Enthalpy of formation, also known as heat of formation, is a thermodynamic property that measures the heat energy change when one mole of a compound is formed from its constituent elements in their standard states. It is a fundamental concept in the field of thermochemistry, which studies the heat energy changes associated with chemical reactions.

Enthalpies of formation are determined under standard conditions, which include a temperature of 25 degrees Celsius (298 K), a pressure of 1 atmosphere, and the compound in its most stable form at that temperature and pressure. The enthalpy of formation is typically expressed in units of energy per mole, such as kilojoules per mole (kJ/mol).

The enthalpy of formation provides valuable information about the stability and energy content of a compound. A positive value for ΔHf indicates that the formation process is endothermic, meaning it requires the input of heat energy. Conversely, a negative value for ΔHf indicates an exothermic process, where heat energy is released during the formation.