CHEM 361 L

EXPERIMENT 8

The Kinetics of the Bromination of Acetone

(Latest Update: January 20, 2006)

Introduction

The rate of a chemical reaction is usually influenced by the concentration of the reactants, catalysts, and products. In this experiment, you will follow the aqueous (acid catalyzed) reaction of bromine and acetone. The overall reaction is:

H₃CC(O)CH₃ + Br₂ = H₃CC(O)CH₂Br + Br^- + H⁺

where the hydrogen ion concentration must be considered. You will follow the rate of reaction spectrophotometrically by measuring the disappearance of the bromine as a function of time. From the measured effect of changing the concentration of each reagent, you will determine the rate law (the order with respect to each reagent) for this reaction, and then determine the rate constant at room temperature. The rate law expression is given by:

RATE = k [(CH₃)₂CO]^a [Br₂]^b [H⁺]^c

where a, b, and c may be determined from pairs of runs in which only one concentration is changed.

Procedure

Each group will perform measurements at all concentration values assigned. Prepare 100 mL each of ~ 0.02 M bromine (the person preparing this solution should be double-gloved), 1.0 M hydrochloric acid, and 4.0 M acetone. Determine the λ_max value for the bromine solution: this will be the wavelength at which all further absorbance measurements will be made. The molar absorptivity of aqueous bromine is 160 L mol^-1 cm^-1 at 400 nm, 100 L mol^-1 cm^-1 at 450 nm, determine the actual concentration of the bromine solution using the absorption at these wavelengths.

The reactions will be performed in spectrophotometer cuvettes (4 mL total volume). Add 1 mL each of the acetone solution and distilled water, 0.5 mL of the HCl solution, then place the cuvette in the sample compartment of the spectrophotometer (a matched (water-filled) cuvette in the reference compartment). Add 1.0 mL of the bromine solution, stir, and begin monitoring the solution absorbance as a function of time. Repeat this procedure, cutting one reagent aliquot at a time in half, keeping the remaining aliquots constant, and making up the difference in volume with distilled water.

Calculations

Determine the rate law exponents for H⁺, acetone, and Br₂. Round these to the nearest integer, and determine the average rate constant for this reaction. Are these consistent with proposed mechanisms for this reaction?