notes_162_Exp_2_Notes_041108

notes_162_Exp_2_Notes_041108 - EXPERIMENT 2 Part I A Clock...

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EXPERIMENT 2 Part I: A Clock Reaction ( Method of Initial Rates – measure rate before the [reactants] change significantly ) Part II: Crystal Violet - Hydroxide Reaction ( Integrated Rate Approach – rate laws expressing [reactant] as a function of time )
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BrO 3 (aq) + 6I (aq) + 6H + (aq) Æ Br (aq) + 3I 2 (aq) + 3 H 2 O(l) Use I 2 to follow rate of reaction Rate = Δ [Br ] = 1 Δ [I 2 ] = - Δ [ BrO 3 ] = -1 Δ [ I ] = -1 Δ [ H + ] Δ t 3 Δ t Δ t 6 Δ t 6 Δ t Rate law = k[BrO 3 ] b [I ] i [H + ] h k is the rate constant and depends on T b, i and h are the orders with respect to each of the reagents (These must be experimentally determined.) Reaction of interest in Part I…
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Goals of Part I: i) Determine the order of reaction with respect to each reagent (b, i, and h in the rate law) ii) Determine the rate constant (k) at room temp and 0°C iii) Determine the activation energy (E a ) iv) Determine the effect of a catalyst on the rate constant and the activation
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How do we determine the initial rate of the reaction? BrO 3 (aq) + 6I (aq) + 6H + (aq) Æ Br (aq) + 3I 2 (aq) + 3 H 2 O(l) - rxn of interest I 2 (aq) + 2S 2 O 3 2- (aq) Æ 2I - (aq) + S 4 O 6 2- (aq) - chemical trick Monitor the production of I 2 by including cornstarch in the reaction mixture (combination will produce a blue-purple complex). Use a “chemical trick” to initially suppress the production of I 2 so that you have a reasonable amount of time, between mixing the reagents and the formation of the colored complex, to look at your watch and make an accurate time measurement while keeping the concentrations of reactants almost unchanged. A relatively small, but precisely known, amount of thiosulfate (S 2 O 3 2- ) is added to the rxn mixture and reacts with I 2 at a rate much faster than the reaction of interest. (Consequence: The I 2 is consumed by S 2 O 3 2- before it can react with the cornstarch.) When the S 2 O 3 2- is used up, the I 2 produced in the first reaction begins to react with the pre-added starch indicator resulting in the formation of the blue color. When the color first appears, the rxn of interest has produced an amount of I 2 equal to ½ the amount of S 2 O 3 2- added (see stoichiometry above, rxn of interest is the only source of I 2 ). For this to be a true initial rate measurement, the reagents in the reaction of interest must change by only a negligible amount Æ < 1%.
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Our experiments will be based on the following reagent concentrations: 0.040 M BrO 3 - (KBrO 3 ) 0.100M H + (HCl) 0.010 M I - (KI) 0.00050 M S 2 O 3 2- (NaS 2 O 3 ) A typical run in the experiment is as follows: Tube #1 0.50 mL of 0.040 M BrO 3 - + 0.50 mL of 0.100M H + + starch indicator Tube #2 0.50 mL of 0.00050 M S 2 O 3 2- + 0.50 mL of 0.010 M I - + 0.50 mL H 2 O Mix Test tubes 1 & 2 to start reaction.
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This note was uploaded on 05/08/2008 for the course CHEM 162 taught by Professor N. during the Spring '08 term at University of Washington.

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notes_162_Exp_2_Notes_041108 - EXPERIMENT 2 Part I A Clock...

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