A chemical reaction is a process where one set of chemical substances is transformed into a different set of substances. This transformation occurs through the rearrangement of atoms, involving the breaking of existing chemical bonds and the formation of new ones. This creates new molecules with properties distinct from the originals. Chemical reactions are the basis for nearly every process in the universe, from metabolism to the formation of stars. This process changes the chemical identity of the materials involved, distinguishing it from a physical change like melting or boiling.
The Core Components Reactants and Products
Every chemical reaction has two defining material components: the substances that start the process and the substances that are formed. The starting materials that enter the reaction are known as the reactants; they are the chemical “ingredients” for the change. For a reaction to begin, the bonds holding the atoms together in the reactant molecules must first be broken, which typically requires an input of energy.
The new substances created as a result of the chemical change are called the products. Products are formed when the atoms from the original reactants rearrange and establish new chemical bonds. The properties of the products are distinct from those of the reactants because the arrangement of the constituent atoms is different.
The Language of Chemistry Chemical Equations
Scientists use a standardized notation, the chemical equation, to represent the interaction between reactants and products symbolically. In this representation, the reactants are always written on the left side, and the products are written on the right side, separated by an arrow. Each substance is represented by its chemical formula, which uses elemental symbols and subscripts to show the number of atoms of each element present in a molecule, such as H2O for water.
The arrow in the middle is known as the reaction arrow (\(\to\)), signifying the transformation and often read as “yields” or “produces.” A double arrow (\(\rightleftharpoons\)) indicates a reversible reaction where products can re-form the original reactants, establishing a state of equilibrium. To ensure the equation obeys the Law of Conservation of Mass, large whole numbers called coefficients are placed in front of the chemical formulas. These coefficients specify the number of molecules or moles of each substance involved, ensuring the total number of atoms for every element is identical on both the reactant and product sides.
The physical state of each substance is also communicated using small symbols in parentheses next to the formula. These state symbols include \((s)\) for a solid, \((l)\) for a liquid, \((g)\) for a gas, and \((aq)\) for an aqueous solution, meaning the substance is dissolved in water. This detailed notation provides a complete picture of the chemical change, indicating not only what substances are involved but also their physical conditions and relative amounts.
Modifiers and Energy Flow
Beyond the material components, chemical reactions are affected by outside factors and involve an exchange of energy. A catalyst is a substance that influences the reaction rate without being consumed or permanently changed. Catalysts work by providing an alternate pathway, which lowers the activation energy—the minimum energy required to start the reaction. Catalysts are noted above the reaction arrow in an equation, showing their presence without including them as a reactant or product.
All reactions also involve an energy exchange, classifying them as either exothermic or endothermic. Exothermic reactions release energy, often in the form of heat, because the energy stored in the product bonds is less than the energy stored in the reactant bonds. Conversely, endothermic reactions absorb energy from the surroundings, meaning the products hold more energy than the reactants. Every reaction, regardless of its overall energy release or absorption, must first overcome the activation energy barrier for the initial bond breaking to occur.