Constant Pressure Process-1

A gas in the piston-cylinder assembly is considered as the system of interest (figure 7.1)

Figure 7.1

• The pressure is maintained at a constant value by loading the system with a mass.
  
  
• The cylinder is brought into contact with a heat source.

Energy transfer as heat takes place reversibly. The work is done by system when it changes from the initial state (1) to the final state (2).

(7.1)

Applying the first law, we get

(7.2)

or

(7.3)

or

(7.4)

The quantity is known as enthalpy, H (a property) of the system. The specific enthalpy h is defined as

(7.5)

The molar enthalpy is , where N is the mole number of the substance.