Thermochemistry is the study of heat changes associated with chemical reactions, physical transformations and changes in the state or composition of matter. It is closely connected with thermodynamics but concentrates particularly on quantities describing heat transfer and energetic changes, including enthalpy, internal energy, heat capacity and reaction enthalpy. An exothermic process releases heat to its surroundings, while an endothermic process absorbs heat. Thermochemical measurements can describe combustion, neutralisation, dissolution, formation, phase transitions and many other processes.
The discipline provides quantitative methods for calculating energy changes even when a reaction cannot be measured directly. Thermochemistry is important in fuels, industrial reactors, materials processing, metabolism, environmental chemistry, batteries and energy systems. It must not be confused with kinetics because a reaction can release substantial heat while proceeding very slowly, and it must not be confused with spontaneity because an exothermic reaction is not automatically spontaneous under every condition.
The historical development of Thermochemistry accompanied the development of quantitative chemical measurement and thermodynamics. Early investigations of combustion demonstrated that chemical transformations were associated with measurable heat release. Antoine Lavoisier and Pierre Laplace studied heat effects in chemical and biological processes, while later work by Germain Hess established the principle now known as Hess law.
Hess law follows from the state function nature of enthalpy and allows the overall heat change of a reaction to be calculated by adding the enthalpy changes of suitable intermediate reactions. Later thermodynamic developments introduced internal energy, entropy and Gibbs energy and clarified the relationship between heat, work, equilibrium and spontaneity. Modern Thermochemistry combines calorimetric measurement with thermodynamic databases, statistical mechanics and computational chemistry. It therefore provides both an experimental and theoretical framework for understanding the energetic consequences of chemical and physical transformations.
Enthalpy is one of the most important quantities in Thermochemistry. Enthalpy is defined as H equal to U plus pV, where U is internal energy, p is pressure and V is volume. Under constant pressure conditions, when only pressure volume work is relevant, the heat transferred is equal to the change in enthalpy. Standard enthalpy changes are specified for defined standard states and can describe formation, combustion, neutralisation, solution, atomisation, hydration and phase transitions.
Standard enthalpies of formation are especially useful because reaction enthalpies can be calculated from the formation enthalpies of products and reactants. Because enthalpy is a state function, its change depends only on the initial and final states and not on the route taken between them. This allows chemists to combine experimentally measured values into thermochemical cycles and calculate quantities that would otherwise be difficult to determine directly.
Calorimetry is the principal experimental method for measuring heat changes. A calorimeter measures heat transferred between a chemical system and a defined surroundings or reference system. In simple constant pressure calorimetry, often called coffee cup calorimetry, the temperature change of a solution is used to calculate heat transfer when appropriate assumptions are satisfied. Bomb calorimetry operates approximately at constant volume and is widely used for combustion measurements.
The measured heat can be related to changes in internal energy and, with suitable corrections, enthalpy. Differential scanning calorimetry measures differences in heat flow as a sample is heated, cooled or held under controlled conditions and is widely used for polymers, pharmaceuticals, metals and other materials. Accurate calorimetry requires calibration and consideration of heat capacity, heat loss, sample purity, phase changes and experimental uncertainty.
Bond energies provide an approximate molecular interpretation of thermochemical behaviour. Breaking chemical bonds requires energy, whereas formation of bonds generally releases energy. Approximate reaction enthalpies can therefore be estimated by summing bond dissociation energies for bonds broken and subtracting the corresponding energies for bonds formed. Such estimates are limited because tabulated bond energies are usually averages and individual bond energies depend on molecular environment. More accurate thermochemical values can be obtained from measured enthalpies of formation or advanced computational methods.
Phase changes also involve characteristic enthalpy changes because intermolecular interactions must be disrupted or established. Enthalpies of fusion, vaporisation and sublimation are important in understanding melting, evaporation, boiling, condensation and sublimation. These quantities are directly relevant to refrigeration, distillation, atmospheric processes, materials science and energy engineering.
Thermochemistry interacts with but is distinct from other parts of chemical thermodynamics. Reaction enthalpy describes heat effects under appropriate conditions, whereas entropy describes changes in energy dispersal and Gibbs energy combines enthalpy and entropy to determine equilibrium tendencies under specified temperature and pressure conditions. Kinetics describes reaction rates and activation barriers. A highly exothermic reaction can remain effectively inert for long periods if its activation barrier is large.
Catalysts can reduce activation barriers and accelerate reactions without changing the overall enthalpy difference between reactants and products. Temperature also affects thermochemical quantities because heat capacities and equilibrium properties can vary with temperature. Thermochemical equations must therefore specify physical states and, where necessary, temperature and pressure. Without these conditions, numerical values can be ambiguous or incorrectly compared.
Industrial applications of Thermochemistry are extensive because chemical processes often involve substantial heat generation or absorption. Combustion systems require knowledge of fuel heating values and reaction enthalpies to determine energy output and operating temperatures. Industrial reactors handling strongly exothermic reactions require adequate cooling to prevent thermal runaway, while endothermic reactions may require continuous heat input. Cement manufacture, metallurgy, petroleum refining, polymerisation, food processing and materials synthesis all involve thermochemical calculations.
Thermochemistry is also relevant to batteries and energy storage because chemical and electrochemical transformations can generate or absorb heat. In biological systems, metabolic pathways involve energetic changes, although cellular energy transfer cannot be described by heat alone because chemical free energy and coupling processes are also important. Environmental processes such as atmospheric reactions, evaporation, condensation and aerosol formation likewise involve measurable thermal effects.
Modern Thermochemistry increasingly integrates precise experimental measurements with computational chemistry and statistical thermodynamics. Statistical mechanics provides a microscopic interpretation of thermodynamic properties by relating molecular energy levels and populations to macroscopic quantities. Computational methods can estimate reaction energies, molecular enthalpies, heat capacities and other properties, although accuracy depends on the treatment of electron correlation, molecular vibrations, basis sets and environmental effects. Thermochemical databases provide standardised values for use in research and engineering calculations.
Important limitations include dependence on temperature, pressure, physical state, concentration and composition, as well as experimental uncertainty and possible variation between measurement methods. The major significance of Thermochemistry is that it gives chemistry a quantitative energy framework, allowing molecular transformations to be connected with measurable heat flows and enabling scientists and engineers to calculate energy requirements, reaction hazards, fuel performance, material behaviour and the thermal consequences of chemical processes.
