If the reaction is carried out at constant pressure, it involves the change in volume against constant pressure. Work is done by the system in expanding against the atmospheric pressure. It has been found that the heat changes occurring at constant pressure and constant temperature are not only due to change in internal energy but also include energy changes due to expansion or contraction in volume against the atmospheric pressure.
Enthalpy (H) or heat content of the system is defined as the sum of the internal energy and the P-V energy of the system. Under a given set of conditions
H = E + PV
Just like E and V, H is also a state function. Its value depends on the initial and final state of the system. As a result, absolute value of enthalpy cannot be determined but DH can be experimentally calculated as,
DH = Hfinal – Hinitial
Enthalpy is also an extensive variable.
From the definition of enthalpy, the change in enthalpy (DH) is given by,
DH = DE + PDV (at constant P)
From first law of thermodynamics
DE = q + w
DE = q – PDV
Substituting this value of DE in equation DH = DE + PDV, we get,
DH = q – PDV + PDV
DH = qP.
Relation between DH and DE
For constant pressure for initial system: H1 = E1 + PV1
For final system: H2 = E2 + PV2
H2 – H1 = (E2 – E1) + P (V2 – V1)
DH = DE + PDV
DH = DE + Dng RT
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