Exp3 - Kinetics - A KINETIC STUDY OF THE DECOLORIZATION OF...

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1 A KINETIC STUDY OF THE DECOLORIZATION OF PHENOLPHTHALEIN IN VERY BASIC SOLUTIONS EXPERIMENT 3 INTRODUCTION Reactions can occur over a wide variety of time intervals. Some reactions are very slow (copper in moist air slowly reacts to form green basic carbonate), others are very fast, almost instantaneous (acids and bases react very fast with indicators). Although, the vast majority of reactions fall somewhere in between on the time scale. By studying the parameters affecting the rate of a reaction, chemists can find ways to speed up or slow down a reaction. In this experiment, you will study the reaction of phenolphthalein in strongly basic solution. Phenolphthalein changes from colorless to pink at a pH of about 8, it is a fast reaction, represented by equation 1. C C O O OH HO C C O O - O O - + 2 OH - + 2 H 2 O H 2 P , colorless P 2- , pink The letter P is for the whole phenolphthalein molecule, H 2 P is the acid form, and P 2 the basic form. However, if the pH is increased to 11 or more, the pink color fades away. That reaction is not instantaneous, and therefore the color change is not instantaneous. The reaction is: C C O O - O O - C C O O - - O OH O - + OH - P 2- , pink P OH 3- , colorless The rate law for that reaction is: rate = k [OH ] m [ P 2 ] n , (3) where k is the rate constant for the reaction, n is the order with respect to phenolphthalein ion ( P 2 ), and m is the order with respect to the hydroxide ion. When the hydroxide ion concentration is more (1) (2)
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2 than 1000 times as large as the concentration of P 2 , [OH - ] does not change appreciably during the reaction. Therefore, considering that [OH ] remain constant with time, equation (3) can be simplified to: rate = k 1 [ P 2- ] n (4) where k 1 = k [OH - ] m (5) In this experiment, you will observe the reaction represented by equation (2). As the reaction proceeds, you will see the pink color slowly fade away to yield a colorless solution. Using the green (565 nm) light source of the Colorimeter, you will monitor the absorbance (ABS) of the phenolphthalein with time. Since ABS is proportional to the concentration of P 2 (Beer law), ABS will be used in place of [ P 2 ] in plotting the graphs. Once the order with respect to P 2 has been determined, you will also be finding the rate constant k , and the order m for this reaction. The order of a reaction can only be determined experimentally. Often, chemists will assume that a reaction is a particular order and then test the validity of the assumption. In this experiment you will successively assume that the reaction is: zero order, first order, then second order. you will test each assumption by plotting the appropriate graph. If the reaction is zero order
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This note was uploaded on 04/04/2011 for the course SCIENCE CHEM 120 taught by Professor Fenster during the Winter '11 term at McGill.

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Exp3 - Kinetics - A KINETIC STUDY OF THE DECOLORIZATION OF...

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