Explain change in enthalpy.

Explanation

Consider a process, `A rarr B`, taking place at constant pressure P.
Initial enthalpy is given by `H_(1) = U_(1) + PV_(1)`
Final enthalpy is given by `H_(2) = U_(2) + PV_(2)`
`:.` Change in enthalpy during the process `= H_(2) - H_(1) = (U_(2) - U_(1)) + P(V_(2) - V_(1))`
i.e., `dH = d U + P.dV`
If the magnitude of change is large, the equation can be written as, `Delta H = Delta U + P.Delta V`
where `Delta U` represents, changes in internal energy and `Delta V` represents change in volume.