Question

For the given reaction, choose the correct expression of \(K_c\)​ from the following: \(Fe_{(aq)}^{3+}​+SCN_{(aq)}^{−}​⇌(FeSCN)_{(aq)}^{2+}\)​

• A. \( K_c = \frac{[\text{FeSCN}^{2+}]}{[\text{Fe}^{3+}][\text{SCN}^-]} \)
• B. \( K_c = \frac{[\text{Fe}^{3+}][\text{SCN}^-]}{[\text{FeSCN}^{2+}]} \)
• C. \( K_c = \frac{[\text{FeSCN}^{2+}]}{[\text{Fe}^{3+}]^2[\text{SCN}^-]^2} \)
• D. \( K_c = \frac{[\text{FeSCN}^{2+}]^2}{[\text{Fe}^{3+}][\text{SCN}^-]} \)

Answer 1

The Correct Option is A

The equilibrium constant, \(K_c\), for a reaction is given by the ratio of the concentration of the products to the concentration of the reactants, each raised to the power of their respective stoichiometric coefficients in the balanced chemical equation.

For the reaction: 

\(\text{Fe}_{(aq)}^{3+} + \text{SCN}_{(aq)}^{-} \rightleftharpoons (\text{FeSCN})_{(aq)}^{2+}\),

the balanced chemical equation shows one mole of each \( \text{Fe}^{3+} \) and \( \text{SCN}^- \) reacting to form one mole of \( \text{FeSCN}^{2+} \). Thus, for this reaction, the expression for the equilibrium constant \( K_c \) is:

\(K_c = \frac{[\text{FeSCN}^{2+}]}{[\text{Fe}^{3+}][\text{SCN}^-]}\).

Now, let's evaluate the given options:

• \(K_c = \frac{[\text{FeSCN}^{2+}]}{[\text{Fe}^{3+}][\text{SCN}^-]}\): This expression matches our derived formula based on the stoichiometry of the reaction. This is the correct option.
• \(K_c = \frac{[\text{Fe}^{3+}][\text{SCN}^-]}{[\text{FeSCN}^{2+}]}\): This option represents the inverse of the correct expression, which is incorrect.
• \(K_c = \frac{[\text{FeSCN}^{2+}]}{[\text{Fe}^{3+}]^2[\text{SCN}^-]^2}\): This option incorrectly squares the concentrations of the reactants, which is not in line with the stoichiometry of the reaction.
• \(K_c = \frac{[\text{FeSCN}^{2+}]^2}{[\text{Fe}^{3+}][\text{SCN}^-]}\): This option incorrectly squares the concentration of the product, which is not reflected in the balanced chemical equation.

Therefore, the correct expression for the equilibrium constant \( K_c \) is:

\(K_c = \frac{[\text{FeSCN}^{2+}]}{[\text{Fe}^{3+}][\text{SCN}^-]}\).

Answer 2

The equilibrium constant \( K_c \) is given by the ratio of the concentration of products to the concentration of reactants.

\[ K_c = \frac{[\text{FeSCN}^{2+}]}{[\text{Fe}^{3+}][\text{SCN}^-]} \]