spontaneous process and entropy

Spontaneous process (Irreversible process)

 Definition:-

 The process which is take place on its own without the external influence known as Spontaneous process.

Ex. flow of heat from hot body to cold body

Ex. Water flow from higher level to lower level.

          i.e. all the example show that the process occur spontaneously in one direction and cannot occurs spontaneously in opposite direction.

          The spontaneous process is still continuous up to the equilibrium reached.

Ex.- example of spontaneous process which is endothermic.

          Generally spontaneous process is exothermic but the many process which is endothermic and spontaneous. 

Ex;- Dilution of NaCl_(s) in water is spontaneous and endothermic process in which heat absorbed from surrounding.

         NaCl_(s) + aq  ---------->     Na⁺_(aq) + Cl^-_(aq)      ∆_soluH⁰ = +3.9 Kj/mol 

Ex. conversion of ice into water  

            Ice (H₂O_(s)) ---------->    water (H₂O_(l))      ∆_fusH⁰ = + 6.01KJ/mol

Entropy

  The molecular disorder or randomness is measured by thermodynamic property called entropy.

   It is denoted by S. greater is the randomness higher is the entropy.

          It is a state function and extensive property and unit is J/K

Ex.- When solid I₂  dissolved in water

                    I_2(s) ----------->     I_2(aq)               ∆S = positive

     Order state               disorder state                

Ex.- When H_2(g)  is converted in H_(g)

                 H_{2(g)---------------->}   2H_(g)                  ∆S = positive

         Disorder state           more disorder state 

Ex        H_{2 (g)} + O_{2 (g)}  ----------------> 2 H₂O_(l)   ∆S = Negative

More disorder state                       less disorder state

Quantitative definition of entropy (Entropy change):-        

          The entropy change (∆S) of a system in a process is equal to heat transferred to it in reversible manner divided by the absolute temperature (T) at which transfer takes placed. 

Entropy change = heat transfer reversibly/ absolute temp. of heat transfer

                          ∆S = qrev/T    

          Unit of entropy are JK^-1 in SI unit and cal K^-1 in C.G.S. system

Note: - The disorder of the system increases entropy increases i.e. solid has less entropy than liquid which has less entropy than vapour.absolute temp. of heat transfer

Entropy and spontaneity (second law of thermodynamics)

                    The total entropy of system and its surrounding increase in a spontaneous process.

                   ∆S_universe = ∆S _total = ∆S_sys + ∆S_surr >0

                             

Significance of ∆S

i)            ∆S total  >0 , process is spontaneous

ii)           ∆S total  <0 , process is non spontaneous

iii)         ∆S total  =0 , process is equilibrium