Aim of experiment: To determine  DG, DS and DH of a galvanic cell using potentiometric measurement.

Theory: The cell reaction is,

                2 Fe3+(aq) + 2 Hg (s) + 2 Cl- (aq) -------  2 Fe2+(aq) + Hg2 Cl2 (s)

The cell to be used is,

                Hg (s) | Hg2 Cl2 (s) |  Cl- (sat. aq.) | | Fe3+(0.1M) , Fe2+ (0.1M) | Pt

The e.m.f. of cell is:

                Ecell  = E ( Fe3+ , Fe2+ | Pt )  - E SCE  - (2.303 RT / n F) log ([Fe2+]2 / [Fe3+]2)  (where n = 2)

Again, decrease in free energy is given by,

                - DG = n Ecell F

                So DG =  - 2 Ecell F   (since  n=2)

The value of DS and DH can be found according to the following two equation :

                    DS = n F (d Ecell / dT )

                          = n F [{ E cell (h.t. )  - E cell (r. t. )}/ ( Th.t.  - T r.t.) ]

Where h.t. denotes a (somewhat) higher temperature and r.t. denotes room temperature .

And,             DH = DG + T DS     where T = room temperature.

Requirements:   1)Galvanic cell

                            2)Potentiometer

                            3)0.1 M Fe2+ , 0.1 M Fe3+ in 0.1 M  H2SO4

Procedure: 1) The potentiometer is set up and standardized using Weston Cadmium Cell.

                    2) The e.m.f. of the cell is then measured at three temperatures  viz.  room temperature,   room temperature + 100 C,  room temperature + 200 C.

Observation and calculation: 

                    Room temperature: ...............

 Table for readings of potentiometric measurements :

   No. of observation       Temperature   Null point reading ( Lcell)  (in cm)
     1    
     2    
     3    

Again, 

                    E cell  = ( Estd / Lstd). L cell

where, Estd  = e.m.f. of the standard (Weston cadmium) cell 

            Lstd  = Length of null point for the standard  (Weston cadmium) cell

Lstd can be determined from the potentiometer by replacing the galvanic cell  by Weston cadmium cell and Estd can be found out from literature.

            From literature , Estd = ........

             From potentiometric measurements, Lstd  = .........

Determination of E cell at the three temperatures :

Using   E cell  = ( E std / L std )   . L cell

1)................................

2)........................

3)  .....................  

Determination of DG:

DG = - n E cell F  ; E cell = e.m.f. of the cell at room temperature.

       = ....... ............

Determination of DS:

 Sl. no.  Temperature (0C)   E cell

   ( V )

 dE / dT = 
[{E h.t - E r.t. }/ (Th.t - Tr.t ) ]		     D S    Mean
  1
  2
  3

                 So the value of D S :.........

Determination of DH:

We know that ,  DH = DG + T DS ; T = room temperature.

So  DH = ............................ (putting the values of  DG ,T and  DS ).