Ascorbic Acid Critical Thinking
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A reaction in which a certain property of a reactant changes after a predicted time is known as a “clock reaction”. In this study, the effect of increasing the volume of ascorbic acid to the time elapsed before a colour change was determined. It was found that when the volume of ascorbic acid is increased, the time elapsed before the appearance of a black-blue colour also increased. The graph plotted for time elapsed against volume of ascorbic acid added shows a positive linear relationship.
The more the amount of ascorbic acid was present in the reaction mixture, the longer it took to be consumed through a reaction with triiodide ions, produced by the reaction of hydrogen peroxide and potassium iodide.
Using the reagents provided, 5mM ascorbic acid, 10 ml 3% hydrogen peroxide solution and 10 ml 1% potassium iodide solution, it was determined that it took 36 seconds to completely use up 104.0µL and 46 seconds to completely up ascorbic acid. Possible sources of error in the experiment would include inaccurate measurement of reaction times.
The iodine “clock reaction” is a reaction between two solutions, which contain, among other things, starch and iodine. When these two solutions are combined, a series of reactions follow, ending with the release of iodine in its elemental form (Weiner & Harrison, 2009). When iodine is combined with starch, they make a black-blue complex as shown in the following reaction:
2I3-(aq) + starch (aq) starch- I5- complex (aq) + I-(aq)
Triiodide ions can be formed when potassium iodide (KI) is oxidised by hydrogen peroxide (H2O2) as in the following reaction:
2H+(aq) + 3I- + H2O2 I3- + 2H2O (slow)
However, when ascorbic acid (C6H8O6) is added to the reaction mixture containing starch, it immediately reacts with the triiodide ions reducing them back to iodide...