HShin_Experiment_6

HShin_Experiment_6 - Author Hoon Shin Austin Kwon Branden...

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Author: Hoon Shin, Austin Kwon, Branden Heilman Group: F TA: Eileen Chen Section: A05 Class: Chem100A, UCSD (University of California San Diego) Date: 5/19/08 Oxidation of Ethanol by Dichromate Abstract The purpose of this lab is for one to calculate or determine the rate of the oxidation of ethanol by observing the disappearance of dichromate. Using the highest wavelength where the absorbance of [Cr 2 O 7 2- ] is highest, 445nm, The value for X,Y, and Z, where Rate = k [CH 3 CH 2 OH] x [Cr 2 O 7 2- ] y [H + ] z , was 1,1, and 2 respectively. The average K was -0.00205 which gives equation Rate = -0.00127 [CH 3 CH 2 OH][Cr 2 O 7 2- ][H + ] 2 (Rounded K value used) For the second part of the lab, %(w/v) in the unknown, unknown #2, was 0.123% where the standard deviation is 0.00353%. Introduction The main goal of this experiment, for part one, is for one to find the rate of the oxidation of ethanol which can be measured by disappearance of dichromate. For part two, is to find the amount of alcohol in an unknown sample. In the first part of the exp, the f F irst reaction to consider is following: 16 H + + 2 Cr 2 O 7 2- + 3 CH 3 CH 2 OH → 3 CH 3 COOH + 4 Cr 3+ + 11 H 2 O For the rate of this reaction is expressed in the following equation: Rate = k [CH 3 CH 2 OH] x [Cr 2 O 7 2- ] y [H + ] z
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constant of the reaction. To determine the orders for each reaction, one must use the technique called “flooding.” By adding much more but known amounts of the two species, one can determine the speed of disappearance of the other specie which can be consider as limiting reagent. When depending on [Cr 2 O 7 2- ], excess amount of [CH 3 CH 2 OH] and [H + ] are used so that the initial concentration would be almost same as final concentration. Then the new constant can be obtained which is called the observed rate constant,(k obs ): Rate = k obs [Cr 2 O 7 2- ] y Where k obs = k [CH 3 CH 2 OH] x [Cr 2 O 7 2- ] y The change of [Cr 2 O 7 2- ] can be observed by a spectrophotometer, since the color of [Cr 2 O 7 2- ] solution changes from bright yellow to pale green where the other solutions are colorless. This absorbance is equal to the concentration of dichromate due to the following equation: I W log = A Where A, W, and I are absorbance, intensity of light entering sample, and intensity of light passing through the sample, respectively. Thus, by using this absorbance, the equation of concentration vs time data can be obtained to calculate the rate law. For the second part of this experiment, one is asked to determine the amount of alcohol in the unknown solution that will be provided by the lab. The procedures are same except for the fact that the limiting reagent is an ethanol because that is what we must find out. Excess of dichromate will be titrated with ferrous ethylenediammonium sulfate, where ferroin is used as an indicator. Where the following reaction occurs: K 2 Cr 2 O
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This note was uploaded on 10/16/2008 for the course CHEM 100A taught by Professor Dai during the Spring '06 term at UCSD.

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HShin_Experiment_6 - Author Hoon Shin Austin Kwon Branden...

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