For example, H and p can be controlled by allowing heat transfer, and by varying only the external pressure on the piston that sets the volume of the system. They are suitable for describing processes in which they are experimentally controlled. H = ∑ k H k, are said to be the natural state variables in this representation. For inhomogeneous systems the enthalpy is the sum of the enthalpies of the composing subsystems: As intensive properties, the specific enthalpy h = H / m is referenced to a unit of mass m of the system, and the molar enthalpy H m is H / n, where n is the number of moles. Where U is the internal energy, p is pressure, and V is the volume of the system.Įnthalpy is an extensive property it is proportional to the size of the system (for homogeneous systems). The enthalpy H of a thermodynamic system is defined as the sum of its internal energy and the product of its pressure and volume: H = U + pV, Real gases at common temperatures and pressures often closely approximate this behavior, which simplifies practical thermodynamic design and analysis. The enthalpy of an ideal gas is independent of its pressure or volume, and depends only on its temperature, which correlates to its thermal energy. For endothermic (heat-absorbing) processes, the change Δ H is a positive value for exothermic (heat-releasing) processes it is negative. The temperature does not have to be specified, but tables generally list the standard heat of formation at 25 ☌ (298 K). The value does not depend on the path from initial to final state since enthalpy is a state function.Įnthalpies of chemical substances are usually listed for 1 bar (100 kPa) pressure as a standard state. This quantity is the standard heat of reaction at constant pressure and temperature, but it can be measured by calorimetric methods even if the temperature does vary during the measurement, provided that the initial and final pressure and temperature correspond to the standard state. In chemistry, the standard enthalpy of reaction is the enthalpy change when reactants in their standard states ( p = 1 bar usually T = 298 K) change to products in their standard states. When transfer of matter into or out of the system is also prevented and no electrical or shaft work is done, at constant pressure the enthalpy change equals the energy exchanged with the environment by heat. In practice, a change in enthalpy is the preferred expression for measurements at constant pressure, because it simplifies the description of energy transfer. The total enthalpy of a system cannot be measured directly because the internal energy contains components that are unknown, not easily accessible, or are not of interest in thermodynamics. Other historical conventional units still in use include the calorie and the British thermal unit (BTU). The unit of measurement for enthalpy in the International System of Units (SI) is the joule. As a state function, enthalpy depends only on the final configuration of internal energy, pressure, and volume, not on the path taken to achieve it. Therefore, enthalpy is a stand-in for energy in chemical systems bond, lattice, solvation and other "energies" in chemistry are actually enthalpy differences. The pressure-volume term is very small for solids and liquids at common conditions, and fairly small for gases. to make room for it by displacing its surroundings. The pressure–volume term expresses the work required to establish the system's physical dimensions, i.e. It is a state function used in many measurements in chemical, biological, and physical systems at a constant pressure, which is conveniently provided by the large ambient atmosphere. Enthalpy / ˈ ɛ n θ əl p i/ ( listen) is a property of a thermodynamic system, and is defined as the sum of the system's internal energy and the product of its pressure and volume.