**Enthalpy** is defined as follows

Enthalpy is the sum of the internal energy

and pressure times volume.

We cannot measure the enthalpy of a system, but we can look at changes in enthalpy.

to make life easier we will make certain that Pressure is held constant...

So, the change in enthalpy, H, is equal to the change in internal energy, E, plus PV work, PV.

The expression for H
can be simplified.

Since,

and work is - PV

q is given the subscript "p"
because

we are holding the pressure constant.

Substituting for E in H = E + PV gives

So, at constant pressure allowing for only PV work (no other kind of work occurring)

Enthalpy was defined so that we can talk about the amount of

DH < 0 then q_{p} is
negative which means heat is leaving the system. A process which
releases heat is exothermic.

DH > 0 then q_{p} is
positive which means heat is being absorbed by the system. A process
which absorbs heat is endothermic.

Remember this does not represent the change in the total energy content (internal energy, E) it just tells us how much heat energy is released during a change occuring under very specific conditions.