A reversible reaction never reaches the completion stage but results in an equilibrium state in which concentrations of all the reactants and products become constant. This equilibrium is dynamic in nature and it is the result of two opposite reactions occurring simultaneously and at the same rate. However, no net change occurs in the system.

According to Le Chatelier’s Principle: when any system at equilibrium is disturbed by changing concentrations, temperature or pressure, the equilibrium shifts either in forward or backward direction and a net change occurs in it so as to decrease the effect of the disturbing factor.

Consider the following equilibrium

A + B ⇌ C + D

The equilibrium constant for it is given by

K = [C][D]/[A][B]

When concentration of A or B is increased, the equilibrium will shift in forward direction, so that some quantity of A and B is consumed and corresponding quantities of products C and D are formed. When concentration of A or B is decreased, the equilibrium will shift in backward direction. Now quantity of C and D is consumed and corresponding quantities of reactants A and B are produced.

Similarly when concentration of products C or D is increased, the equilibrium will shift in the backward direction and when it is decreased it will shift in the forward direction.

Ferric ions react with thiocyanate ions to form a blood red coloured complex, [Fe(CNS)]2+, and the following equilibrium is established.

Fe3+ (aq) + CNS- (aq) ⇌ [Fe(CNS)]2+ (aq)

The effect of change in concentration of Fe3+ or CNS ions on this equilibrium can be studied easily. If this equilibrium shifts in forward direction, more quantity of the complex will be formed and the colour of the solution will become more intense. Similarly, when the equilibrium shifts in backward direction, some quantity of the complex will break and the colour of the solution will become light.

Experiment: To study the shift in equilibrium between ferric ions and thiocyanate ions by increasing or decreasing the concentration of these ions.

How To Perform Experiment

1. Take five clean and dry test tubes, number them from 1 to 5 and keep them in a test tube stand. Mark the test tube number 3 as Reference Tube.

2. Take 10 mL Fe(NO3)3 solution (0.02 M) in a clean and dry 50 mL measuring cylinder. Fill it up with distilled water upto 40 mL mark and stir to mix. Take 5 mL of this diluted (0.005 M) Fe(NO3)3 solution in each of the five test tubes.

3. In a clean and dry 10 mL measuring cylinder, take 10 mL KCNS solution (0.02 M). Out of it, add 5 mL solution to test tube number 1.

4. The measuring cylinder now contains 5 mL of 0.02 M KCNS solution. Add distilled water to make the volume up to 10 mL mark in the measuring cylinder with a wash bottle and stir to mix. You get KCNS solution of 0.01 M concentration. Pour 5mL of 0.01 M KCNS solution into test tube number 2.

5. Add distilled water in the remaining solution to make the volume upto 10 mL which will give a solution 0.005 M and transfer 5 ml of this solution to test tube number 3.

6. Again add distilled water in the remaining solution in the measuring cylinder to obtain a 10 mL solution of 0.0025 M KCNS and transfer 5 mL of this solution to test tube number 4.

7. Similarly, get 10 ML of 0.00125 M solution by adding distilled water to the measuring cylinder containing 0.0025 M KCNS solution. Transfer 5 mL of this solution to the fifth test tube.

8. Shake all the five test tubes to mix the solutions and keep them back on test tube stand.

9. Compare the intensity of red colour in test tube number 1 with that in Reference Tube (Number 3). For this purpose, hold the two tubes against white background of a glazed tile or a plain white paper. Find out whether the colour intensity is more or less than the Reference tube and record it.

10. Similarly, compare the intensity of red colour in the remaining test tubes numbered 2, 4 and 5.

11. Similarly, you can study the effect of [Fe3+] on the equilibrium by performing another experiment where you vary the concentration of Fe3+ while keeping the concentration of [CNS-] constant. For this, use KCNS solution in step 2 and Fe(NO3)3 solution in steps 3 to 8.

Precautions

  1. All glass apparatus (test tubes and measuring cylinders) should be thoroughly clean, otherwise the solutions will stick to their sides and not mix properly.
  2. Volumes of solutions and distilled water should be measured carefully so that the solution of desired concentration can be obtained.
  3. Mixing of solution and water or of two solutions should be done by gentle shaking and avoiding their splashing.