Recall our original definition of enthalpy:
H = E + PV
Then for a change in enthalpy:
DH = D E + D(PV)
If we set P
constant, then:
DH = D E + P D V
Since
qp = DE + PDV
Then
DH = qp
The change in enthalpy, D H, is then
equal to the heat transferred
at constant pressure, qp.