Exp 6 - Lab Manual: Oxidation of Ethanol by Dichromate Ion...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Lab Manual: Oxidation of Ethanol by Dichromate Ion 1 The Oxidation of Ethanol by Dichromate Ion (I) Reaction Kinetics (II) Determination of Blood Alcohol Content WebCT > Lab Manual > Appendix – Kinetics INTRODUCTION & THEORY Part I – Kinetics This experiment deals with the rate of a reaction and how it is affected by changes in the reactant concentrations at a constant temperature. The rate of the oxidation of ethanol is measured by following the disappearance of K 2 Cr 2 O 7 (aq), which has a bright yellow color, using a spectrophotometer. As the dichromate reacts, the Cr(VI) is reduced to Cr(III). Cr(III) is pale green in color and does not absorb at the same wavelength as the dichromate ion. All other reactants and products are colorless. The (unbalanced) reaction is 23 32 2 7 3 CH CH OH Cr O CH COOH Cr   When balancing this reaction, H + will have to be introduced. You will determine the order of the reaction with respect to each of the three reactants (i.e., CH 3 CH 2 OH, Cr 2 O 7 2- , and H + ), and calculate the specific rate constant. This can be cleverly accomplished using only six experimental runs if you "flood" the system (add large, but known, excess amounts) with two of the three reactants. The two reagents in large excess (CH 3 CH 2 OH and H + ) will remain essentially constant throughout the course of the oxidation so that an observed rate constant (k obs ) can be determined by monitoring the change of the color of 2 7 2 O Cr . In different runs of the experiment, you will use different concentrations of alcohol and acid (still in excess of the dichromate ion) in order to observe how they affect the rate of the reaction and ultimately determine the rate law of the reaction, 2 2 7 [] [ ] [ ] x yz rate k CH CH OH Cr O H  The technique of "flooding" the system with all reagents except one is commonly used by kineticists and results in a pseudo-order reaction with respect to the limiting reagent. "Pseudo" is a prefix from the Greek word that means to deceive. In fact, you are tricking the system into a critical dependence on only one substance. This can be shown mathematically as follows. Starting with the above rate law, if large amount of CH 3 CH 2 OH and H are present (relative to [Cr 2 O 7 2 ]), then the initial and final concentrations of these two species remain constant during the course of the reaction. As a rule-of-thumb, the limiting reagent should have a concentration ~100 times less than the others. Since [CH 3 CH 2 OH] and [H ] are relatively constant, they can be combined with k, [CH 3 CH 2 OH] x and [H ] z as a new constant called the observed rate constant, (k obs ). 2 27 y obs rate k Cr O where [ ] x z obs k k CH CH OH H Thus, in some experiments you will be seeking data with which to calculate y and k obs . In turn, variations in k obs with [CH 3 CH 2 OH] and [H ] will allow you to calculate x, z, and k.
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Lab Manual: Oxidation of Ethanol by Dichromate Ion 2 Spectrophotometry
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 06/02/2011 for the course CHEM 100A taught by Professor Dai during the Winter '06 term at UCSD.

Page1 / 9

Exp 6 - Lab Manual: Oxidation of Ethanol by Dichromate Ion...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online