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Unformatted text preview: 1 Laboratory IV: Chemical Kinetics (Two-Week Lab) Pre-lab reading assignment: Textbook, Chapter 14 (all sections) Ocean Optics handout This laboratory handout !! You will have a pre-lab quiz each week !! Introduction / Purpose The pH-indicator compound crystal violet (and its analogs) is used as a textile dye and a tool for the identification and disinfection of certain bacteria. It has a purple color in acidic solution and is colorless in more basic solution. Your task in the laboratory is to determine the rate law for the reaction of crystal violet with hydroxide. C + N(CH 3 ) 2 N(CH 3 ) 2 N(CH 3 ) 2 + OH _ C N(CH 3 ) 2 N(CH 3 ) 2 N(CH 3 ) 2 OH PURPLE COLORLESS This reaction can be represented as follows: CV + OH- CVOH colored colorless The kinetics of this reaction can be monitored with a spectrophotometer by observing the decrease in absorbance of crystal violet, which can be used as a measure of the rate to determine the rate law through the following relationships: rate of disappearance of CV = rate of appearance of CVOH = [CV] x [OH- ] y (1) Chemical Kinetics Chemical reactions occur at varying speeds with a vast spectrum of rates, ranging from very slow to extremely fast. For example, the rusting of iron is fairly slow, whereas the decomposition of TNT proceeds explosively fast. Experiments have shown that the rate of a homogeneous reaction in solution depends upon the nature of the reactants, their concentrations, the temperature of the system, and the use of catalysts. 2 Consider the hypothetical reaction: A + B C + D The rate of this reaction may be measured by observing the rate of disappearance of the reactants A or B, or the rate of appearance of the products C or D. The species that is observed is a matter of convenience. For example, if A, B, and C are colorless and D is colored, the rate of appearance of D can be conveniently measured by observing an increase in the intensity of the color of the solution as a function of time. Mathematically, the rate of reaction may be expressed as follows: In general, the rate of the reaction depends upon the concentration of one or more of the reactants. Thus, the rate of the hypothetical reaction above may be expressed as: Rate = k[A] x [B] y where [A] and [B] are the molar concentrations of A and B, x and y are the powers to which the respective concentrations must be raised to describe the rate, and k is the specific rate constant. The values of x and y must be determined experimentally. For example, if x = 2 and y = 1, then the rate law is: Rate = k[A] 2 [B] This reaction is first order in B , meaning that doubling the concentration of B while keeping A constant causes the reaction rate to double. Simultaneously, this reaction is second order in A , meaning that doubling the concentration of A while keeping B constant causes the rate to increase by a factor of four since the rate of the reaction is proportional to the square of the concentration of A. The overall order of the reaction is the sum of the exponents: or thirdconcentration of A....
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