Presentation on theme: "Experiment 22: Colorimetric determination of an equilibrium constant"— Presentation transcript:
1 Experiment 22: Colorimetric determination of an equilibrium constant PURPOSETo determine the value of the equilibrium constant for the equilibrium system involving Fe3+(aq), SCN–(aq) and FeSCN2+(aq) using colorimetric analysis.
2 THEORYIn aqueous solution, Fe3+ ions react with SCN– ions to form the blood red coloured FeSCN2+ ion:Fe3+(aq) (pale yellow) + SCN–(aq) (colourless) FeSCN2+(aq) (red)The colour of the solution is directly proportional to the concentration of FeSCN2+ ions present. The concentration of these ions can be determined by measuring the absorbance of the solution and comparing it with the absorbance of a solution of known concentration. If the initial concentrations of Fe3+ and SCN– are known, a value for the equilibrium constant, K, for the reaction can be calculated.Colorimeters measure the amount of light that is transmitted or absorbed by a solution. A description of how they work can be found in your text book
3 SAFETY PROCEDURESFollow all instructions for using the equipment in this activity.2. Wear safety glasses and a laboratory coat for this experiment.3. Potassium thiocyanate is irritating to the skin and eyes. Avoid contact.4. The iron(III) nitrate solution used in this experiment contains nitric acid. The solution is irritating to skin and body tissues.
4 Part 1. Preparation of Equilibrium Solutions Label six ml flasks 1-6Pipet ml of .2M Fe(NO3)3 into each flaskThen pipet 1.00, 2.00, 3.00, 4.00, and 5.00 ml of .002M (NaSCN)- into flasks 2 – 6 respectivelyAdd .1M nitric acid to each flask to make a total of ml per flask and stopper each flask
5 Part 1. Preparation of Equilibrium Solutions Calculate the final Fe(NO3)3 concentration and report them in part ACalculate the final Fe(NCS)+2 concentration in each flaskNaSCN ml soln123456Initial [SCN-] M4.0x 10-5MEquil FeNCS+2MPercent T10078.5Absorbacnce0.105
6 Part 1. Preparation of Equilibrium Solutions Obtain 2 cuvettes, rinse one cuvette with soln 1, and discard in waste jarFill the rinsed cuvette with soln 1, insert cuvette into spectrophotometer and adjust 10 0 absorbance and 100 transmittance.Rinse the second cuvette with soln 2 and discard in waste jar and measure the absorbance and transmittance of soln 2 at 447nmRepeat procedures for soln 3 -6
7 calibration curvePrepare calibration curve by plotting absorbance vs concentration
8 Part B: Determination of the calibration curve Label six clean dry test tubes 1- 6Pipet ml of .002M Fe(NO3)3 into each test tubeThen pipet 1.00, 2.00, 3.00, 4.00, and 5.00 ml of .002M (NaSCN)- into flasks 2 – 6 respectivelyAdd .1M nitric acid to each flask to make a total of ml per flask and stopper each flask
9 Part B: Determination of the calibration curve Measure and record the absorbance and transmittance of these solns at 447nmFrom your calibration curve determine the equilibrium concentration of Fe(NCS)+2
10 Part B: Determination of the calibration curve NaSCN ml soln123456Volume of (FeNO3)3 ml5.00 ml5.00 mVolume of NaSCN ml1.002.00Initial Fe +3 M1.0 x 10-3MInitial SCN -1M2.0 x 10-4MAbsorbance.164
11 Part B: Determination of the calibration curve NaSCN ml soln123456Equil [FeNCS+2] M3.28 x 10-5Equil [Fe+3] M1.0 x 10-3M9.67 x 10-4Equil [SCN-] M1.6 x 10-4MKeq-211