Kinetics of the Depolymerization of Diacetone Alcohol

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Experiment N|
Kinetics of the Depolymerization of Diacetone Alcohol via Basic Catalysis| |
Ingrid Tafur -5672578|

Partner: Laura Marrongelli
Demonstrator: Cheryl McDowall

The rate constant of the depolymerization of diacetone alcohol via basic catalysis was determined by monitoring the change in volume as a function of time at constant temperature of a pseudo first order reaction where the species in excess was sodium hydroxide. This was accomplished by using a dilatometer as the apparatus and following both methods: isolation and initial rates in conjunction. Introduction

Depolymerization is the process in which a compound is converted into one of a smaller molecular weight and of different physical properties, without changing the percentage relations of the elements composing it. The depolymerization or simply, the decomposition of diacetone alcohol into acetone molecules takes place via basic catalysis. The catalyst is capable of directing and accelerating thermodynamically the reaction while remaining unaltered at the end of the reaction. Diacetone alcohol is catalyzed by the hydroxide ions. The reaction rate of a chemical reaction is the rate of decrease of the concentration of a reactant or the rate of increase of the concentration of a product. The rate law is an equation that expresses the rate of a reaction as a function of the concentration of all the species present in the overall chemical reaction at some time. The rate law is often found to be proportional to the concentration of the reactants raised to a power. For the depolymerization of diacetone alcohol the empirical rate equation is -∂x∂t=kxn[OH-]m (1)

X= concentration of diacetone alcohol , t=time , k=rate constant, n+m= order of the reaction Where concentration is in moles per litre, time is in seconds and the dimensions for k depend on the overall reaction order. The rate constant ‘k’ is independent of the concentrations but dependent on temperature. The rate of depolymerization is first-order with respect to the concentrations of both diacetone alcohol and hydroxide ion. However, since hydroxide ion is a catalyst its concentration remains constant during the reaction. That is, the rate law can be simplified by the isolation method; the concentrations of all the reactants except one are in large excess. Thus, if a large excess of hydroxyl ions is present, the rate law equation becomes -∂x∂t=kobsxn (2)

Where kobs is the hydroxyl-dependent rate constant
kobs=k[OH-]m0 (3)
m= reaction order with respect to hydroxide ions, 0= initial The overall reaction now appears first order. Since the true rate law has been modified into first-order by assuming that the concentration of hydroxide ions is in excess, the rate law becomes a pseudo-first-order rate law. In order to determine the rate constant for this reaction, a more useful relationship is needed. It follows that the integrated rate law of the pseudo-first-order rate law is ln[x][x]0=-kobst ⁡ (4)

Where [x] is the concentration at time t and [x]0 is the initial concentration. Since the overall reaction is therefore a first-order, the rate constant can be determined by measuring any property of the system that undergoes a change which is proportional to the extent of the reaction. In this experiment, the rate constant law is determined by measuring the change in volume with respect to time using a dilatometer. A dilatometer measures the expansion or contraction of a liquid. It consist of a glass bulb holding approximately less than 150mL of liquid joined to a fine capillary tube with a narrow and uniform bore. The height of the capillary tube is measured by a cylindrical cover marked to the 50mL mark. When the bulb is filled with the solution, the liquid enters the capillary tube. As the solution expands, it expands into the capillary tube. The volume change is obtained by measuring the distance up the capillary tube that the...
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