Question

How would you use Le Chatelier’s principle to explain different colors found in the following equilibria:-
$N{{i}^{2+}}_{(aq)}+6N{{H}_{3(aq)}} \to {{[Ni{{(N{{H}_{3}})}_{6}}]}^{2+}}_{(aq)}$

Answer 1

We know that $N{{i}^{2+}}$ has light green color and when it forms ${{[Ni{{(N{{H}_{3}})}_{6}}]}^{2+}}$ complex on addition of ammonia, the solution turns to purple/blue color. This color change can be disrupted easily by addition of different reactants.

Complete step-by-step answer: “Le Chatelier’s principle states that a change in any of the factors that determine the equilibrium conditions of a system will cause the system to change in such a manner so as to reduce or counteract the effect of the change”. This principle is applicable to all the physical and chemical equilibria.
We have been provided the following chemical equilibria:-
$N{{i}^{2+}}_{(aq)}+6N{{H}_{3(aq)}} \to {{[Ni{{(N{{H}_{3}})}_{6}}]}^{2+}}_{(aq)}$
Here we observe that, when we have an aqueous solution of nickel nitrate, it dissociates into $N{{i}^{2+}}_{(aq)}\text{ and }N{{O}_{3}}{{^{+}}_{(aq)}}$and gives light green color. When $N{{H}_{3}}$ is added in the solution, it starts giving blue/purple color due to the formation of ${{[Ni{{(N{{H}_{3}})}_{6}}]}^{2+}}$ complex and at equilibrium, blue/purple color becomes constant. This equilibrium can be shifted in either forward or backward direction depending on our choice of adding a reactant or a product. This is called the effect of concentration on the equilibrium.
So here we use Le Chatelier’s principle as follows:-
“When the concentration of any of the reactants or products in a reaction at equilibrium is changed, the composition of the equilibrium mixture changes so as to minimize the effect of concentration changes.”
Let us apply the same to the given chemical equilibria:-
The equilibrium can be shifted in backward direction by the removal of $N{{i}^{2+}}$ ions or $N{{H}_{3}}$ molecules.
-If we add HCl in the solution, it reacts with $N{{H}_{3}}$ to give $N{{H}_{4}}Cl$ and hence decrease the concentration of $N{{H}_{3}}$ molecules in the solution. To remove the concentration stress, there occurs the dissociation of ${{[Ni{{(N{{H}_{3}})}_{6}}]}^{2+}}$ and hence it replenishes back the concentration of $N{{H}_{3}}$molecules (the reaction moved in backward direction). Due to this the concentration of ${{[Ni{{(N{{H}_{3}})}_{6}}]}^{2+}}$ decreases and $N{{i}^{2+}}$ increases. Therefore the color of the solution goes back from dark blue/purple to light green color.
- Similarly, if we add ammonia or a nickel nitrate to the solution, the reaction goes in a forward direction and hence gives us dark blue/purple color.

Note: Always remember that we can shift the equilibrium in any direction just by adding or removing the concentration of the reactant or product because according to the Le Chatelier’s principle it will always want to remove the concentration stress of addition or removal of reactant/product.