In strongly acidic solution, the dark red complex Fe(phen)3 2+ dissociates to the nearly colorless F — Kinetics Chemistry Question
Problem Context
In strongly acidic solution, the dark red complex Fe(phen)3 2+ dissociates to the nearly colorless Fe(H2O)6 2+.
Fe(phen)3 2+ + 3 H3O+ + 3 H2O → Fe(H2O)6 2+ + 3 phenH+
phen =
The reaction was carried out at 42.3 ºC and the concentration of Fe(phen)3 2+ was determined using visible absorption spectroscopy. Plots of the natural logarithm of [Fe(phen)3 2+] as a function of time are shown at sulfuric acid concentrations of 0.50 M (circles) and 0.10 M (squares).
What is the order of the reaction with respect to Fe(phen)3 2+? Explain your answer.
Model Answer
ln([Fe(phen)3 2+]) decreases linearly with time, which is characteristic of a 1st-order process, so the order in Fe(phen)3 2+ is 1.
What is the order of the reaction with respect to H3O+? Explain your answer.
Model Answer
The slope of the line does not depend on the concentration of H+, which says that the rate is independent of [H+]. Therefore the order in H+ is zero.
What is the value of the rate constant at 42.3 °C?
Model Answer
The slope of the line is –k. From the graph, the slope is about (–4)/(30 min), so k = 0.13 min-1 = 2.2 × 10-3 s-1.
The experiment is run again, but at 52.3 °C. The rate constant is determined to be 3.5 times larger than the rate constant at 42.3 °C. What is the activation energy for the reaction?
Model Answer
ln(3.5) = (Ea/8.314 J/mol•K)([1/315.4 K] – [1/325.4 K])
Ea = 107 kJ/mol
Coordination complexes can be viewed as arising via a Lewis acid-base interaction. Sketch the structure of the complex Fe(phen)3 2+ and identify the Lewis acid and Lewis base. What is the coordination geometry around the iron?
Model Answer
Phen is the Lewis base, Fe2+ the Lewis acid. The coordination geometry at Fe is octahedral.