Calorimetry is the experimental measurement of heat absorbed or released during a chemical reaction or physical process. It relates observable signals, such as temperature changes or compensating electrical power, to the heat exchanged by a sample. Calorimetric measurements provide data on reaction energetics, thermal properties, and molecular interactions. The apparatus used is called a calorimeter, whose design depends on the process, operating conditions, and required sensitivity. (goldbook.iupac.org)
Thermodynamic principles
Calorimetry rests on the first law of thermodynamics, which expresses conservation of energy. A thermodynamic system must be distinguished from its surroundings: heat released by the sample is absorbed by the measuring apparatus and surrounding medium, apart from exchanges with the external environment. With the usual thermodynamic sign convention, heat entering the system is positive and heat leaving it is negative. Heat is energy transferred across a boundary, not a substance stored inside a body. (openstax.org)
The relationship between measured heat and a thermodynamic property depends on the experimental constraints. At constant volume, heat equals the change in internal energy, provided no other work occurs. At constant pressure, heat equals the change in enthalpy when pressure–volume work is the only work:
[ q_V=\Delta U,\qquad q_P=\Delta H. ]
These conditions matter because an expanding reaction mixture can transfer energy through work as well as heat. A constant-volume combustion result therefore cannot automatically be treated as a constant-pressure reaction enthalpy. (openstax.org)
Measuring heat from temperature changes
In temperature-rise calorimetry, heat is inferred from a change in temperature and a known heat capacity. If heat capacity is approximately constant over the interval,
[ q=C\Delta T=mc\Delta T, ]
where (C) is total heat capacity, (m) is mass, and (c) is specific heat capacity. The relevant capacity may include the sample container, liquid, thermometer, and other components that warm together. If heat capacity varies appreciably, its temperature dependence must be included rather than represented by one constant value. (openstax.org)
For an ideally isolated reaction experiment,
[ q_{\mathrm{reaction}}+q_{\mathrm{surroundings}}=0. ]
Consequently, a warming calorimeter usually indicates an exothermic reaction, whereas cooling indicates an endothermic reaction. This interpretation requires a defined system boundary and accounting for the apparatus’s own thermal response. (openstax.org)
Not every heat exchange produces a temperature change. During a phase transition, energy may be absorbed or released as latent heat. Melting ice, for example, can absorb heat while the temperature remains at the melting point under fixed-pressure equilibrium conditions. Heat is conventionally reported in joules, while heat capacities are expressed in joules per kelvin. (openstax.org)
Constant-pressure and combustion calorimetry
Solution calorimetry commonly measures mixing, dissolution, and reactions in liquids. A simple insulated cup approximates constant atmospheric pressure, while research instruments use more carefully controlled vessels and temperature sensors. The measured temperature change, together with the calibrated thermal capacity, yields the heat associated with the process. (openstax.org)
A bomb calorimeter measures combustion in a sealed, rigid vessel containing oxygen. The vessel is surrounded by a heat-absorbing medium, often water, and the assembly’s temperature rise determines the energy released. Its rigid reaction chamber makes the measurement fundamentally constant-volume. Applications include determining the combustion energies of fuels and other combustible substances. (openstax.org)
Accurate combustion measurements require calibration of the apparatus’s energy equivalent. Certified benzoic acid is an established reference material because its combustion energy is well characterized. Precision determinations also account for ignition energy and differences between the actual bomb conditions and the thermodynamic reference conditions used to report results. (tsapps.nist.gov)
Differential scanning calorimetry
Differential scanning calorimetry (DSC) compares the energy input to a sample with that to a reference while both undergo a controlled temperature programme. The resulting curve records their differential thermal response during heating, cooling, or temperature holds. Depending on instrument design, the response is obtained through differential heat-flow measurement or differences in supplied power. (goldbook.iupac.org)
DSC characterizes melting, crystallization, curing reactions, and glass transitions. In polymers, it helps determine transition temperatures and examine effects of processing or ageing. Melting and crystallization commonly produce peaks whose integrated areas, after calibration and baseline correction, give transition enthalpies. A glass transition generally appears as a change in the heat-capacity baseline rather than as a latent-heat peak. (analyzing-testing.netzsch.com)
Isothermal titration calorimetry
Isothermal titration calorimetry (ITC) measures heat generated or absorbed as one substance is added incrementally to another at controlled temperature. A common application is the binding of a ligand to a protein. Integrating each injection’s heat signal and fitting the resulting binding curve can yield binding affinity, stoichiometry, and binding enthalpy. These measurements do not require fluorescent labels or immobilization of the binding partners. (malvernpanalytical.com)
From the association constant and enthalpy, Gibbs free energy and entropy changes can be calculated using appropriate thermodynamic conventions. They are derived quantities rather than independent direct heat measurements. Interpretation depends on the binding model and sample concentrations; solvent composition and heat from dilution also require attention. (malvernpanalytical.com)
Calibration and related usage
Different calorimetric methods require different reference materials and test procedures. Electrical calibration supplies a known energy input, while chemical calibration uses a characterized reaction. Thermal leakage, baseline behaviour, and incomplete equilibration can influence the measured response; calorimetric results therefore depend on both instrument calibration and experimental conditions. (nvlpubs.nist.gov)
In particle physics, “calorimetry” also denotes measuring particle energy through absorption in a detector. Such calorimeters commonly read electrical or optical signals rather than bulk temperature changes. Electromagnetic calorimeters measure electrons and photons, while hadronic calorimeters measure particles that interact through nuclear processes. (home.cern)