H is the symbol for the enthalpy of a system. The change of enthalpy at constant pressure is equivalent to the amount of heat added to the system, and is equal to the change in internal energy plus the pressure multiplied by the change in volume of the system.
Enthalpy is defined as: H = U + PV.
H is the enthalpy of the system, U is the internal energy of the system and P and V are the pressure and volume of the system.
At constant pressure, the enthalpy of the system equals the amount of heat added to the system. The change in enthalpy in this kind of system is only dependent on the change in internal energy and the change in volume of the system.
(Change in enthalpy) = (change in internal energy) + (pressure)*(change in volume)