All chemical reactions begin with a reactant, the general term for one or more substances entering the reaction. Sodium and chloride ions, for example, are the reactants in the production of table salt. One or more substances produced by a chemical reaction are called the product. Chemical reactions follow the law of conservation of mass, which means that matter cannot be created nor destroyed in a chemical reaction. The components of the reactants—the number of atoms and the elements—are all present in the product(s). Similarly, there is nothing in the products that are not present in the reactants.
Chemical reactions require sufficient energy that causes the matter to collide with enough precision and force to break old chemical bonds and form new ones. In general, kinetic energy is the form of energy powering any type of matter in motion. Potential energy is the energy of position or the energy matter possesses because of the positioning or structure of its components. All atoms have kinetic energy as they are always in motion. The energy needed to break the chemical bonds of the reactants and start a reaction is called the activation energy. The concentration and temperature of the reactants can influence the rate of a chemical reaction.
The convention for writing chemical equations involves placing reactant formulas on the left side of a reaction arrow and product formulas on the right side. By this convention and the definitions of "reactant" and "product," a chemical equation represents the reaction proceeding from left to right. Reversible reactions, however, may proceed in both forward (left to right) and reverse (right to left) directions. When the rates of the forward and reverse reactions are equal, the concentrations of the reactant and product species remain constant over time, and the system is at equilibrium. The relative concentrations of reactants and products in equilibrium systems vary greatly; some systems contain mostly products at equilibrium, some contain mostly reactants, and some contain appreciable amounts of both.
For example, in human blood, excess hydrogen ions (H+) bind to bicarbonate ions (HCO3–), forming an equilibrium state with carbonic acid (H2CO3). If we added carbonic acid to this system, some of it would convert to bicarbonate and hydrogen ions.
However, biological reactions rarely obtain equilibrium because the concentrations of the reactants or products or both are constantly changing, often with one reaction's product a reactant for another. To return to the example of excess hydrogen ions in the blood, forming carbonic acid will be the reaction's major direction. However, the carbonic acid can also leave the body as carbon dioxide gas (via exhalation) instead of converting back to bicarbonate ion; this drives the reaction to the right by the law of mass action. These reactions are important for maintaining homeostasis in our blood.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 2.3: Chemical Reactions, Openstax, Chemistry 2e, Section 13.1: Chemical equilibria and Openstax, Biology 2e, Section 2.1: Atoms, Isotopes, Ions and Molecules: The Building Blocks
