THEORY:
Ferric ions and thiocyanate ions react to establish the following equilibrium with the formation of the blood red ferric thiocyanate complex [Fe (CNS)]++
Fe+++(aq) + CNS– (aq) —right left harpoons——–> [Fe (CNS)]++(aq) …. (1)
pale yellow colorless blood red
The equilibrium constant K, for the above equilibrium is given by
Kc = [Fe(CNS)]++ / [Fe+++] [CNS–] …..(2)
Where, [Fe+++], [CNS–] and [Fe(CNS)]++ represent the concentrations of the respective ions at equilibrium. We know that for a particular reaction, the value of the equilibrium constant Kc remains constant at a particular temperature irrespective of the changes in the concentrations of the species involved.
(i) Increasing the concentration of ferric ions :
When ferric ions are added to the above system at equilibrium, the concentration of [CNS–] should decrease and/or the concentration of [Fe(CNS)]++ should increase. Then only the value of Kc remains constant as shown by equation (2) above. Now, this becomes possible only if more of CNS– ions combine with Fe+++ ions to give more of the [Fe (CNS)]++ complex.
Hence, increasing the concentration of Fe+++ ions shifts the equilibrium to the right direction (forward direction). As a result, the intensity of the blood red colour increases.
(ii) Increasing the concentration of thiocyanate ions :
Similarly, we can see that increasing the concentration of thiocyanate ions also shifts the equilibrium to the right direction. Hence, in this case also, the intensity of the blood red colour increases.
(iii) Increasing the concentration of potassium ions :
When K+ ions are added in the form of KC1 to the above system (1), the concentration of K+ ions increases. As a result, the ionisation of KCNS is lowered with consequent decrease in the concentration of CNS– tons (Common ion effect).
KCNS —right left harpoons—–>K+ + CNS– …….(3)
KCI ————->K+ + Cl– …………… (4)
Then, some of the (Fe (CNS)]++ complex dissociates back to supply some CNS ions. That is, the equilibrium (1) is shifted to the left. Hence, the intensity of the blood red colour decreases.
APPARATUS :
1) Test tubes
2) Test tube stand
3) Beakers
4) Measuring cylinders 5ml, 10 ml and 50ml
CHEMICALS:
1) FeCl3 solution (0.1M)
2) KCNS solution (0.1M)
3) KCI solution (0.1M)
PROCEDURE:
i) Take 10 ml of 0.1M FeCl3 in a clean 250ml beaker with the help of the measuring cylinder.
ii) Add 10 ml of 0.1M KCNS to it with the help of the same measuring cylinder when deep blood red color is obtained.
iii) Dilute the above deep red color by the addition of 50 ml of distilled water.
iv) Now, label four test tubes as 1, 2, 3, 4 and take 10 ml of the diluted red solution into each of them. Arrange the test tubes in the test tube stand.
v) Add 5 ml of distilled water to test tube No. 1
……………….0.1M FeCl3……………… 2
……………….0.1M KCNS……………….3
……………….0.1M KCL …………………4
Shake all the test tubes well.
vi) Use the color in test tube No. 1 as the reference color, and compare the intensity of colors in test tubes No. 2. 3 and 4 with that of test tube No. 1.
RESULTS:
| Test tube No. | Vol. of solution added to Equilibrium (1) | Change in colour | Change in concn of [Fe(CNS)]++ | Equilibrium shifts |
| 1 | 5 ml of water | Reference color | ||
| 2 | 5 ml of 0.1 m FeCl3 | Intensity increases | Increases | To the right |
| 3 | 5 ml of 0.1 m KCNS | Intensity increases | Increases | To the right |
| 4 | 5 ml of 0.1 m KC1 | Intensity decreases | Decreases | To the right |
CONCLUSION:
Increasing the concentration of any of the reactants (Fe+++ or CNS– ions) shifts the equilibrium to the right and decreasing the concentration of any of the reactants shifts the equilibrium to the left.