A = km (6) A = k [FeSCN 2+ ] KNOWN [FeSCN 2+ ] MEASURED A (Spec 20) CALCULATED k 3.0 x x x x 10-5 AVERAGE k =

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1 Method I. Analysis by Spectrophotometric Measurement We ll be using the spectrophotometer ( Spec 20 ) to compare absorbances (A) indicated by the equipment and known concentrations of iron(iii) thiocyanate complex ion (FeSCN 2+ ). As you might suspect, higher concentrations of (red) FeSCN 2+ make the solution less clear or more opaque. The measurement of opaqueness by the Spec 20 is called the absorbance (A). The absorbance and the concentration of the absorbing specie in the solution are related by a constant, and our lab manual uses k. (We ll call this little k.) The relationship is: For our specific experiment: A = km (6) A = k [FeSCN 2+ ] For this first part, we need to determine k so that we can calculate [FeSCN 2+ ] from the absorbances we ll measure in the experimental procedure ( Method I, p. 32 in the lab manual). Using the solutions of known FeSCN 2+ concentration, measure the absorbance of each solution using the Spec 20 (per your instructor s direction). Note that you will have to make the 5.0 x 10-5 mol/l solution by diluting one of the other solutions. KNOWN [FeSCN 2+ ] MEASURED A (Spec 20) CALCULATED k 3.0 x x x x 10-5 AVERAGE k = After you have calculated an average value for k, go on to the experimental procedure. Use your value of k to calculate [FeSCN 2+ ] values (on p. 2). (You will NOT be using a calibration curve or equation provided by your instructor.) Page 1 of 6 Printed 10:34 AM 4/20/2011

2 Method I. Analysis by Spectrophotometric Measurement MIXTURE Vol. (ml) of 2.00 x 10-3 M Fe(NO 3 ) 3 Vol. (ml) of 2.00 x 10-3 M KSCN Vol. (ml) of water (Method I) Absorbance [FeSCN 2+ ] 1. Measure the absorbance (A) for each of the experimental mixtures. 2. Calculate [FeSCN 2+ ] for each of your experimental mixtures using equation 6 and the value you calculated for k (on the previous page). Page 2 of 6 Printed 10:34 AM 4/20/2011

3 For each mixture: Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) STEP 1: calculate initial moles of Fe 3+ and SCN - (equation 3) STEP 2: enter [FeSCN 2+ ] mol/l at equilibrium (from page 2) calculate moles of FeSCN 2+ at equilibrium (equation 3) STEP 3: calculate moles of Fe 3+ and SCN - at equilibrium STEP 4: calculate [Fe 3+ ] and [SCN - ] mol/l at equilibrium (eq. 3) in L mix solution STEP 5: calculate K Mixture 1 Fe 3+ SCN - FeSCN 2+ Mixture 2 Fe 3+ SCN - FeSCN 2+ Page 3 of 6 Printed 10:34 AM 4/20/2011

4 Mixture 3 Fe 3+ SCN - FeSCN 2+ Mixture 4 Fe 3+ SCN - FeSCN 2+ Mixture 5 Fe 3+ SCN - FeSCN 2+ Page 4 of 6 Printed 10:34 AM 4/20/2011

5 Could the reaction be = Fe 3+ (aq) + 2SCN - (aq) + (aq)? (5) For each mixture: STEP 1: calculate initial moles of Fe 3+ and SCN - (equation 3) STEP 2: enter [FeSCN 2+ ] mol/l at equilibrium (from page 2) calculate moles of FeSCN 2+ at equilibrium (equation 3) STEP 3: calculate moles of Fe 3+ and SCN - at equilibrium STEP 4: calculate [Fe 3+ ] and [SCN - ] mol/l at equilibrium (eq. 3) in L mix solution STEP 5: calculate K Mixture 1 Fe 3+ SCN - + EQUILIBRIUM concentrations mol/l mol/l mol/l K 1 = Mixture 2 Fe 3+ SCN - + EQUILIBRIUM concentrations mol/l mol/l mol/l K 2 = Page 5 of 6 Printed 10:34 AM 4/20/2011

6 Mixture 3 Fe 3+ SCN - + EQUILIBRIUM concentrations mol/l mol/l mol/l K 3 = Mixture 4 Fe 3+ SCN - + EQUILIBRIUM concentrations mol/l mol/l mol/l K 4 = QUESTION / CONCLUSION: On the basis of the results from Part A (pp. 3-4) and Part B (pp. 5-6), what is the balanced chemical equation for the formation of the iron(iii) thiocyanate ion? Briefly explain your answer. Page 6 of 6 Printed 10:34 AM 4/20/2011