Chapter 14: Q14.3-51 E (page 831)

Which of the following will increase the percentage of \({\rm{N}}{{\rm{H}}_3}\)that is converted to the ammonium ion in water (Hint: Use Le Châtelier's principle)?
(a) Addition of \(NaOH\)
(b) Addition of \(HCl\)
(c) Addition of \(N{H_4}Cl\)

Expert verified

The addition of \({\rm{HCl}}\) (b) will increase the percentage of ammonia.

Step by step solution

The Le Chatelier's principle says that when a system is put out of balance by a change of any kind, the system reacts in a way that minimizes that same change.
It is clear that increasing the number of products will "push" the equilibrium towards the products, and increasing the number of products will "push" the equilibrium towards the reactants.
Similarly, decreasing the number of products or reactants work in the opposite way.
Hence, if we intend to increase the percentage of ammonia that is converted to the ammonium ion in water, we need to either increase the concentration of one of the reactants or decrease the concentration of one of the products.

a) The addition of \({\rm{NaOH}}\)does not increase, but decrease the percentage of ammonia converted as the increasing number of products \(\left( {{\rm{O}}{{\rm{H}}^ - }} \right)\)pushes the equilibrium towards the reactants.
b) The addition of \({\rm{HCl}}\)does increase the percentage of ammonia converted because the \({{\rm{H}}^ + }\)ions from the hydrochloric acid react with the \({\rm{O}}{{\rm{H}}^ - }\)ions and decrease their concentration. Hence, adding the \(HCl\)decreases the number of products and thus, pushes the equilibrium towards the products.
c) The addition of \({\rm{N}}{{\rm{H}}_4}{\rm{Cl}}\) does not increase, but decreases the percentage of ammonia converted as increasing the number of products \(\left( {{\rm{NH}}_4^ + } \right)\) pushes the equilibrium towards the reactants.
Hence, option (b) is the right answer. The addition of \({\rm{HCl}}\) will increase the percentage of ammonia.

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