The result expressed in eqn 2.19 states that, when a system is subjected to constant
pressure and only expansion work can occur, the change in enthalpy is equal to the
energy supplied as heat. For example, if we supply 36 kJ of energy through an electric
heater immersed in an open beaker of water, then the enthalpy of the water increases
by 36 kJ and we write ΔH = +36 kJ.