Advanced Placement (AP) · Chemistry
Enthalpy & Calorimetry
Why it matters
Heat, q = mcΔT and enthalpy change.
Advanced20m readingPrerequisite: None
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.
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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.