The complexion of Fe^2+ with the chelating agent dipyridyl has been studied kinetically in both the — Chemical Equilibrium Chemistry Question
Question
The complexion of Fe^2+ with the chelating agent dipyridyl has been studied kinetically in both the forward and reverse directions: Fe^2+ + 3 dipy ⇌ [Fe(dipy)3]^2+ For this reaction, the rates of forward and reverse reactions are (1.45 × 10^13 M^-3 s^-1)[Fe^2+][dipy]^3 and (1.22 × 10^-4 s^-1)[Fe(dipy)3^2+], at 25°C. What is the stability constant of the complex?
💡 Solution & Explanation
Reaction: $\text{Fe}^{2+} + \text{dipyridyl} \rightleftharpoons [\text{Fe(dipyridyl)}^{2+}]$ (simplified) At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction: \[ k_f [\text{Fe}^{2+}][\text{dipyridyl}]^3 = k_b [\text{complex}] \] The stability constant (formation constant) $K_{\text{stab}}$ equals the ratio of forward to reverse rate constants: \[ K_{\text{stab}} = \frac{[\text{complex}]}{[\text{Fe}^{2+}][\text{dipyridyl}]^3} = \frac{k_f}{k_b} \] Given: $k_f = 1.45\times10^{13}\ \text{M}^{-3}\text{s}^{-1}$, $k_b = 1.22\times10^{-4}\ \text{s}^{-1}$ \[ K_{\text{stab}} = \frac{1.45\times10^{13}}{1.22\times10^{-4}} = \frac{1.45}{1.22}\times10^{17} \approx 1.19\times10^{17}\ \text{M}^{-3} \] \textbf{Answer: C} — $K_{\text{stab}} \approx 1.19\times10^{17}\ \text{M}^{-3}$