Fe^3+ ions in water form [Fe(H_2 O)_6]^3+. This colored complex can then undergo

Question

Fe^3+ ions in water form [Fe(H_2 O)_6]^3+. This colored complex can then undergo an equilibrium reaction with (SCN^-) to form a red complex with the chemical formula [Fe(H_2 O)_5(SCN)]^2+. In order to make this reaction occur, the two chemicals need to be mixed in 0.5 M nitric acid. This equilibrium reaction is shown below: Fe(H_2 O)_6^3+ (aq) + SCN^-(aq) Fe(H_2 O)_5(SCN)^2+ + H_2 O(l) a) When recording the absorbance values of these equilibrium mixtures, what should be used as a blank solution and how should its data be treated? b) K_eq is quoted to have a value of 250 at 20 degree C. When an initial KSCN concentration of 2 times 10^-4 M was used, the red (Fe(H_2 O)_5(SCN)]^2+ complex had an absorbance value of 0.5. If the molar coefficient of the red complex is 5400 M^-1 cm^-1 and a 1 cm was used, calculate the equilibrium concentrations of all three compounds present in solution: (Fe(H_2 O)_6^3+]_eq, [SCN^-]_eq, and (Fe(H_2 O)_5(SCN)^2+]_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?

Explanation / Answer

For the given experiment,

a) The blank solution would be Fe(H2O)6^3+ solution. The absorbance of this solution would be subtracted from the absorbance values of the reaction solution to get correct values.

b) Keq = 250

molar absorptivity = 5400 M-1.cm-1

[SCN-] = 2 x 10^-4 M

absorbance = 0.5

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

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

So,

[Fe(H2O)6]3+eq = 9.26 x 10^-5/250 x 1.07 x 10^-4

                           = 3.46 x 10^-3 M

c) If increasing the temperature speeds up the formation of complex, such that we have higher absorbance value for solution, it is an endothermic reaction. If however, increase in temperature reduces the absorbance of solution, it is an exothermic reaction.

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