What does enthalpy mean?
Enthalpy is a fundamental concept in thermodynamics, defined as the total heat content of a system: its internal energy plus the product of its pressure and volume. Denoted by the symbol H, it becomes especially useful when studying processes that occur at constant pressure, such as chemical reactions carried out in open vessels, because the change in enthalpy directly equals the heat absorbed or released. When the enthalpy of the products is lower than that of the reactants, heat is given off and the reaction is exothermic; when it is higher, heat is absorbed and the reaction is endothermic. Chemists routinely tabulate quantities such as the enthalpy of formation, combustion, or neutralization to predict energy changes in reactions. The term was introduced relatively recently, around 1909, by Dutch physicist Heike Kamerlingh Onnes, drawing on Greek roots meaning 'to put heat into.' Though technical in flavor, enthalpy is indispensable across chemistry, physics, and engineering wherever heat flow matters.
nounA thermodynamic property of a system equal to its internal energy plus the product of its pressure and volume, symbolized H. It is especially useful for measuring heat changes in processes occurring at constant pressure.
- The thermodynamic quantity equal to a system's internal energy plus the product of its pressure and volume (H = U + PV), representing total heat content.
"The enthalpy change of the reaction tells us how much heat is released at constant pressure."
"The standard enthalpy of formation of liquid water is about −286 kilojoules per mole."
"In an exothermic reaction at constant pressure, the enthalpy of the products is lower than that of the reactants."
Rare in practice; the plural typically appears when comparing enthalpy values across multiple substances, phases, or reactions.
"The table lists the enthalpies of formation for several common compounds."
The word 'enthalpy' was only coined around 1909 by Dutch physicist Heike Kamerlingh Onnes — even though scientists had been measuring the concept for decades.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of enthalpy
The word 'enthalpy' was coined in the early twentieth century, around 1909, by the Dutch physicist Heike Kamerlingh Onnes. It derives from the Greek 'enthalpos,' meaning 'putting heat into,' formed from the prefix 'en-' ('in') and 'thalpos,' related to the verb 'thalpein' ('to warm'). The term entered scientific English shortly after its coinage and has since become standard vocabulary in thermodynamics worldwide.
Related word forms
How enthalpy is actually used
Strictly technical and scientific in register, used almost exclusively in chemistry, physics, and engineering. It appears most often in the phrase 'enthalpy change' (ΔH) when discussing reactions at constant pressure.
Easily confused with enthalpy
Enthalpy measures a system's total heat content under constant pressure, while entropy measures the degree of disorder or randomness in a system.