Determination of the equilibrium constant for the reaction

The product complex ion is the only one of the three species which has an appreciable color (blood-red).

6.  Relevant Equations/Chemical Reactions Involved :
                Fe3+(aq) + SCN-(aq) ⬄ FeSCN2+(aq)

7.  Chemicals :
   0.002 M   KSCN(aq)        50 cm3
   0.2 M     Fe(NO3)3(aq)   20 cm3

8.  Apparatus and equipment :

9. Procedure :

1. 0.2 M Fe(NO3)3(aq) was used and 10 cm3 of 0.08 M, 0.032 M, 0.0128 M, 0.00512 M Fe(NO3)3(aq) were prepared respectively.
2. The solutions were added by using 10 cm3 measuring cylinder to 5 clean boiling tubes in a rack as below.

3. Tubes 1 and 2 were wrapped around with a strip of paper so that light was excluded from the side.
4. The solutions towards the lamp were looked vertically down to compare the colour intensity.
5. Some of the solution from the standard tube 1 was removed with a dropper until the colour intensities of solutions in both tubes were the same. And the removed liquid was put into a clean and dry beaker.
6. The depth of the solution in tube 1 was then measured.
7. Steps 3 to 6 were repeated with tubes 1 and 3, 1and 4 and finally 1and 5.

10. Observations :

The colour intensities of the solutions in the two tubes being observed were the same.

11. Data, Calculation and Results :

• Height of solution in original tube 1 = 59 mm
• Height of solution in tube 1 having the same colour intensity as tubes 2, 3, 4 and 5 are :

12. Conclusion :

 The equilibrium constant for the reaction : Fe3+(aq) + SCN-(aq) ⬄ FeSCN2+(aq) was found to be 82.1 M-1.

13. Discussion :

1. By M1V1 = M2V2,

For tube 1, initial concentration of SCN-(aq) = 0.001 M

         initial concentration of Fe3+(aq) = 0.1 M

For tube 2, initial concentration of SCN-(aq) = 0.001 M

         initial concentration of Fe3+(aq) = 0.04 M

For tube 3, initial concentration of SCN-(aq) = 0.001 M

         initial concentration of Fe3+(aq) = 0.016 M

For tube 4, initial concentration of SCN-(aq) = 0.001 M

         initial concentration of Fe3+(aq) = 0.0064 M

For tube 5, initial concentration of SCN-(aq) = 0.001 M

         initial concentration of Fe3+(aq) = 0.00256 M

2. Concentration of FeSCN2+ in tube 1 = 0.001 M
3. Ratio of depth in the colour comparison with tube 2
   = 45 /59
   = 0.763
   Ratio of depth in the colour comparison with tube 3
   = 35 /59
   = 0.593
   Ratio of depth in the colour comparison with tube 4
   = 28 /59
   = 0.475
   Ratio of depth in the colour comparison with tube 5
   = 23 /59
   = 0.390

Equilibrium concentration of thiocyanoiron(III) ions:

In tube 2 = 0.001 x (45 / 59) = 7.63 x 10-4 M

In tube 3 = 0.001 x (35 / 59) = 5.93 x 10-4 M

In tube 4 = 0.001 x (28 / 59) = 4.75 x 10-4 M

In tube 5 = 0.001 x (23 / 59) = 3.90 x 10-4 M

        4.  [Fe3+]equil = [Fe3+]initial - [FeSCN2+]equil
                  = 0.04 - 7.63 x 10-4
                            = 0.0392 M
          [SCN-]equil = [SCN-]initial - [FeSCN2+]equil

                    = 0.001 –7.63 x 10-4
                      = 2.37 x 10-4 M

5.  Room temperature = 16 oC
           K = [FeSCN2+]equil / ([Fe3+]equil[SCN-]equil)

      = 7.63 x 10-4 / [(0.0392)(2.37 x 10-4)]
     = 82.1 M-1

        6.  There are few sources of error in this experiment.
(1) Determination of colour intensity in the two tubes observed might       not be accurate.
(2) Taking reading when measuring the depth of liquid.
  Error estimation -
  When taking initial reading, error is + 0.05 cm3.
  When taking final reading, error is also + 0.05 cm3.
  Therefore, error is + 0.1 cm3.