Question

consider the equilibrium system below.
nh₄cl(s)↔nh₄cl(g)

if the system is at dynamic equilibrium, which statement is true?
○ sublimation of the nh₄cl crystals stops.
○ solidification of the nh₄cl vapors stops.
○ the rate of sublimation of the nh₄cl crystals is equal to the rate of solidification of the nh₄cl vapors
○ the rate of sublimation of the nh₄cl crystals is higher than the rate of solidification of the nh₄cl vapors

Explanation:

In a dynamic equilibrium for a phase - change process like the sublimation - solidification of $\ce{NH4Cl}$ (where $\ce{NH4Cl(s)
ightleftharpoons NH4Cl(g)}$), the key concept is that the forward and reverse reactions occur at the same rate. Sublimation is the forward process (solid to gas) and solidification (gas to solid) is the reverse process.

• Option 1: In dynamic equilibrium, sublimation does not stop. The solid is still subliming, so this is incorrect.
• Option 2: Solidification of vapors also does not stop. The gas is still solidifying, so this is incorrect.
• Option 3: At dynamic equilibrium, the rate of the forward reaction (sublimation of $\ce{NH4Cl}$ crystals) is equal to the rate of the reverse reaction (solidification of $\ce{NH4Cl}$ vapors). This matches the definition of dynamic equilibrium.
• Option 4: If the rate of sublimation was higher than solidification, the system would not be at equilibrium (more solid would be turning into gas over time), so this is incorrect.

Answer:

The rate of sublimation of the $\ce{NH4Cl}$ crystals is equal to the rate of solidification of the $\ce{NH4Cl}$ vapors