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 U_{1} and having volume V_{1} which is being chemically reacted at constant pressure and temperature and give rise to another chemical system having internal energy U_{2} and having volume V_{2}. Let us take H_{1} be the enthalpy of the first system and H_{2} 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.