# Misunderstanding in definition of Enthalpy

Enthalpy is defined as $$H = U + PV$$, but internal energy is given by $$U = \frac{3}{2} nk_B T = \frac{3}{2}PV$$ for an ideal gas, so why don't we just write

$$H = \frac{5}{2} PV$$

for enthalpy? Is this equation even correct? If so, what significance $$H = U + PV$$ serve that $$H = \frac{5}{2} PV$$ does not?