What was the evidence for a shift in equilibrium when conc. hydrochloric acid wa

Question

What was the evidence for a shift in equilibrium when conc. hydrochloric acid was added to the cobalt chloride solution? (a) Write the equilibrium equation for this system. (b) State whether the concentration of each of the following substances was increased, decreased, or unaffected when the cone, hydrochloric acid was added to cobalt chloride solution. (a) What did you observe when ammonium chloride was added to cobalt chloride solution? (b) What did you observe when this mixture was heated? (c) Explain why heating the mixture caused the equilibrium to shift. (d) What did you observe when the mixture was cooled? (e) Explain why cooling the mixture caused the equilibrium to shift.

Explanation / Answer

1) on addition of concentrated HCl the pink coloured solution will turn to blue coloured solution. So it shows that the equilibrium shifts toward forward direction.

2) the balanced chemical equation will be

[Co(H2O)6]2+(aq)+ 4Cl-(aq) === [CoCl4]2-(aq) + 6H2O(l)

       (pink)                                    (blue)

3) As chloride ions are on the left hand side of equilibrium and we have added more of chloride ions to the equilibrium , then as per Le-Chatelier's principle the equilibrium will shift in the direction where it can nullify the change.

so more of [Co(H2O)6]2+(aq) will react with Chloride ions to give more of [CoCl4]2-

hence

concentration of [Co(H2O)6]2+will decrease while that of [CoCl4]2-   will increase.

The concentration of Cl- is increased thats why the equilibrium is shifting, till it becomes constant

3) the addition of ammonium chloride will cause oxidation of Co(II) complex to Co(III) complex

the reaction will be

4 CoCl2·6H2O + 4 NH4Cl + 20 NH3 + O2 4 [Co(NH3)6]Cl3 + 26 H2O

so blue colour will disapper slowly

b) The reaction of conversion of [Co(H2O)6]2+ to [CoCl4]2- is an endothermic reaction so on heating the solution the blue colour will get intensified as the equilibrium will shift in forward direction.

c) The reaction is endothermic so heating causes shift in the equilibrium

d) If we cool the mixture then the equilibrium will shift in backward direction, and pink colour will appear

e) As it is an endothermic reaction, so cooling will shift the equilibrium where the temperature can be rasied (heat can be released) which is possible in backward direction.

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