Enthalpy

Science

Definition

A thermodynamic quantity representing the total heat content of a system at constant pressure. Changes in enthalpy (ΔH) tell us whether a reaction releases heat (exothermic, ΔH < 0) or absorbs heat (endothermic, ΔH > 0).

Examples

  • Combustion of methane releases heat (ΔH = −890 kJ/mol)
  • Dissolving ammonium nitrate in water absorbs heat, making a cold pack
  • Photosynthesis is endothermic, requiring energy input from sunlight
Key Fact

ΔH = H_products − H_reactants; negative ΔH = exothermic

Related Terms

Hess's law Gibbs free energy Entropy Activation energy

Study This Concept

Practice enthalpy with free review games in these units:

AP Chemistry Unit 6: Thermodynamics