The heat content or enthalpy of systems (H) is a thermodynamic property

                                                                                                           (2)

P is pressure and V is volume of the system. With the help of equation 1, first law is written for any thermodynamic change of state:

                                                                          (3)    
 
For a constant pressure process, only expansion work is done by system on surrounding and is equal to

 also                                                           (4)

                                                                                                     (5)

By 3, 4 and 5 we get

 or                                                                                      (6)

In equation  is heat absorbed by system in changing from state 1 to state 2 by a path of constant pressure. A change under constant pressure is 1 called “ISOBARIC process”.

If an isobaric processes involves work other than expansion

                                                                                                       (7)

W1 does not include expansion work. For example in an electric process at constant pressure – is input of electrical work to the system.

The equation 6 is very useful since  is related to the heat content (H). The change in heat content  depends on the initial and final states only; hence heat absorption or evolution can be evaluated form the data on the properties of system in the two states.

Various heat effects

1. Effect of temperature  

The variation of heat content in pure compounds with temperature can be determined by

                                                                     (8)

 is increase in heat content in cal/mole as the substance is heated from 298K to T. In all calculations the reference temperature is 298K  T is temperature in K and  a, b, c and d are constants and depends on state or aggregation of the system. The quantity  is called sensible heat,`