Science
Enthalpy.
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
Real-world.
- 1 Combustion of methane releases heat (ΔH = −890 kJ/mol)
- 2 Dissolving ammonium nitrate in water absorbs heat, making a cold pack
- 3 Photosynthesis is endothermic, requiring energy input from sunlight
Key Fact
ΔH = H_products − H_reactants; negative ΔH = exothermic