Enthalpy is nothing but the heat content of a system. When a chemical reaction occurs in an open vessel under constant external pressure, energy change obtained is equal to the total of internal energy (E) and the product of its volume (V). This is called as enthalpy and denoted as ‘H’.

H = E + PV (1)

If a state of a system changes, then enthalpy of a system also changes.

Δ H = ΔE + Δ(PV)

Δ H = ΔE + PΔV + VΔP

Reaction occurs at constant external pressure ΔP = 0, therefore:

ΔH = ΔE + PΔV (2)

Recall ΔE = q + w (3) and w = - P ΔV (4) from the first law of thermodynamics, if we insert (3) into (2), then we have:

ΔH = q + w + PΔV

But if equation (4) is applied then:

ΔH = q - PΔV+ PΔV

ΔH = q

Therefore when a reaction occurs at constant pressure, the change in enthalpy of the system is equal to the value of heat obtained or lost by the system.