Chemical Reaction

A chemical reaction is a process in which one or more substances are transformed into new substances with different properties. In this process, atoms are neither destroyed nor created; only the chemical bonds holding atoms together are altered. The starting substances are called reactants, and the substances produced are called products.

Reactions are written in chemical notation, for example 2H₂ + O₂ → 2H₂O. The equation must be balanced: the number of atoms of each type on the left side must equal the number on the right side. This follows from the law of conservation of mass, which states that the total mass of a closed system does not change during a reaction.

Types of Reactions

There are common categories based on how atoms are rearranged. Synthesis combines two substances into one. Decomposition breaks one substance into two or more. Single displacement occurs when one element displaces another from a compound. Double displacement involves the exchange of ions between two compounds. Combustion occurs when a substance reacts with oxygen, releasing heat and light. Also important are redox reactions, based on the transfer of electrons, and acid-base reactions, based on the transfer of protons.

Energy and Equilibrium

Every reaction involves an energy change. Reactions that release heat are called exothermic, while those that absorb heat are endothermic. For a reaction to begin, activation energy is usually required to reach a transition state. Many reactions do not go to completion but reach equilibrium, a state in which the rates of the forward and reverse reactions are equal, so the concentrations of substances stop changing.

Reaction Rate and Factors

The study of reaction rates (kinetics) examines how quickly substances transform. Higher temperature generally increases the rate because atoms and molecules collide more vigorously. Higher concentration, increased surface area of reacting substances, and a catalyst—a substance that speeds up a reaction without being consumed—all affect the rate. Enzymes in living organisms are natural catalysts that make life processes possible.

Importance

Chemical reactions are the basis of industry, including the production of fertilizers, medicines, and petroleum products. They are also fundamental to life processes, such as respiration and photosynthesis. Understanding reactions allows scientists to predict the products and energy required.