KINETICS AND EQUILIBRIUM Writing the concentration equilibrium expression for heterogeneous equilibrium Write the concentration equilibrium constant expression for this reaction (aq) + H2O (aq) → HF (aq) + H2O (l)
The Correct Answer and Explanation is:
Correct answer:
The concentration equilibrium expression for the reaction is:
Kc = [HF] / [F⁻]
Explanation
In chemical equilibrium, the equilibrium constant expression (denoted as Kc when using concentrations) is derived from the balanced chemical equation. It represents the ratio of the concentrations of products to reactants, each raised to the power of their stoichiometric coefficients.
In this case, the reaction is:
F⁻ (aq) + H₂O (l) ⇌ HF (aq) + OH⁻ (aq)
Note: It seems there was an issue in the question formatting. Since the original question shows HF and water on the right-hand side and “aq” on the left, this is most likely the fluoride ion reacting with water:
F⁻ (aq) + H₂O (l) ⇌ HF (aq) + OH⁻ (aq)
However, if the reaction given is:
F⁻ (aq) + H₂O (l) ⇌ HF (aq) + H₂O (l)
That would be chemically inconsistent, as water would not appear unchanged on both sides with HF formation. So, we assume the proper form is:
F⁻ (aq) + H₂O (l) ⇌ HF (aq) + OH⁻ (aq)
This is a heterogeneous equilibrium because water is a pure liquid. In equilibrium expressions, pure solids and pure liquids (like water in liquid form) are not included because their concentrations remain constant throughout the reaction.
Therefore, the equilibrium expression is written as:
Kc = [HF][OH⁻] / [F⁻]
If the original reaction only involved F⁻ and water producing HF, as in:
F⁻ (aq) + H₂O (l) ⇌ HF (aq)
Then the equilibrium expression would simply be:
Kc = [HF] / [F⁻]
This follows the same rule: exclude water since it is a liquid. The square brackets indicate molar concentration. This expression allows chemists to calculate or predict concentrations of species at equilibrium.
