Enthalpy
For measuring heat lost or gained at constant pressure, a new state function, called enthalpy, is defined. It is denoted by the symbol H.
H = U + p V
Enthalpy change, ΔH, is given by
ΔH = ΔU + Δ(pV)
ΔH = ΔU + pΔV + VΔp
If the change is carried out at constant pressure, then Δp = 0.
ΔH = ΔU + pΔV
Now, ΔU = q – pΔV
ΔH = q – pΔV+ pΔV = q (at constant pressure)
ΔH = qp
By measuring heat lost or gained at constant pressure, you can measure enthalpy change for any process.
Relation between ΔH and ΔU
For liquids and solids, the difference between ΔH and ΔU is not significant but for gasses, the difference is significant.
Let VA be the total volume of the gaseous reactants, VB the total volume of the gaseous products, nA the number of moles of gaseous reactants, and nB the number of moles of gaseous products, at constant pressure and temperature.
Then, using ideal gas law,
pVA = nART
pVB = nBRT
Subtracting the first equation from the second equation,
p(VB – VA) = (nB – nA)RT
pΔV = ΔnRT
At constant pressure
ΔH = ΔU + pΔV
Therefore, ΔH = ΔU + ΔnRT