Effect of Concentration on Ferric & Thiocyanate Equilibrium

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

K_c = [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 K_c 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 K_c 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.