Chemistry 2740 Laboratory Experiment 2
A KINETIC STUDY OF THE BASE CATALYZED CLEAVAGE OF DIACETONE ALCOHOL USING A DILATOMETER The decomposition of diacetone alcohol into two molecules of acetone is catalyzed by hydroxide ions and is an example of an aldol condensation in reverse. O OH OHO 2CH3-C-CH3
The rate of decomposition is first-order with respect to the concentrations of both diacetone alcohol and hydroxide ion: Rate = k[OH-][diacetone alcohol] (1)
However, since hydroxide ion is a catalyst its concentration remains constant during the reaction. The overall reaction appears first-order (i.e. is a “pseudo first order reaction”) and follows the observable rate law Rate = k' [diacetone alcohol] where k' = k [OH-] (2)
Since the overall reaction is first-order we can study the kinetics of the reaction by measuring any property of the system that undergoes a change which is proportional to the extent of reaction. Such a property in this case is the volume of the reaction solution. The effective volume of one molecule of diacetone alcohol is not the same as the effective volume of two molecules of acetone and as a result the total volume of the reaction solution changes as the reaction proceeds. In this case the solution expands although in some reactions it contracts. A simple instrument for measuring volume changes is a dilatometer which consists of a glass bulb to which is attached a tube with a stopcock (for filling the bulb) and also a piece of long capillary tubing. The bulb is filled with reaction solution to the point where liquid just enters the capillary tube and then the stopcock on the filling tube is closed. As the solution expands it does so into the capillary tube causing the meniscus in the tube to rise. By measuring the distance up the capillary tube that the meniscus travels one has a measure of the volume change. One can determine the actual volume change if the crosssectional area of the capillary is known but even that is not necessary in this experiment. Since the position of the meniscus in the capillary column can be measured accurately using a cathetometer, this is a good experiment to test the Guggenheim method for determining first-order rate constants (refer to Appendix A on “First-order Reactions”). In this method readings are generally made at times t0, t1, t2, t3, etc., with each reading Page 2 - 1
Chemistry 2740 Laboratory Experiment 2 taken at a constant, accurately determined time interval after the preceding measurement. The resulting data list is divided into equal halves. For example, if there are 20 readings taken at times t 0 – t19 with corresponding measurements P0 – P19, the data would be divided in two between readings P9 at t 9 and P10 at t10. Next, the differences between the measurements in the two data sets are taken, i.e., P0-P10, P1-P11, P2-P12, etc. Notice that the time interval between each pair of readings is constant. Finally a plot of the natural logarithm of the differences against time, i.e., ln(P0-P10), ln(P1-P11),… vs. t0, t 1,… should yield a straight line of slope -k, the first-order rate constant.
Apparatus Cathetometer, 3 dilatometers, timer. A dilatometer is a device for measuring the expansion (or contraction) of a liquid. Ours is of relatively simple design and was made locally by Luis Delgado from various pieces of glassware. It consists of an expansion bulb to which is attached a fine capillary tube with a narrow and hopefully uniform bore. The expansion tube is connected at the other end to a filling tube through a stopcock. When the stopcock is closed, a solution in the expansion tube can only expand up the capillary tube. The volume of liquid in a capillary or cylinder is given by the cross-sectional area, A, of the cylinder times its length, l (V = A x l). Thus by measuring the travel, Δl, of the liquid up the capillary tube one has a quantity...