Fe^3+ ions in water form a colorless hexa-aquo complex with the chemical formula
ID: 530240 • Letter: F
Question
Fe^3+ ions in water form a colorless hexa-aquo complex with the chemical formula [Fe(H_2 O)_6]^3+. This colored complex can then undergo an equilibrium reaction with thiocyanate (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 reaction,
a) Blank solution would be [Fe(H2O)6]3+ solution.
The absorbance value of blank is substracted from the absorbance values of indivual reaction solutions to get correct adjusted values.
b) Keq = 250
[SCN-]initial = 2 x 10^-4 M
Absorbance = 0.5
[Fe(H2O)5SCN]2+eq = 0.5/5400 = 9.26 x 10^-5 M
[SCN-]eq = 2 x 10^-4 - 9.26 x 10^-5 = 1.07 x 10^-4 M
and,
[Fe(H2O)6]3+eq = 9.26 x 10^-5/250 x 1.07 x 10^-4
= 3.45 x 10^-3 M
c) If increasing the temperature of reaction increases formation of complex, that is higher value of absorbance reading is obtained, it is an endothermic reaction.
If however, increasing the temperature decreases the formation of complex, such that lower value for absorbance is observed, it is an exothermic reaction. This is according to the LeChatellier;s principle.
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