Enthalpy

Enthalpy
Eric Gaba (Sting - fr:Sting) · Public domain · source
Name	Enthalpy
Unit	joule (J)
Dimension	energy

Enthalpy Enthalpy is a state function in classical thermodynamics describing the total heat content of a thermodynamic system under constant pressure conditions. It is central to analyses of heat transfer in chemical reactions, phase transitions, and engineering processes, and it appears in the formulations used by scientists working in fields associated with James Prescott Joule, Lord Kelvin, Ludwig Boltzmann, Josiah Willard Gibbs, and institutions such as the Royal Society and the National Institute of Standards and Technology.

Definition and Thermodynamic Formulation

Enthalpy is defined in macroscopic thermodynamics by the relation H = U + pV, where U appears in work associated with Sadi Carnot-inspired heat engines, pV terms occur in descriptions used by Rudolf Clausius and Willard Gibbs, and the formulation is applied in analyses influenced by the Second Law of Thermodynamics and the First Law of Thermodynamics. In open- and closed-system treatments adopted by researchers at the Max Planck Society and the American Chemical Society, enthalpy changes ΔH link to heat flow at constant pressure in calorimetric experiments pioneered by figures like Antoine Lavoisier and Hermann von Helmholtz. The differential formulation dH = TdS + Vdp connects enthalpy to entropy and pressure, an approach employed in textbooks used at University of Cambridge, Massachusetts Institute of Technology, and University of Oxford.

Units and Measurement

The SI unit of enthalpy is the joule, founded on the experimental work of James Prescott Joule and the standards developed by International Bureau of Weights and Measures and International Organization for Standardization. Practical measurements of enthalpy changes use calorimeters and bomb calorimetry techniques standardized by organizations including the American Institute of Chemical Engineers and the International Union of Pure and Applied Chemistry, and data are tabulated in handbooks published by the National Institute of Standards and Technology and the Royal Society of Chemistry. Engineering contexts often express enthalpy per unit mass or per mole consistent with practices in materials tested at facilities such as Sandia National Laboratories and Lawrence Livermore National Laboratory.

Types of Enthalpy (Specific, Molar, Standard)

Specific enthalpy (h), molar enthalpy (Hm), and standard enthalpy (ΔH°) are distinctions used across curricula at California Institute of Technology, Imperial College London, and ETH Zurich. Specific enthalpy is used in thermodynamic cycles studied in the context of the Brayton cycle and Rankine cycle, while molar enthalpy appears in chemical thermodynamics treatments linked to work by J. Willard Gibbs and experimentalists at Brookhaven National Laboratory. Standard enthalpies of formation and combustion are compiled following conventions set by the International Union of Pure and Applied Chemistry and used in databases maintained by the National Aeronautics and Space Administration and the European Space Agency.

Relationship to Other Thermodynamic Functions

Enthalpy interacts with internal energy, Gibbs free energy, Helmholtz free energy, and entropy in the frameworks developed by Josiah Willard Gibbs, Hermann von Helmholtz, and contemporaries at institutes such as the Max Planck Institute for Biophysical Chemistry. The relation G = H − TS links enthalpy to Gibbs free energy used in phase equilibrium studies influenced by the Clausius–Clapeyron relation and phase diagrams from research at Argonne National Laboratory and Oak Ridge National Laboratory. Thermochemical cycles, including Hess’s law applied in analyses dating to Germain Hess and thermodynamic tables produced by the International Thermodynamics Commission, rely on additive properties of enthalpy and its compatibility with conservation principles championed by figures like Émile Clapeyron.

Applications in Chemistry and Engineering

Enthalpy underpins calorimetry in organic and inorganic chemistry labs at University of California, Berkeley and University of Tokyo; it is used in reaction engineering for designing reactors at companies such as Siemens and General Electric and in energy systems modeled by organizations like the U.S. Department of Energy. In metallurgy and materials science programs at MIT and University of Cambridge enthalpy changes inform phase transformation studies, while in aerospace engineering at NASA and European Space Agency enthalpy is critical for propulsion and re-entry thermal protection analyses. Environmental assessments by groups such as the Intergovernmental Panel on Climate Change and industrial process optimization at Shell and Chevron also employ enthalpy-based energy balances.

Temperature and Pressure Dependence

Enthalpy dependence on temperature and pressure is quantified through heat capacities (Cp, Cv) with foundations in experimental calorimetry by James Joule and theoretical treatments by Ludwig Boltzmann and Max Planck. Kirchhoff’s law for reaction enthalpies and temperature dependence is used in chemical kinetics research at Lawrence Berkeley National Laboratory and industrial practice at BASF and DuPont. For fluids and supercritical substances, equations of state like those of Van der Waals and models developed by researchers at the National Physical Laboratory and Institut Français du Pétrole provide enthalpy as a function of T and p for simulations in computational fluid dynamics codes used at Rolls-Royce and Boeing.

Category:Thermodynamics