The question
Name: ______________________ Date: ____________ Measuring K for iron(III) thiocyanate with a spectrophotometer (10 points)
Experimental setup
A colored equilibrium measured with Beer’s law
Iron(III) ions react with thiocyanate ions to form a red complex ion:
Fe3+(aq) + SCN−(aq) ⇌ FeSCN2+(aq)
FeSCN2+ is the only colored species at the wavelength used. A student mixes 5.00 mL of 2.00 × 10−3 M Fe(NO3)3 with 5.00 mL of 2.00 × 10−3 M KSCN at 25 °C, waits for the color to stop changing, and measures the absorbance in a 1.00 cm cuvette. From a calibration curve, the molar absorptivity of FeSCN2+ at this wavelength is 4.50 × 103 M−1 cm−1.
| Quantity | Value |
|---|---|
| Volume of 2.00 × 10−3 M Fe(NO3)3 | 5.00 mL |
| Volume of 2.00 × 10−3 M KSCN | 5.00 mL |
| Absorbance of the equilibrium mixture | 0.297 |
(a) Write the expression for the equilibrium constant, Kc, for the reaction. [1 point]
(b) (i) Calculate the concentration of Fe3+ in the mixture immediately after mixing, before any reaction. (ii) Calculate [FeSCN2+] in the equilibrium mixture. [2 points]
(c) Calculate the value of Kc at 25 °C. Show how you found each equilibrium concentration. [2 points]
(d) Identify the piece of glassware the student should use to measure each 5.00 mL volume, and justify your choice. [1 point]
(e) A few drops of concentrated Fe(NO3)3 solution are added to the equilibrium mixture at 25 °C. (i) Predict whether the absorbance will increase, decrease or stay the same once equilibrium is re-established. (ii) Justify your prediction in terms of Q and K. [2 points]
(f) (i) A classmate says, "Kc is greater than 1, so the equilibrium mixture contains more FeSCN2+ than Fe3+." Use your results to evaluate the claim. (ii) When the equilibrium mixture is warmed to 40 °C, its absorbance decreases. Determine whether the forward reaction is exothermic or endothermic, and justify your answer. [2 points]