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 pottasium 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) Keq is quoted to have a value of 250 at 20deg celcius. When an initial pottasium 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 1cm 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

For the reaction,

Fe3+ + SCN- <==> [FeSCN]2+

a) Blank reading is taken with a solution prepared as usual without an SCN- in in. So it has only Fe3+ in the solution.

b) absorbance = 0.5

concentration = 0.5/5400 = 9.26 x 10^-5 M

So,

[FeSCN]2+eq = 9.26 x 10^-5 M

[SCN-]eq = 2 x 10^-4 - 9.26 x 10^-5 = 1.074 x 10^-4 M

[Fe(H2O)5SCN]2+ = 9.26 x 10^-5 M

c) If the reaction is exothermic, heating the solution would disfavor the reaction and color of complex formed would decrease. If the reaction is endothermic heating would make the soluiton darker as more complex is formed.