Intermediate Topic 338 of 1102 Thermodynamics & Energetics
Enthalpy
Heat content H equal to U plus PV; the change equals heat at constant pressure.
Practise Thermodynamics & Energetics All Thermodynamics & Energetics topics
In-depth explanation of Enthalpy
Enthalpy, symbolized H, is defined as H equals U plus PV, a combination of internal energy and the pressure-volume product that is itself a state function. Its practical importance comes from the fact that at constant pressure the change in enthalpy equals the heat exchanged, delta H equals q_p, when the only work is pressure-volume work. Most laboratory and industrial reactions are carried out in open vessels at atmospheric pressure, so enthalpy is the natural quantity for reporting heats of reaction. A negative delta H indicates that heat is released to the surroundings, while a positive delta H indicates that heat is absorbed. Enthalpy changes are tabulated for many reactions, formations and phase transitions, which lets energy budgets be estimated quickly.
Going deeper: Enthalpy
Key ideas: Remember that delta H equals q only at constant pressure and with no non-expansion work. Enthalpy changes are extensive, reverse sign when a reaction is reversed, and depend on the physical states. Use delta H equals delta U plus delta n R T to move between the two for gas reactions. The standard symbol delta H degree refers to standard conditions.
Common mistakes and connections: Do not read the sign backwards: negative means heat released. Also do not assume a negative delta H guarantees a spontaneous reaction, since entropy matters too. Enthalpy is the basis for Hess's law, formation data and bond enthalpy estimates later in this area.
Visualization of Enthalpy
Enthalpy H = U + PV is the heat exchanged at constant pressure. Here products sit 90 kJ/mol below reactants, so the change in H is negative. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The combustion of methane has a standard enthalpy change of about -890 kJ per mole, meaning large quantities of heat are released when it burns at constant pressure.
Frequently asked questions about Enthalpy
What is Enthalpy?
Heat content H equal to U plus PV; the change equals heat at constant pressure.
Is Enthalpy a beginner, intermediate or advanced topic?
Enthalpy is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
What should I learn before Enthalpy?
Study these earlier topics in Thermodynamics & Energetics first: Work in Chemistry, Internal Energy, First Law of Thermodynamics.
What comes after Enthalpy?
The next topics in Thermodynamics & Energetics are Exothermic Reactions and Endothermic Reactions.
Can I practise Enthalpy with quizzes?
Yes. The interactive lesson includes a self-check question on Enthalpy, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Where Enthalpy fits in Thermodynamics & Energetics
Enthalpy is topic 10 of 91 in Thermodynamics & Energetics.