Question

Fe(NO3)₃. 9H₂O in 0.5M HNO₃ forms a colorless hexa-aquocomplex with the chemical formula {Fe(H2O)₆}³⁺ (aq).

This complex can then undergo an equilibrium reaction with potassium thiocynate (KSCN) to form a red complex with the chemical formula {Fe(H2O)5SCN}²⁺ (aq). In order to make this reaction occur, the two chemicals need to be mixed with 0.5M nitric acid. This equilibrium reaction is shown below:

{Fe(H₂O)₆}^₃₊ (aq) + SCN^-(aq) <--------> {Fe(H₂O)₅SCN}²⁺(aq) + H₂O (l)

a) When recording the absorbance values, what should be used as a blank slution and how show its data be treated?

b) K_eq is quoted to have a value of 250 at 20°C. When an initial potassium thiocynate concentration of 2x10^-4M was used, the red {Fe(H2O)₅SCN}²⁺ (aq) complex had an absorbance value of 0.5 . If the molar absorptivity coefficient of the red complex is 5400 cm-¹M^-1 and a 1 cm cuvette was used, calculate the equilibrium concentrations of [{Fe(H20)₆}³⁺(aq)]eq, [SCN^-]eq , [{Fe(H2O)₅SCN}²⁺_(aq)]_eq

c) Briefly describe the experiment you might conduct to determine if this reaction is endothermic or exothermic. What sort of results would lead you to conclude it was one or the other?

Answer 1

Complexometric reaction

a) The black for the reaction would be a solution made only of Fe(NO3)3.9H2O and HNO3 in water.

This solution will not form any red complex and thus the complexed solutions with SCN- would have higher absorbance which is corrected by substrating the blank absorbance from it.

b) Concentration = absorbance/molar absorptivity x path length

So the the given data,

equilibrium concentration of [Fe(H2O)5(SCN)]2+ = 0.5/5400 x 1 = 9.26 x 10^-5 M

equilbrium concentration of [SCN-] = 2 x 10^-4 - 9.26 x 10^-5 = 1.07 x 10^-4 M

let the initial concentration of [Fe(H2O)63+ be 2 x 10^-4 M

so,

equilibrium concentration of [Fe(H2O)6]3+ = 1.07 x 10^-4 M

Keq = 9.26 x 10^-5/(1.07 x 10^-4)^2 = 8088

c) To determine If the complex formation reaction is exothermic (giving off heat) or endothermic (consumption of heat),

When the complex formed in solution is heated, the color changes from deep-red (due to [Fe(H2O)5(SCN)]2+) to lighter (due to [Fe(H2O)6]3+ formation). Thus the reverse reaction occurs when the temperature is raised, that is the forward reaction is exothermic in nature. this is according to LeChatellier's principle. The reverse reaction is endothermic as it proceeds when temperature is increased by heating the solution.

Similarly, cooling the reaction solution makes the color even darker due to more formation of colored complex, thus the forward reaction is exothermic in nature as cooling reduced the temperature and to reequilbriate, forward reaction occurs and generates more energy.