Chemical reaction

A chemical reaction is the process by which one set of chemical substances is transformed into another. This transformation involves the rearrangement of atoms, and it always comes with an energy change as new products form. Traditionally, these reactions only affect the positions of electrons—breaking and forming chemical bonds between atoms—without altering the atomic nuclei, meaning the elements themselves stay the same. Such reactions are often represented by a chemical equation.

Chemical equilibrium

The starting substances are called reactants or reagents, and they produce one or more products, which typically have different properties than the reactants did. Chemical reactions often happen through a series of smaller steps known as elementary reactions; the detailed path of these steps is called the reaction mechanism. They are described by chemical equations that show the starting materials, final products, and sometimes intermediate products and reaction conditions. Each reaction proceeds at a characteristic rate, which depends on temperature and chemical concentration.

Thermodynamics

Some reactions release heat and are called exothermic, while others need heat to occur and are called endothermic. Generally, raising the temperature increases the reaction rate because more thermal energy is available to reach the activation energy needed to break bonds. Reactions can be classified as redox (involving oxidation and reduction) or non-redox (without oxidation or reduction).

Most simple redox reactions fall into categories like combination, decomposition, or single displacement. In chemical synthesis, various reactions are used to obtain a desired product. In biochemistry, a linked series of reactions—where the product of one becomes the reactant for the next—forms metabolic pathways. These are often sped up by protein enzymes, which allow reactions that would be impossible under normal cellular conditions to occur at the temperature and concentrations found inside a cell.

History

The concept of a chemical reaction has also been extended to processes involving entities smaller than atoms, such as nuclear reactions, radioactive decays, and reactions between elementary particles, as described by quantum field theory. People have known about chemical reactions since ancient times, including combustion in fire, fermentation, and reducing ores to metals. Early theories of material transformation came from Greek philosophers, like Empedocles’ Four-Element Theory, which held that everything is made of fire, water, air, and earth. During the Middle Ages, alchemists studied chemical transformations, especially trying to turn lead into gold using reactions of lead and lead-copper alloys with sulfur.

A major goal for medieval alchemists was the artificial production of chemical substances. For example, works attributed to Jābir ibn Ḥayyān (around 850–950) describe synthesizing ammonium chloride from organic substances, and later alchemists, starting around 1300, produced mineral acids like sulfuric and nitric acids by heating sulfate and nitrate minerals such as copper sulfate, alum, and saltpeter. In the 17th century, Johann Rudolph Glauber made hydrochloric acid and sodium sulfate by reacting sulfuric acid with sodium chloride. The lead chamber process (1746) and the Leblanc process allowed large-scale production of sulfuric acid and sodium carbonate, bringing chemical reactions into industry.

Further improvements led to the contact process in the 1880s for sulfuric acid and the Haber process (1909–1910) for ammonia synthesis. From the 16th century, researchers like Jan Baptist van Helmont, Robert Boyle, and Isaac Newton tried to create theories for observed chemical transformations. In 1667, Johann Joachim Becher proposed the phlogiston theory, which claimed that a fire-like element called “phlogiston” was inside combustible bodies and released during burning.

This was proven wrong in 1785 by Antoine Lavoisier, who correctly explained combustion as a reaction with oxygen from the air. In 1808, Joseph Louis Gay-Lussac recognized that gases always react in fixed relationships with each other. Building on this and John Dalton’s atomic theory, Joseph Proust developed the law of definite proportions, which led to the concepts of stoichiometry and chemical equations. In organic chemistry, it was long believed that compounds from living organisms were too complex to make synthetically.

This idea, called vitalism, held that organic matter had a “vital force” that separated it from inorganic materials. That separation ended in 1828 when Friedrich Wöhler synthesized urea from inorganic precursors. Other major contributors to organic chemistry include Alexander William Williamson, who synthesized ethers, and Christopher Kelk Ingold, who, among many discoveries, established the mechanisms of substitution reactions. Some chemical reactions have specific characteristics, such as...