Solid ammonium chloride decomposes reversibly on heating to form two gases, in a sealed container at constant temperature: NH4Cl(s) NH3(g) + HCl(g) This is a heterogeneous equilibrium, since the solid and the two gases are not all in the same phase. Which expression is correct f…

AS Chemistry 9701: Writing Kc for a Heterogeneous Equilibrium

Step 1: Identify which species belong in the Kc expression

For a heterogeneous equilibrium, only species whose concentration can genuinely change, gases and species in solution, appear in the $K_c$ expression. A pure solid (or pure liquid) has an essentially fixed “concentration” (its density divided by its molar mass), which does not change as the reaction proceeds. This constant value is absorbed into $K_c$ itself, so the solid is omitted from the expression entirely, not written as a factor of 1.

Here, $\text{NH}_4\text{Cl(s)}$ is a pure solid, while $\text{NH}_3\text{(g)}$ and $\text{HCl(g)}$ are both gases, so only the two gases appear.

Step 2: Write the expression, products over reactants

Following the usual rule of products over reactants, each raised to its stoichiometric coefficient (both equal to 1 here), with the solid excluded:

$K_c=[\text{NH}_3\text{(g)}][\text{HCl(g)}]$

Why the other options are wrong

A: incorrectly includes the solid $\text{NH}_4\text{Cl}$ in the denominator; a pure solid’s concentration is constant and must not appear in the expression.
C: has the expression inverted, with the solid (which shouldn’t appear) in the numerator and the gaseous products in the denominator. Both features are wrong.
D: includes only the solid and omits both gaseous products, the opposite of the correct approach.

Final answer

$K_c=[\text{NH}_3\text{(g)}][\text{HCl(g)}]$ , option B.