What Are Chemical Reactions?

Chemical reactions are processes in which one set of substances is converted into another by the breaking and forming of chemical bonds, with the atoms conserved and only their arrangement altered. A reaction is written as an equation whose stoichiometric coefficients express the fixed proportions in which reactants combine and products appear. Whether a transformation is favorable and how far it runs toward products are settled together by thermodynamics, since the equilibrium constant follows from the standard Gibbs energy change. How fast the system reaches that endpoint is a separate question, and it belongs to kinetics.

The study of reactions underpins chemical technology, from the synthesis of bulk commodity chemicals to the operation of batteries, fuel cells, and combustion engines. Engineering practice depends heavily on measured data, because neither the energy released by a reaction nor the speed at which it runs can be predicted from a structural formula alone.

Reaction Classes and Mechanisms

Reactions are grouped by what happens to the participating species. Oxidation-reduction reactions involve transfer of electrons between species, and chemical reduction, the gain of electrons by a species, always occurs paired with an oxidation elsewhere in the same system. Acid-base reactions transfer protons, hydrolysis cleaves a bond by inserting the elements of water, and precipitation reactions remove ions from solution as an insoluble solid. Behind each overall equation lies a mechanism: an ordered sequence of elementary steps, each involving one, two, or rarely three species, connected by short-lived intermediates and transition states. Determining a mechanism means finding a step sequence consistent with the observed rate law, isotope labeling results, and detected intermediates.

Energetics, Kinetics, and Solvation

The Gibbs energy change of a reaction determines the position of equilibrium, while the activation energy determines the rate at which equilibrium is approached. Both calculations rest on tabulated property data, which the NIST Chemistry WebBook supplies as reaction thermochemistry and thermophysical properties. Arrhenius and transition-state expressions relate rate constants to temperature, and reaction order is established experimentally rather than read off the stoichiometry, which is why compilations of measured values such as the NIST Chemical Kinetics Database remain the working reference for gas-phase rate coefficients. Reactions carried out in solution are strongly shaped by solvation: solvent molecules organize around ions and polar species, stabilizing charged reactants, intermediates, and transition states to different degrees, so the same reaction can change rate by many orders of magnitude between solvents. Energy released by a reaction usually appears as heat, but chemiluminescent reactions release part of it as visible photons from an electronically excited product, and ignition describes the runaway condition in which heat release from an exothermic reaction outpaces heat loss and drives a self-sustaining combustion wave.

Catalysis

A catalyst increases the rate of a reaction by opening a lower-energy pathway, without being consumed and without changing the overall standard Gibbs energy change, which means it accelerates the approach to equilibrium but cannot shift its position. The IUPAC Gold Book entry for catalysis distinguishes homogeneous catalysis, in which catalyst and reactants share a single phase, from heterogeneous catalysis, in which the reaction proceeds at an interface, typically on the surface of a supported metal or oxide. Enzymes are the biological case, achieving rate enhancements of many orders of magnitude with strict selectivity. Industrial catalysis dominates large-scale chemistry: ammonia synthesis, catalytic cracking, and automotive exhaust treatment all depend on engineered catalyst formulations and on managing deactivation by sintering, coking, and poisoning.

Reaction Engineering

Turning a reaction into a process requires choosing a reactor and a set of operating conditions. The continuous-stirred tank reactor, or CSTR, assumes perfect mixing so that composition is uniform and equal to the outlet stream, while the plug-flow reactor assumes no axial mixing and a composition that varies along its length. Batch and semi-batch vessels are used where residence time must be controlled directly. Bioreactors apply the same design framework to cell cultures and enzymatic conversions, adding constraints on shear, oxygen transfer, sterility, and pH. Across all of these, heat removal, mass transfer, and residence time distribution govern whether the yield achieved in a laboratory flask survives scale-up, and a comparative analysis of scale-down bioreactor configurations shows how nutrient and dissolved-gas gradients absent in a small vessel emerge at production volume.

Applications

Chemical reactions have applications in a wide range of fields, including:

• Petrochemical refining and bulk chemical manufacture
• Pharmaceutical and fine chemical synthesis
• Batteries, fuel cells, and electrolyzers
• Combustion, propulsion, and energy conversion
• Semiconductor etching and thin-film deposition
• Water treatment, environmental remediation, and analytical sensing