2PurposeThe purpose of this experiment is to determine the rate of a chemical reaction (potassium permanganate, KMnO4, + oxalic acid, H2C2O4) as the concentrations are varied and to determine the rate law for the reaction.
5Different Rates Average (this experiment) Initial (Experiment 17) Instantaneous
6Rate LawRate law---a relationship between concentrations and rate. For a reaction aA + bB productsthe rate law often takes the formRate = k[A]x[B]y
7Rate = k[A]x[B]y [A],[B]: molarities of A and B in solution x, y: orders with respect to A and B, respectively. (These orders might not correspond to coefficients from the balanced equation!)k: rate constant
8What We’re Running 2 MnO4- + 5 H2C2O4 + 6 H+ 2 Mn2+ + 10 CO2 + 8 H2O We assume a rate lawRate = k [MnO4-]m[H2C2O4]nThe rate law and rate constant are not affected by concentration.
9StrategyOne pair of experiments (assignments 1 and 2): [MnO4-] is constant and [H2C2O4] doubles.In another pair (1 and 3), [MnO4-] doubles and [H2C2O4] is constant.For each of these pairs, divide the rate measured in one experiment by that from another.
10Comparing assignments 1 and 2: This can be rewritten asSince rates and concentrations are known, n (the order with respect to oxalic acid) is available.
11Similarly, comparing assignments 1 and 3 gives And, therefore,From this, m (the order with respect to MnO4-) is available.
12Once the orders are known, we can calculate the rate constant from the rate law. Since rates depend on temperature. we will also look at the effect of temperature on the rate of this reaction.
13Safety Aprons and glasses. KMnO4 is a strong oxidant (and also stains skin and clothing); oxalic acid is poisonous.Waste into waste bottles.
14Safety 2If you use the Bunsen burner for heating water, keep hair, clothing, paper, and other flammable material away.Shut off burner before mixing high-temperature samples.
15Procedure Work in pairs. Check out pipettes and bulbs from stockroom. Needed equipment: medium-sized test tubes; 250- and 400-mL beakers. May also need ring stand, ring, wire gauze, and Bunsen burner.
16Quantities of Reactants Used (P. 15-6)Assignment numbers#1#2#3Oxalic Acid, mL5.010.0KMnO4, mL1.02.0Water, mL6.0
17Mark an X on a piece of white paper. Get 75 mL oxalic acid and 15 mL KMnO4 solutions; record concentrations.Two tubes for each assignment; pipet desired volumes of oxalic acid and water into each tube. For use of volumetric pipets, review Expt. 11 from CHEM 1031.
18Prepare four additional tubes for Assignment #1; also pipet 1 Prepare four additional tubes for Assignment #1; also pipet 1.0 mL of KMnO4 solution into each of four small test tubes. Save these for temperature study---last part of experiment.Start with first oxalic acid-water tube for Assignment 1. Place the paper behind the test tube.
19Pipet KMnO4 solution into tube; begin timing when half the solution has been added. When you can see the X through the test tube, record elapsed time on your data sheet.Repeat the run you have just completed; then do duplicate runs for the other two assignments.
20Effect of temperature: Place two oxalic acid tubes and two KMnO4 tubes into a beaker containing warm water (10oC above room temperature). (If tap water is not sufficiently warm, use Bunsen burner.)After tubes have been in warm water for 5 minutes or so, add KMnO4 to an oxalic acid-water mix; record elapsed time for X to become visible. Repeat.
21Make cool water bath (ice in water) to get 10oC below room temperature. Cool other two oxalic acid-water mixes and KMnO4 samples. Again mix, record elapsed time as before. Repeat.
22CalculationsConcentrations of oxalic acid and KMnO4 from dilution formula:Example: mL of M oxalic acid diluted to mL gives M.
23For each assignment, average the times for the two runs. D[MnO4-] = [MnO4-]f – [MnO4-]i = -[MnO4-]i
24From measured rates, determine orders: Round the orders to the nearest integers.
25Go back to the rate law:Rate = k [MnO4-]m[H2C2O4]nYou now know rates, concentrations, and orders. Calculate k for each assignment and average.
26The effect of temperature Rate is proportional to DConcentration/Dtime---if the time decreases by a factor of 3 (say), the rate correspondingly increases by a factor of 3.