Enthalpy

Enthalpy is a thermodynamic state function defined as a system’s internal energy plus pressure multiplied by volume. At constant pressure, its change equals heat transferred when only pressure–volume work occurs.

Pressure–volume work: Work transferred when a system expands or contracts against pressure. This work term explains why pressure multiplied by volume appears in enthalpy.
Standard enthalpy of formation: The enthalpy change when one mole of a compound forms from its elements in their standard states. Formation values let reaction enthalpies be calculated from tabulated data.
Thermodynamic system: A chosen quantity of matter or region in space analyzed using thermodynamic properties and exchanges. Enthalpy is assigned to a defined system and its state.
Helmholtz free energy: A thermodynamic potential equal to internal energy minus temperature multiplied by entropy. Unlike Gibbs free energy, it is especially useful for constant-volume conditions.
First law of thermodynamics: The energy-conservation principle stating that a system’s energy change equals heat added minus work done. It relates enthalpy changes to heat while accounting for work.
Hess's law: The principle that a reaction’s enthalpy change is independent of the route taken between initial and final states. It permits reaction heat to be found by adding known enthalpy changes.
State variable: A measurable property that specifies a system’s thermodynamic state. Pressure, volume, temperature, and enthalpy describe system states.
Heat: Energy transferred between systems because of a temperature difference. Heat is path-dependent energy transfer; enthalpy is a state property.
Heat capacity at constant pressure: The heat required to raise a system’s temperature by one degree while pressure remains constant. It measures how enthalpy changes with temperature at fixed pressure.
Calorimetry: The experimental measurement of heat transferred during physical or chemical processes. Calorimeters infer enthalpy changes from measured heat under controlled conditions.

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