Bleaching experiment Essay
Both potassium iodide and ethanoic acid act as excess reagent, so that it is not necessary to know the accurate amount, what are the most concerned are that the accurate amount of the limiting reagents. For a more clear explanation, it could be accounted for by these ways: First, it is unnecessary to measure the accurate amount of potassium iodide because: Iodide ions must be in excess amount so as to ensure the chlorine gas evolved in reaction (1) to be reduced to chloride ions in reaction (2) provided that chlorine gas evolved in reaction(1) is easy to escape from the solution in the conical flask.
Thus, in the excess amount of potassium iodide, it can reduce the maximum amount of chlorine gas. What is more is that; potassium ion is a extremely weak oxidizing agent which would not partake any reaction in this experiment and so would not make any inaccuracies in the experiment even they are in the excess. Second, it is also not necessary to measure the accurate amount of ethanoic acid because: Ethanoic acid is needed to be excess for speeding up the completion of reaction (1) and (2). In reaction (1) and reaction (2), both need H+ ions for the completion of the redox reactions.
Thus, ethanoic acid is needed to be speed up the two reactions or it will make the great errors for the experiment. Apart from it, chlorine gas evolved in reaction (1) is easy to escape from the solution in the conical flask, which needs a short time for reaction to minimize the escaping of chlorine gas. In this way, the acid in excess amount can achieve this. Furthermore, NO3- , which is a very weak oxidizing agent compare to chlorine gas, so that it will not make any inaccuracy for the experiment though in excess amount.
(2) What is the function of starch solution? Why we should NOT add the starch solution at the beginning of the titration? The starch solution acts as indicator, whose end point is to indicate the completion of reaction. In fact, there is the color change from iodine to iodide. It, however, cannot be used to accurately detect the end point (the change in color ‘brown–>yellow–>colorless’ is very difficult to observe), thus, starch (preserved with salicylic acid) is used as the indicator.
Starch + iodine Blue ‘complex’ Starch solution should not be added at the beginning of the titration because: Since starch irreversibly combines with iodine at a high concentration of I2(aq) ( so that I2 will not be released from starch at the end point), the starch solution should be added at the later stage of the titration (when the solution just turns from brown to pale yellow). After the addition of starch, the mixture turns deep blue. Then the end point is shown by the complete decolorization of the blue color.
University/College: University of Arkansas System
Type of paper: Thesis/Dissertation Chapter
Date: 10 July 2017
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