starting compounds and elements that will be transformed are known as reactants.
Defining the Reactant
A reactant is the starting material in any chemical reaction. These substances are present at the beginning and are consumed as the reaction proceeds. Think of a chemical reaction like baking a cake: the flour, sugar, and eggs are the reactants, the initial ingredients transformed in the oven.
For a chemical change to occur, reactants must possess the necessary chemical activity. They often require specific conditions, such as a certain temperature or pressure, to initiate the process. When these conditions are met, reactant molecules collide with enough energy to start the atomic rearrangement. Once the reaction begins, the reactants are used up to create the final substances.
Reactants in Chemical Notation
Chemists represent these transformations using a standardized shorthand called a chemical equation. In this notation, the reactants are always written on the left-hand side, signifying they are the substances initiating the reaction.
When multiple reactants are involved, they are separated by a plus sign (+), indicating they are combined or reacted together. For example, the notation shows sodium (Na) plus chlorine gas (\(\text{Cl}_2\)) as the starting materials for table salt. Following the reactants, an arrow points toward the right, signifying the direction of the change.
A number known as a stoichiometric coefficient is placed directly in front of a reactant’s chemical formula. This coefficient indicates the relative quantity, or the specific number of molecules, required for the reaction. This value ensures the equation is balanced, reflecting the precise ratio of reactants needed to complete the chemical transformation.
The Transformation: Reactants to Products
The chemical reaction is a dynamic process where reactants transition into new chemical entities called products. The difference is that reactants are used up, while products are the new substances formed. This transformation involves the rearrangement of atoms within the reactant molecules.
For the rearrangement to happen, the chemical bonds holding the atoms together in the reactants must first be broken. Energy must be put into the system to break these existing bonds, which is an energy-absorbing process. The atoms then become temporarily separated, allowing them to form new partnerships.
New chemical bonds are formed as these atoms recombine in different ways to create the product molecules. The formation of these new bonds releases energy back into the system. For instance, when hydrogen gas (\(\text{H}_2\)) and oxygen gas (\(\text{O}_2\)) react to form water (\(\text{H}_2\text{O}\)), the bonds in the gas molecules break, and new bonds form between the atoms.
Despite the change in chemical identity, the total mass of the reactants equals the total mass of the products afterward. This principle, known as the law of conservation of mass, means that no atoms are created or destroyed during the transformation, only rearranged. The reactants are consumed, resulting in substances with entirely different properties, marking the completion of the chemical change.