Question

I am doing a lab on the determination of the equilibrium constant for the formation of Fe(SCN)+2. We used a spectrometer to measure the absorption of solutions with a varying initial concentration of Fe+3, but constant concentrations of SCN-. The reaction equation of Fe+3 + SCN- —> Fe(SCN)+2. Along with the SCN- and Fe+3 stock solutions, we also combined them with .3 M HNO3. We then calculated an average molar absorptivity and the equilibrium concentration of Fe(SCN)+2. From that concentration we could find the equilibrium concentrations of the reactants, and then finally the equilibrium constant of the reaction by plugging in the concentrations into the Kc equation. I ended up calculating a Kc value of 774.41. My molar absorptivity was 2120.14. Do these numbers seems reasonable for the reaction?

I also need to know:

1) Why is the reaction done in .3 M HNO3?

2) Why is there an excess of Fe+3 over SCN- in all of the solutions?

3) What results can be expected?

Answer 1

1) 0.3 Nitric acid (HNO₃) is added because Ferric ions (Fe⁺³) form coloured complex ions with the hydroxide ions (OH^-). So in order to neutralise these hydroxide ions nitric acid is added because these coloured complex ions may interfere with the required absorbance. This prevent iron hydrolysis.

   Fe⁺³(aq) + 6OH^-(aq) ------> [Fe(OH)₆]⁺³(aq)

[Fe(OH)₆]⁺³(aq) + SCN^-(aq) ----> [Fe(OH₂)₅SCN]⁺²(aq) + H₂O(aq)

   HNO₃(aq) + OH^-(aq) -----> H₂O(aq) + NO₃^-3(aq)

2)    Fe⁺³(aq) + SCN^-(aq) $\rightleftharpoons$ FeSCN⁺²(aq)

   K_eq = [FeSCN⁺²]eq/[ Fe⁺³]eq [SCN^-]eq

When the concentration of Fe⁺³ than SCN^- , the equilibrium shifts to the right side i.e. product side according to LeChatelier's principle until all the thiocyanate ions convert to FeSCN⁺² . So that equilibrium concentration of FeSCN⁺² is almost equal to that of SCN^- initial concentration in the solution.

3) Actual accepted literature value of equilibrium constant of [FeSCN⁺²] is K_eq(lit) = 290, but the calculated value in the above question Keq(cal) = 774.41.

Percent error = [measured value-accepted value]/accepted] x 100

Percent error = [774.41 - 290]/290 ] x 100 = 167 %

Theoretical value of molar absorptivity = 3550 M^-1cm^-1 but the calculated value of molar absorptivity was 2120.14.

Percent error = [measured value-accepted value]/accepted] x 100

Percent error = [2120.14. - 3550]/3550] x 100 = 40.28 %

These percent errors might be due to high absorbance reading indicating higher concentration of [FeSCN⁺²] which is due to addition of Ferric ions over SCN^- ions that leads to production of more [FeSCN⁺²] ions and high Kc value. These might be due to some other experimental errors too.