Experiment 27 Rates & Mechanisms of Reations Method I visual Clock Reaction A. Concentration effects on reaction Rates 2- Log Log Time Temp Log S₂Og (time) (°c) Run mol/L [S₂O82] Time mo/L (sec) 1a 41.0 23.3 1b 34.0 22.9 1c 56.0 21.6 lavg 54.7 23.0 80.0 22.8 2 3 The stoichiometry of the reaction explored: 2 2 I² + S₂0 g²² = I 2 (aq) + 2504 2- For a reaction between two Iodides & a persulfate a possible sequence would be the following: I + S₂ 08² = 504² + 504I For t 2- 2- mechanisumy S04 I + I = I2 (aq) +504²- rate expression for mechanism 1-a rate = k [I][5,0% ] mechanisum 2: 2- 2I === I2- 2- 2 2- I2²+S₂ Og² = I₂caq) + 2504² rate expression 2a rate = 2k [I]2 rale expression 2-b rate = k [I-] 2 [S2082-] Experiment 27 - 333 A. Concentration effects on reaction rates. For convenience, five 50 mL burets should be cleaned and filled with solutions ae. If 5 groups of 2 students each set up one buret, considerable time should be saved. A dropper bottle should be available containing solution f. Perform run 1 three times. For runs 1-8, add with a buret the required amounts of solutions a - d to a 125 or 250 mL Erlenmeyer flask (for run 8 also add 1 drop of the copper(II) sulfate solution). Measure the required amount of solution e into a second flask. Quickly add the contents of the second flask to the first. Simultaneously begin timing and swirl until thorough mixing is achieved. Record the time elapsed until the purple color appears. For each run after the purple appears, record the temperature. Solution Run Number (mL of solution) 8 a 0.20 M KI b 0.0050 M Na2S2O3 10 in a 0.4% starch soln. la lb lc 2 3 4 5 6 7 120 10 20 200 20 15 10- 51 10 5 20 10 10 10 10 10 20 220 220 20 10 - 00. I C 0.2 M KCI d 0.1 M K2SO4 e 0.10 M K₂S₂O8 20 20 of 0.1 M CuSO4 I I 20 I 5 10 15 5 10 15 20 20 15 10 02 20 20 551 20-1l 10 - 500 al 20 1 drop too 2000 B. Temperature effects on reaction rates. To study the effect of temperature on the reaction rate, run 1 will be repeated at several temperatures. The flask containing solutions a and b will be heated or cooled and then solution e (at room temperature) will be added. It will be assumed that quick mixing will result in the same temperature for the whole solution. The temperature will be measured immediately after the purple color appears. For each run, add 20 mL of solution a and 10 mL of solution b to the first flask. Add 20 mL of solution e to the second flask. Change the temperature of the first flask to about the value indicated below. Quickly add solution e, commence timing and stir. Record the time elapsed for the purple to appear and measure the temperature of the solution. Repeat the procedure for the remaining runs. To raise the temperature of a flask, suspend the flask in a beaker of water and heat the beaker over a wire gauze with a Bunsen burner. To lower the temperature of a flask, swirl the solution in an ice bath until the desired temperature is achieved. approximate temperature for flask 1 (°C) room temperature (already performed above) run number la, lb, lc 2' 35 200d 1000ml 3' 4' 50 5 METHOD 2 - Spectroscopic Monitoring. The solutions for each of the kinetic runs are the same as for the Method 1 (except the sodium thiosulfate (Na2S2O3) solution is not needed). You will need a spectrometer capable of doing measurements down to at least 350 nm and a couple of ©2015 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.