Enthalpy (H) is a measure of the total heat content of a system at constant pressure. It represents the sum of the system’s internal energy (the energy stored in chemical bonds and particle motion) plus the energy required to make room for it by displacing its surroundings (pressure–volume work).
In simple words: Enthalpy tells us how much heat energy a system contains and can give out or absorb during a reaction at constant pressure.
Key Points:
- Enthalpy is a state function – it depends only on the current state (temperature, pressure, amount of substance), not on the path taken.
- The change in enthalpy (ΔH) is what matters in reactions:
- ΔH < 0 → Exothermic reaction (heat released, surroundings warm up).
- ΔH > 0 → Endothermic reaction (heat absorbed, surroundings cool down).
- Enthalpy change is what we commonly measure in calorimetry.
Examples:
- Combustion of fuels (like burning wood or petrol) releases heat → negative ΔH.
- Photosynthesis absorbs sunlight to form glucose → positive ΔH.
- Neutralization of acid and base usually releases heat → negative ΔH.
In short: Enthalpy is the heat content of a system at constant pressure, and its change (ΔH) tells us whether a process absorbs or releases energy.