Question

Which of the following is not a case of chemical equilibrium?

• A. ${\mathrm{N}}_2{\mathrm{O}}_4\mathrm{gives}2{\mathrm{NO}}_2$
• B. ${\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{aq}\right)\mathrm{is}\mathrm{aqueous}\mathrm{solution}\mathrm{of}{\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{s}\right)$
• C. ${\mathrm{N}}_2+{\mathrm{H}}_2\mathrm{gives}2{\mathrm{NH}}_3$
• D. ${\mathrm{PCl}}_5\mathrm{gives}{\mathrm{PCl}}_3+{\mathrm{Cl}}_2$

Answer 1

The correct option is B

${\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{aq}\right)\mathrm{is}\mathrm{aqueous}\mathrm{solution}\mathrm{of}{\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{s}\right)$

Explanation for correct option:

B. ${\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{aq}\right)\mathrm{is}\mathrm{aqueous}\mathrm{solution}\mathrm{of}{\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{s}\right)$

Chemical equilibrium

• In the case of chemical equilibrium, the rates of the forward and reverse reactions become equal.
• In this case, the concentrations of the reactants and the products remain constant.
• Chemical equilibrium is dynamic in nature wherein concentrations of both reactant and product change constantly with equal speed.
• It is a reversible reaction in which a change in a substance can be reversed by changing the conditions.
• Thus, neither the forward nor the reverse reaction has stopped.

The reaction for the conversion of an aqueous solution of ammonium chloride to solid ammonium chloride does not represent equilibrium as the rate of forward and backward reaction is not equal.

$$\begin{gathered} {\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{aq}\right)\to {\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{s}\right) \\ (\mathrm{Aqueous}\mathrm{solution}\mathrm{of}(\mathrm{Ammonium}\mathrm{chloride}) \\ \mathrm{Ammonium}\mathrm{chloride}) \end{gathered}$$

Explanation for incorrect options:

A. ${\mathrm{N}}_2{\mathrm{O}}_4\mathrm{gives}2{\mathrm{NO}}_2$

• There exist an equilibrium between dinitrogen tetraoxide and nitrogen dioxide
• The chemical equilbrium can be depicted as:

$$\begin{gathered} {\mathrm{N}}_2{\mathrm{O}}_4\left(\mathrm{g}\right)\rightleftharpoons {\mathrm{NO}}_2\left(\mathrm{g}\right) \\ (\mathrm{Dinitrogen}\mathrm{tetraoxide})(\mathrm{Nitrogen}\mathrm{dioxide}) \end{gathered}$$

C. ${\mathrm{N}}_2+{\mathrm{H}}_2\mathrm{gives}2{\mathrm{NH}}_3$

• The reaction for the formation of ammonia shows equilibrium.
• The chemical equilbrium can be depicted as:

$$\begin{gathered} {\mathrm{N}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right)\to 2{\mathrm{NH}}_3\left(\mathrm{g}\right) \\ \left(\mathrm{Nitrogen}\right)\left(\mathrm{Hydrogen}\right)\left(\mathrm{Ammonia}\right) \end{gathered}$$

D. ${\mathrm{PCl}}_5\mathrm{gives}{\mathrm{PCl}}_3+{\mathrm{Cl}}_2$

• There exist an equilibrium when Phosphorus pentachloride dissociates into phosphorus trichloride and chloride.
• The chemical equilibrium can be depicted as:

$$\begin{gathered} {\mathrm{PCl}}_5\left(\mathrm{g}\right)\rightleftharpoons {\mathrm{PCl}}_3\left(\mathrm{g}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right) \\ (\mathrm{Phosphorus}\mathrm{pentachloride})(\mathrm{Phosphorus}\mathrm{trichloride})\left(\mathrm{Chlorine}\right) \end{gathered}$$

Hence, the correct option is B i.e. ${\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{aq}\right)\mathrm{is}\mathrm{aqueous}\mathrm{solution}\mathrm{of}{\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{s}\right)$.