Kinetic study of the reaction of KI with FeCl3

KINETIC STUDY OF THE REACTION OF KI WITH FeCl3 INTRODUCTION Concentration of reactants is one factor which influences the rate of a chemical reaction. The following reaction will be investigated: 2 I-(aq) + 2 Fe3+(aq) ─────────> I2(aq) + 2 Fe2+(aq) The concentration of I- and Fe3+ will be systematically varied, and the effect of these changes in solution absorbancies will be determined using a visible spectrophotometer. The rate expression for the reaction will then be calculated: Rate = k[I-]x [Fe3+]y, x,y = order with respect to reactants PURPOSE 1. 2. 3. To become familiar with the use of the visible spectrophotometer. To determine the order of the reaction with respect to each reactant. To write an overall rate expression for the reaction. SAFETY CONSIDERATIONS Goggles must be worn in the lab. PRE-LAB Consider the hypothetical reaction: A + B ─────────> product(s) The rate expression is: Rate = k[A]x[B]y Determine the values for x and y, from the following data: [A] 0.10 0.10 0.20 MATERIALS [B] 0.10 0.20 0.10 Rate 2.0 4.0 8.0 3-5 mL pipets (graduated to 0.1 mL) small test tubes visible spectrophotometer cuvettes 0.020 M KI 0.020 M FeCl3 distilled water safety goggles PROCEDURE 1. Warm up thespectrophotometer. Set wavelength at 450 nm. Prepare a blank by mixing 2 mL of distilled water + 2 mL KI solution in a cuvette. Set % transmittance = 0 with cuvette holder empty and set % transmittance = 100 with the blank solution. 2. Using the quantities listed in the Data Table, place the required amount of KI solution and distilled water in another cuvette, place the required amount of FeCl3 solution in a small test tube. 3. Pour the FeCl3 solution into the (KI + H2O) solution and begin timing (t=0 sec.). Cover the cuvette and quickly invert 3-4 times to mix the solutions. 4. Insert cuvette into the Spec 20/20D and record the % transmittance 15 sec. after the initial mixing. 5. Continue to record the % transmittance every 15 sec for 2 min (see Data Table). 6. Repeat 3 times for each set of concentrations assigned. 7. Rinse all pipets, cuvettes, and test tubes with distilled water and drain to dry. 2 DATA TABLE time (sec) Mixture 1: 2 mL KI Trial 15 30 1 1 mL H2O 2 1 mL FeCl3 3 average absorbance 2: 2 mL KI 1 1 ½ mL H2O 2 ½ mL FeCl3 3 average absorbance 3: 1 mL KI 1 1 mL H2O 2 2 mL FeCl3 3 average absorbance 4: 1 mL KI 1 2 mL H2O 2 1 mL FeCl3 3 average absorbance 5: 2 mL KI 0 mL H2O 1 2 3 45 60 75 90 105 120 2 mL FeCl3 3 average absorbance 4 CALCULATIONS 1. For each trial, plot average absorbance in the y-axis and time on the x-axis. Determine the best straight line and calculate the slope of the line. 2. From the slope and concentration data for all 5 solutions, determine the x and y values (orders) and write the rate expression for the reaction. CONCLUSIONS/ FOLLOW-UP QUESTIONS 1. (a) Why is it not possible to calculate the specific rate constant, K, from this set of data? (b) What data would be needed to calculate K, and what type of experiments could one use to obtain this data? 5 TEACHERS' GUIDE KINETIC STUDY OF THE REACTION OF KI WITH FeCl3 TYPICAL CLASSROOM USAGE This experiment would be appropriate for second-year chemistry students (AP). CURRICULUM INTEGRATION This experiment fits into the Rates of Reaction topic area. Students should have a basic understanding of collision theory -- why and how particles react with one another. It would be very helpful if a quantitative rate activity were performed first, either as a demonstration or a lab exercise. One could quickly show the effects of temperature, for example, and not attempt this in a quantitative manner at this experimental level. PREPARATION 1. Thespectrophotometer should be warmed up for at least 15 min. prior to use. 2. It requires a maximum of 6 mL of solution for a set of 3 trials. 250 mL of each solution should be sufficient for 50 students working in pairs. TIME GETTING READY 45-60 min to prepare the two solutions and gather equipment. DOING THE ACTIVITY 10-15 min for the students to acquire the data for 3 trials of the same concentration. SAFETY AND DISPOSAL No special safety hazards. Disposal: KI solution -- Flinn Method #26b FeCl3 solution -- Flinn Method #26b SAMPLE RESULTS AND CALCULATIONS 6 1. The slope of the (Absorbance vs. time) curve in proportional to the rate of reaction. 2. The slope of the curve can be determined by: a. Individually plotting the points on graph paper. 3. b. Using Vernier Graphical Analysis III software. c. Using a calculator-installed least-square program. Sample results (From least-square program) Solution No. 1 4. slope 0.00548 2 0.00252 3 0.00478 4 0.00270 5 0.00934 Determination of order (x & y) a. Compare slopes of solutions 1 & 2 and 3 & 4: doubling [Fe3+] ─────────> approximately doubles slope ═════════> " Answer: y = 1 7 " rate b. Compare slopes of solutions 1 & 3: doubling [I-] ─────────> approximately doubles slope ═════════> " " rate Answer: x = 1 c. double-check by comparing solutions 4 & 5: doubling both [I-] and [Fe3+] ─────────> rate approximately quadruples!! 5. Rate expression Rate = k[I-]x[Fe3+]y = k[I-][Fe3+] 6. The specific rate constant, k, cannot be calculated since the rate of reaction cannot be determined. Rate = ∆ concentration ∆ time but we measure ∆ absorbance (= slope) ∆ time and Absorbance ∝ concentration, Absorbance ≠ concentration 8