Eleven Standard >> Concept of enthalpy and some problems based on enthalpy

Enthalpy:

Enthalpy is the sum of internal energy and product of pressure and volume of the system.

  H=U+PV

   Where H=Enthalpy of the system
              U=Enternal energy of the system 
              P=Pressure of the system 
              V=Volume of the system 

Characteristics of Enthalpy: 

1) It is an extensive property of the system
2) It is a state function. That means like internal energy it depends only on the initial stage and final stage but not on the path of transition.

Q. A gas is allowed to expand fixed temperature from 10 L to 100 L at 1 atm pressure. Find the work done, internal Energy generated heat.

Ans: If it is not stated in the problem the gas is allowed to reversibly expand, we always considered the expansion is irreversible. So the process is irreversible. 
    
      \(\tt w=-P_{2}(V_{2}-V_{1})\)
          
             \(\tt P_{2}\)=final pressure =1 atm
             \(\tt V_{1}\)=initial volume =10 L
             \(\tt V_{2}\)=final volume =100 L
   
            \(\tt =-1(100-10)\)
            \(\tt =-90\) L.atm

According to first law of thermodynamic

  \(\tt \triangle U=q+w\)

        Where U=internal energy
                   q=generated heat 
        \(\tt \triangle U\)=0 [\because temperaure is constant
 
  \(0=-90+q\) [\(\because\) \(\tt w=-90\) L.atm]
 \(\therefore\) \(q=90\) L.atm