Enthalpy
Definition
The accountant's energy — internal energy plus the energy cost of making room (pressure × volume). Chemists love it for reactions at constant pressure.
Symbol: · Unit: joule (J)
Technical definition
H = U + PV; at constant pressure ΔH = Q_p, the heat absorbed or released. Enthalpy is a state function — path independent.
Example
Burning 1 mol of methane releases ΔH = −890 kJ — the negative sign means heat flows out (exothermic).
Common confusion
Enthalpy is not a mysterious new energy hiding in matter — it is a bookkeeping combination (U + PV) that is useful because most chemistry happens at constant atmospheric pressure.