AP Chemistry · Chemistry · Lesson 6.1
Enthalpy & Calorimetry
- Level
- Advanced Placement (AP)
- Reading time
- 12 min reading
- Review
- Last reviewed 2026-09-03
- Standard
- Accuracy standard
- Errata
Abstract
Heat, q = mcΔT and enthalpy change.
What is enthalpy in chemistry?
Short answer
Enthalpy (H) is a measure of the total heat content of a system at constant pressure. Chemists work with the enthalpy change of a reaction, ΔH: negative for exothermic reactions that release heat and positive for endothermic reactions that absorb it. ΔH can be found from calorimetry, from bond enthalpies, or by Hess’s law from known reaction enthalpies.
Concept preview
What you will be able to do
- 01Understand the concept of enthalpy and its significance in thermodynamic processes.
- 02Apply the formula to calculate heat changes in calorimetry experiments.
- 03Interpret enthalpy changes in chemical reactions and relate them to bond energies.
- 04Analyse and solve complex calorimetry problems involving multiple steps and substances.
The concept of enthalpy is fundamental in thermodynamics, especially when analysing chemical reactions. Enthalpy, denoted as , is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create the system, and the energy that is due to the system's pressure and volume. Enthalpy is a state function, meaning it depends only on the current state of the system, not on how it got there. This property makes enthalpy particularly useful when dealing with processes involving heat transfer at constant pressure, which is common in chemical reactions. The change in enthalpy, , is especially important because it tells us whether a process is endothermic or exothermic, providing insight into the energy dynamics of chemical processes.
All-Access Plan
Enthalpy & Calorimetry
This lesson's full explanation, worked examples, quiz and practice are locked. Full access is on the All-Access Plan.
Log in or start free and you will return right here.
Related topics
Frequently asked questions
- What is the difference between exothermic and endothermic?
- Exothermic reactions release heat (ΔH < 0); endothermic reactions absorb heat (ΔH > 0).
- What is Hess’s law?
- The enthalpy change of a reaction is the same whether it happens in one step or several, so ΔH values can be added.