Page 34 - Chemistry - XI

Q5. Defi ne a reversible reaction with an example.
Ans. Reversible reaction is a reaction in which products can react to form reactants back, e.g.

Ng () + 3 Hg () 2 NH g ()

2 2 3
EXPERIMENT - 14
Objective

To study the eff ect of concentration on chemical equilibrium between [Co(H O) ] and Cl ions.
–
2+
2 6
Principle/Theory
Co ion reacts with water molecules to form water soluble pink complex [Co(H O) ] .
2+
2+
2 6
Co (aq) + 6H O(l) [Co(H O) ] 2+
2+
2 2 6
Pink
In the reaction between [Co(H O) ] and Cl ions, pink colour of [Co(H O) ] changes to blue colour due
2+
–
2+
2
2
6
6
to the formation of [CoCl ] .
2–
4
[Co(H O) ] + 4Cl [CoCl ] + 6H O
–
2–
2+
2 6 4 2
Pink Blue
This reaction is known as ligand displacement reaction. The equilibrium constant for the reaction will
[[CoCl ] ]
2–
be K = 4
c
[[Co(H O) ] ][Cl ]
2+
– 4
2 6
If we increase Cl ions concentration by adding HCl, the rate of formation of [CoCl ] ions and H O
–
2–
2
4
increases, concentration of [Co(H O) ] ions decreases and therefore, intensity of blue colour increases,
2+
6
2
If concentration of Cl ions is decreased, the equilibrium shifts to the left hand side, the intensity of blue
–
colour decreases and pink colour increases.
If we increase concentration of H O, intensity of blue colour decreases and intensity of pink colour
2
increases.
Materials Required
Boiling tubes, test tubes, test tube stand, graduated dropper, glass rod, beaker, weight box, measuring fl ask,
conc. HCl, and cobalt chloride
Procedure
1. Dissolve 2 g of cobalt chloride in 25 mL of distilled water.
2. Take 10 mL of distilled water in two boiling tubes marked A and B.
3. Add 5 drops of conc. HCl solution in boiling tube A and 5 drops of cobalt chloride solution in boiling
tube B.
4. Shake these two solutions separately and then mix together. A light pink colour will be developed due
to formation of [Co(H O) ]Cl .
2 6 2
5. Take 1 mL of pink colour solution in a beaker and add 20 mL of distilled water and shake the solution
well. well.

6. Divide the solution into seven equal parts in the test tubes a, b, c, d, e, f and g.Divide the solution into seven equal parts in the test tubes a, b, c, d, e, f and g.
6.
7. Keep the solution in the test tube a for comparing its colour with the solution of test tube b.Keep the solution in the test tube a for comparing its colour with the solution of test tube b.
7.
32