A chemical reaction is a process by which the bonds in the atoms of substances are rearranged to generate new substances. Matter cannot be created or destroyed in a chemical reaction—the same type and number of atoms that make up the reactants are still present in the products. Merely the rearrangement of chemical bonds produces new compounds.
A chemical reaction takes starting materials—the reactants—and changes them into different substances—the products. The identities of the elements are the same on both sides of the equation, but they are arranged in different substances after the reaction takes place. In chemical reactions, the bonds between the atoms are broken and reformed which means that the shared electrons among the atoms are rearranged. Reactions can be spontaneous, or they might only occur in the presence of an energy source—such as heat or light. Additionally, macromolecules can act as enzymes—catalysts that greatly speed up chemical reactions. Most biological reactions would take far too long to complete without enzymes.
Some types of reactions will proceed irreversibly until all of the reactants are used up, while others are reversible, with the products able to be converted back into the reactants if conditions change. Certain types of chemical reactions, such as combustion reactions or precipitation reactions that form a solid product from two dissolved substances, typically only proceed in one direction. An example of an irreversible reaction is the combustion of hydrocarbon fuel in the presence of atmospheric oxygen, which produces heat and light energy, carbon dioxide gas, and water. Other reactions occur in either direction until the reactants and products are at equilibrium—the point at which there is no net change in reactants or products.
In a chemical reaction, matter cannot be created or destroyed, a principle known as the Law of Conservation of Matter. However, the products that are formed often contain different ratios of atoms than the reactants. A balanced chemical equation accounts for all of the atoms on both sides of the equation by adding coefficients to the products and reactants until the total number of each type of atom is equal on both sides of the equation. A coefficient applies to all the atoms in a compound, much like a mathematical coefficient applies to all of the variables contained in a parenthetical statement. For example, the reaction that generates water from hydrogen and oxygen gases is:
H2 + O2 → H2O
In this unbalanced equation, there are two hydrogen atoms on each side, but there are unequal numbers of oxygen atoms. To balance the equation, coefficients are added so that there are equal numbers of hydrogen and oxygen atoms on both sides:
2H2 + O2 → 2H2O
In the balanced equation, there are a total of four hydrogen atoms and two oxygen atoms on each side of the equation.
Two important processes that power much of all life on Earth are photosynthesis, which converts sunlight into the six-carbon sugar glucose, and cellular respiration, which converts glucose into usable energy. Both these essential reactions are, at their core, a pair of complementary chemical reactions. Photosynthetic organisms use the energy from sunlight to convert carbon dioxide and water into sugar and molecular oxygen. Cellular respiration is then used by all aerobic organisms to break down that sugar—whether they made it or consumed it—in the presence of oxygen to produce energy for all of their basic needs.
