U is internal energy, which is the sum of heat + work done.
\DeltaU = q + w -----(1) where w = \Delta(PV)
Enthalpy is the internal energy + work done
\DeltaH = \DeltaU + P\DeltaV
At constant pressure,
\DeltaH = q - P\DeltaV + P\DeltaV
\DeltaH = q_{p}
Where q_{p} is used to...