Experiment 15 Chemical Kinetics

Purpose The 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.

Introduction For a reaction a A + b B  c C + d D

Different Rates Average (this experiment) Initial (Experiment 17) Instantaneous

Rate Law Rate law---a relationship between concentrations and rate. For a reaction aA + bB  products the rate law often takes the form Rate = k[A]x[B]y

Rate = 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

What We’re Running 2 MnO4- + 5 H2C2O4 + 6 H+  2 Mn2+ + 10 CO2 + 8 H2O We assume a rate law Rate = k [MnO4-]m[H2C2O4]n The rate law and rate constant are not affected by concentration.

Strategy One 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.

Comparing assignments 1 and 2: This can be rewritten as Since rates and concentrations are known, n (the order with respect to oxalic acid) is available.

Similarly, comparing assignments 1 and 3 gives And, therefore, From this, m (the order with respect to MnO4-) is available.

Once 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.

Safety Aprons and glasses. KMnO4 is a strong oxidant (and also stains skin and clothing); oxalic acid is poisonous. Waste into waste bottles.

Safety 2 If 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.

Procedure 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.

Quantities of Reactants Used (P. 15-6) Assignment numbers #1 #2 #3 Oxalic Acid, mL 5.0 10.0 KMnO4, mL 1.0 2.0 Water, mL 6.0

Mark 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.

Prepare 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.

Pipet 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.

Effect 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.

Make 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.

Calculations Concentrations of oxalic acid and KMnO4 from dilution formula: Example: 5.00 mL of 0.755 M oxalic acid diluted to 12.00 mL gives 0.315 M.

For each assignment, average the times for the two runs. D[MnO4-] = [MnO4-]f – [MnO4-]i = -[MnO4-]i

From measured rates, determine orders: Round the orders to the nearest integers.

Go back to the rate law: Rate = k [MnO4-]m[H2C2O4]n You now know rates, concentrations, and orders. Calculate k for each assignment and average.

The 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.