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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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