NO2 and N2O4 undergo the reaction shown. When a sealed

container of NO2 reaches chemical equilibrium, which must be true?

A) No N2O4 is present.

B) No chemical reactions are occurring.

C) The rates of the forward and reverse reactions are equal.

D) The maximum number of molecules has been reached.

Option C) The rates of the forward and reverse reactions are equal.

Explanation:

NO₂ and N₂O₄ undergo the following equilibrium reaction:

2NO₂ (g) ⇄ N₂O₄ (g)

That is a reversible reaction, i. e. there are two simultaneous reactions: the direct or forward reaction and the reverse reaction:

Direct reaction: 2NO₂ (g) → N₂O₄ (g)

Reverse reaction: 2NO₂ (g) ← N₂O₄ (g)

At the beginning, only NO₂ (g) is in the sealed container. The NO₂ concentration is maximum, and the rate of the forward reaction is maximum.

As the reaction progresses, the concentration of NO₂ diminishes, and, consequently, the rate of the forward reaction decreases.

As soon as the N₂O₄ appears, the reverse reaction starts. At the beginning the rate is low, but as the N₂O₄ concentration increases the rate of the reverse reaction increases.

When both forward and reverse rates become equal the equilibrium has been reached. This is what is called a dynamical equilibrium.

Then, as per the choices, you have that, at equilibrium:

A) No N₂O₄ is present:

False: as explained above, at equilibrium both NO₂ and N₂O₄ are present.

B) No chemical reactions are occurring.

False: as explained above, at equilibrium both forward and reverse reaction are occurring at the same rate.

C) The rates of the forward and reverse reactions are equal.

True: as explained, this is the meaning of dynamic equilibrium.

D) The maximum number of molecules has been reached.

False: the number of molecules of each compound at equilibrium will be given by the constant of equiibrium, Keq = [N₂O₄] / [NO₂]², and this value varies with the temperature.