Exp13 - Experiment 13 CHEMICAL KINETICS: REACTION CRYSTAL...

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Experiment 13 CHEMICAL KINETICS: REACTION CRYSTAL VIOLET WITH NaOH I. Learning Objectives… ± To gain an understanding of chemical kinetics by observing the effects of reactant concentration on reaction rate . ± To experimentally determine the rate law and rate constant for a model reaction (Crystal Violet with NaOH). II. Background Information Chemical kinetics is concerned with the rates and mechanisms of chemical reactions. The rate or velocity of a reaction is described by the change in concentration of the reacting substance(s) or product(s) per unit of time ( e.g. M/s). In a typical chemical reaction the concentration of the reactants decreases with time and the concentration of the products increases with time. For example: A + B ² C + 2 D Experimentally, the rate of the reaction may be based on any reactant or product. Consider in the example reactant A or product D: [] Ät A Ä change for required time A of ion concentrat in change A of ance dissappear of Rate ± = = Or Ät D Ä change for required time D of ion concentrat in change D of appearance of Rate + = = The [ ] represents concentration in moles per liter. In the example above, at a given time interval ³ t, the rate of appearance of D is twice the rate of the disappearance of A. Therefore, () t D t A ± ± + = ± ± ² ] [ 2 1 The determination of reaction rates requires the measurement of concentration and time. The concentration of a substance can be determined in a variety of ways. This
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13-2 experiment measures concentration indirectly, using absorption of light by the reactant as and indicator of concentration ( Beer’s law ). Beer’s Law The mathematical relationship between concentration and absorbance is called Lambert, Bouger and Beer's Law (commonly abbreviated as Beer's Law). Beer’s Law is expressed in linear form as A = ± b c These variables are defined as follows: A = absorbance = - log (I/I o ) I = light intensity of sample and I o = light intensity for the blank solution. The colorimeter uses the relative light intensity of the sample together with the light intensity of the blank solution to determine the absorbance value for each sample. Absorbance has no units ± = molar extinction coefficient (molar aborptivity) defines how strongly the solution being studied absorbs light at a particular wavelength. In this experiment, it is how strongly the Crystal Violet solution absorbs green light. The molar extinction coefficient is unique for each different chemical at each different wavelength. Typically, molar extinction coefficients are expressed with the units M -1 cm -1 . b = the path length of the light through the solution contained in the cuvette. In most experiments the path length of the cuvette is 1.00 cm. The variable has units of centimeters. c = the concentration of absorbing species in the solution. In most cases, concentration is expressed in units of molarity (moles of solute/liters of solution).
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This note was uploaded on 05/15/2011 for the course CHEM 102L taught by Professor N/a during the Fall '07 term at UNC.

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Exp13 - Experiment 13 CHEMICAL KINETICS: REACTION CRYSTAL...

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