3-tBuBr-complete - Determination of the Order Rate Constant...

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Determination of the Order, Rate Constant, Half-Life, and Activation Energy for the Hydrolysis of t -Butyl Bromide INTRODUCTION In this laboratory exercise, we will be studying the kinetics of the hydrolysis of t-butyl bromide: t -butylbromide t -butyl alcohol The above equation provides information about the overall stoichiometry of the reaction, but does not provide any information about the pathway or mechanism of the reaction. However, similar reactions have been shown to proceed in two steps: Step 1 - Slow ionization of t-butyl bromide Step 2 - Fast reaction of the carbocation with water The first step is slow and determines the overall rate of the reaction. It is the rate determining step. We would like to be able to describe the rate of the reaction in terms of the concentrations of the reactants. We could express this relationship in terms of the rate law equation: Rate = k [HOH] m [t-BuBr] n . Because we will study this reaction in aqueous solution, our reaction of study is "zero order" with respect to [HOH]. The amount of water produced or consumed is generally so small in comparison to the total amount of water available that small changes in the water concentration have virtually no effect upon the reaction rate even though water appears as a reactant. Therefore, the rate expression may be simplified to: Rate = k [t-BuBr] n Another feature of most chemical reactions is the need for a little energy "boost" to get the reaction going. For example, placing a match to a piece of paper gets a combustion reaction going; however, once burning, the assistance of the match is no longer needed. Such a necessary "boost in energy" to initiate or activate a reaction is termed the energy of activation, E act for that reaction. For many reactions, including the one we are studying, the heat energy available from the surroundings is sufficient to provide the
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necessary E act . By determining the rate constant of a reaction at two different temperatures, it is possible to calculate the value of E act for a reaction from the Arrhenius equation (see below). Our experiment will involve several determinations. You will first determine the order of the reaction with respect to [BuBr] by measuring the concentration of t-BuBr remaining at various times in the reaction at room temperature. After determining whether the reaction is zero, first, or second order, you will determine the rate law constant, k, and the half-life of the reaction, t 1/2 , for the reaction from the integrated rate equations. In addition, by varying the temperature of the reaction, we can determine the k at low temperature and calculate the activation energy. Part 1: Determination of the order of the reaction for [t-BuBr] We will determine the order of the reaction for t-butyl bromide by monitoring the percent of the original t-BuBr concentration remaining at various times in the reaction. As the hydrolysis reaction proceeds, the amount of remaining t-BuBr will constantly diminish. As there is less reactant, the reaction will slow.
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