The quantity U + PV is known as the enthalpy of the system. It is denoted by H. It represents the total energy stored in the system. Thus
H = U + PV
Where U is a definite property
P and V are also definite properties
Hence H is also definite property i.e. it depends on the state of the system.
Let us suppose a system having internal energy U1 and having volume V1 which is being chemically reacted at constant pressure and temperature and give rise to another chemical system having internal energy U2 and having volume V2. Let us take H1 be the enthalpy of the first system and H2 be the enthalpy of the second system which is obtained from the first system when it is reacted at constant temperature and volume.
Hence at constant pressure ‘P’:
H 1 = U 1 + PV 1
And
H 2 = U 2 + PV 2
Therefore change in enthalpy i.e.
∆H = H 2 – H 1
= (U 2 + PV 2) – (U 1 + PV 1)
= (U 2 – U 1) + P (V 2 – V 1)
Hence, ∆H = ∆U + P∆V
Let the heat exchanged in above chemical reaction i.e. chemical reaction at constant pressure be
q p. Therefore:
∆H = q p
Generally, if the enthalpy of reactants is H r and that of products is H p then:
∆H = H p – H r = q p
Therefore, the change of enthalpy of chemical reaction at constant pressure and at given temperature is given as the difference between the enthalpies of product and reactants.
