A chemical reaction is a process in which one or more substances undergo changes that produce one or more different substances with different chemical compositions and properties. The substances present before the transformation are called reactants, while the substances formed are called products. During a chemical reaction, atoms are rearranged through the breaking, formation or reorganisation of chemical bonds.
The nuclei of ordinary atoms are generally not changed in a chemical reaction, so the identities of the chemical elements are conserved even though their combinations and bonding arrangements change. Chemical reactions can occur in gases, liquids, solids, solutions, plasmas and biological systems. Examples include combustion, acid base neutralisation, precipitation, oxidation reduction, polymerisation and many biochemical transformations. A chemical reaction is fundamentally different from a physical change because a physical change alters a substance's state, shape or arrangement without changing its chemical composition, whereas a chemical reaction changes the molecular or ionic identities present.
At the microscopic level, a chemical reaction occurs because atoms or molecules interact and rearrange into configurations that are chemically different from the initial substances. Existing bonds may need to be broken and new bonds formed. The detailed pathway is called the reaction mechanism. A reaction mechanism can involve several elementary steps, transient intermediates and transition states rather than one simple simultaneous rearrangement.
The activation energy represents an energy barrier that reacting particles must overcome to proceed along a reaction pathway. Heat, light, electrical energy, catalysts or other forms of energy can influence whether sufficient particles can react. A catalyst provides an alternative pathway with a lower activation energy without being consumed overall. The catalyst does not change the overall identity of reactants and products or the thermodynamic equilibrium position by itself, although it can greatly accelerate the rate at which equilibrium is reached.
Chemical reactions can be classified in several complementary ways. Combination reactions produce a more complex substance from simpler reactants, while decomposition reactions break a compound into simpler substances. Substitution and displacement reactions involve replacement of one atom, ion or molecular group by another. Acid base reactions involve proton transfer or, in broader definitions, related electron pair processes. Precipitation reactions produce an insoluble or sparingly soluble solid from substances in solution.
Combustion reactions commonly involve rapid reaction with an oxidising substance, often oxygen, and frequently release substantial heat. Redox reactions involve electron transfer or changes in oxidation state. These categories overlap because a single reaction can simultaneously be described in several ways. Classification is therefore a way of highlighting a particular chemical feature rather than assigning every reaction to one exclusive category.
Chemical reactions can be represented using chemical equations. A chemical equation identifies reactants and products using chemical formulae and uses coefficients to indicate relative amounts. The equation must satisfy conservation of atoms for every chemical element. For example, the reaction between hydrogen and oxygen to form water can be represented as 2H2 plus O2 produces 2H2O when appropriate equation notation is used.
The coefficients show that two molecules or two moles of hydrogen react with one molecule or one mole of oxygen to produce two molecules or two moles of water. A chemical equation does not normally show every microscopic step of the reaction mechanism. It gives the overall stoichiometric relationship. Additional symbols can indicate physical states, catalysts, temperature, pressure or other conditions. Ionic equations can be used when reactions occur in aqueous solution and it is useful to show the ions directly involved in the transformation.
Energy changes accompany chemical reactions because chemical bonding and molecular arrangements change. Breaking bonds generally requires energy, while forming bonds generally releases energy. The overall energy change depends on the balance of these contributions and on the environment. Exothermic reactions release net energy to the surroundings, often as heat, while endothermic reactions absorb net energy. Thermodynamics describes whether a process is energetically favourable under specified conditions, while kinetics describes how quickly it occurs.
A reaction can be thermodynamically favourable but proceed extremely slowly if it has a large activation barrier. Conversely, a catalyst can accelerate a reaction without changing its overall thermodynamic driving force. Temperature, concentration, pressure, surface area and catalysts can influence reaction rates. These distinctions are essential because spontaneity and speed are different chemical concepts.
Chemical reactions occur throughout living systems. Cellular respiration converts energy stored in chemical substances into forms that cells can use, while photosynthesis uses light energy to drive the formation of energy rich organic molecules. Enzymes act as biological catalysts and provide highly selective reaction pathways. Digestion involves hydrolysis reactions that break large molecules into smaller components, while metabolism consists of interconnected networks of reactions that build and degrade biological compounds.
DNA replication, transcription, translation and cellular signalling all depend on carefully regulated chemical transformations. The same fundamental principles of atom conservation, energy transfer, kinetics and molecular interaction apply to biochemical reactions as to laboratory reactions. Biological systems, however, use complex catalysts, compartments and regulatory mechanisms to control reaction pathways with extraordinary precision.
Chemical reactions are also central to industrial technology and natural processes. Combustion powers engines and furnaces, electrochemical reactions enable batteries and fuel cells, polymerisation produces plastics and synthetic materials, and catalytic reactions are used to manufacture fuels, fertilisers, pharmaceuticals and numerous industrial chemicals. Geological processes involve reactions between minerals, water and gases, while atmospheric chemistry includes photochemical reactions driven by sunlight. Corrosion is a collection of chemical and electrochemical processes that transform metals through reactions with their environment. Environmental chemistry examines reactions involving pollutants, atmospheric compounds, soils and natural waters. Understanding reaction conditions is therefore essential for controlling yield, selectivity, safety, energy consumption and environmental effects.
A chemical reaction should not be confused with every observable change in matter. Melting ice changes the physical state of water but does not ordinarily produce a new chemical substance. Dissolving salt in water can be a physical process at the molecular level if the ions remain chemically identifiable, although subsequent reactions may occur in the solution. A chemical reaction produces different chemical species through changes in bonding or electronic structure.
Some reactions are obvious because they generate gas, precipitate, colour change, light or heat, but these observations are not themselves the definition. Some reactions have no dramatic visible sign. The most fundamental criterion is whether the chemical composition and identity of the participating species change. Chemical reactions therefore provide the central framework for understanding how matter transforms while conserving atoms and obeying deeper physical laws.
