# Enthalpy [Heat content]

Let the change of state of a system be brought about at constant pressure. In such a case, there will be a change of volume. Let the volume increase from at constant pressure ‘P’.

Then the work done (w) by the system will be:

…………(1)

Substituting the value of w from equation, we get:

This quantity E + PV is known as **Heat content** or **Enthalpy** of the system and is denoted by H.

Thus,

H = E + PV

Since E, P and V are definite properties, it follows that ‘H’ is also a definite property depending upon the state of the system.

Using the value in equation (3), we get

So, that the increase of the **heat content** of a system is equal to the heat absorbed at constant pressure. Substituting the value of AH for q in equation (3), we get

…..(4)

*I.e. *

*The change in heat content or enthalpy at constant pressure is equal to the sum of the increase in internal energy and mechanical work of the expansion. *

The equation (4) may also be written as :

Where = Difference in no. of moles of gaseous products and reactants

R = gas constant and

T = Temperature in Kelvin.

It is clear that if

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