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Iodine Clock Reaction

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Introduction

Annesha Mohamed Dec. 7th/2004 IODINE CLOCK REACTION The rate law states that r, will always be proportional to the product of the initial concentrations of reactants, where these concentrations are raised to come exponential values. The order of reaction is the exponent value that describes the initial concentration dependence of a particular reactant. If the order of reaction is 0, then doubling the initial concentration of a reactant has no effect on rate; if order of reaction is 1, then doubling initial concentration doubles rate; and if order of reaction is 2, then doubling initial concentration quadruples rate of reaction. "Iodine Clock" refers to a group of reactions which involve the mixing of two colorless solutions to produce a solution which remains colorless for a precise amount of time, then suddenly changes to a deep purple-blue color of starch-iodine complex. The time is controlled by the temperature and/or the concentrations of the reactants. The rate of this reaction depends on the temperature, and on the concentrations of iodate ions. This reaction alone does not give very impressive delays and color changes. The time delay until the appearance of the blue color is inversely related to the rate of the reaction --the faster the reaction, the shorter the delay but the color development is directly related to the rate--a sharp change in the color requires a moderately fast reaction. ...read more.

Middle

The time taken for the change colour to occur was measured from the first drop of Solution A in Solution B to the first appearance of colour in new solution. The pipette was not allowed to touch Solution B so as to eliminate the need for rinsing it after each reaction. o Additionally, the contents of Well 2 of spot plate A were transferred to Well 2 of spot plate B and the time elapsed before colour change was evident was recorded. This process was continued until no visible traces of colour could be detected when different concentrations of Solution A were added to constant concentrations of Solution B. OBSERVATIONS/ANALYSIS: The reactions are: 1. IO3- + 3HSO3- --> 3SO4 2- + 3H+ (slow) 2. 5I- + IO3- + 6H+ --> 3I2 + 3H2O (slow) When all the Bisulfite is used up: 3. I2 + HSO3- + H2O --> 2I- + SO4 2- + 3H+ (fast) 4. I2 + I- --> I3- (fast) 5. I3- + starch --> I3- starch complex (blue black) [KIO3] 0.0522 mol/L [NaHSO3 ] 0.0538 mol/L Time(s) 1 ?t(s) Final concentration of Solution (mol/L) V2 equals 20 drops 0 0 0 0 0 10 drops 10 drops 5.490 0.1821 C1V1 = C2V2 0.0522 x 10 = C2 20 C2 = 0.02610 ...read more.

Conclusion

(i.e. if the concentration of iodate ions is doubled, the rate of reaction will also be doubled. Doing such investigations can be tedious, but the realization that the results are actually beneficial and reliable makes it worthwhile. In this reaction though, errors occur, but not so much that the results were not acceptable. Firstly, the concentration of reactants may have been inaccurate. It is important to add specific volumes of water to the wells for Solution A, so as to dilute the concentration, and yield a volume of 10 drops of solution in the well. The sizes of the water drops were not all equal, even though pipettes were cautiously picked so that all three looked about the same size. When transferring Solution A from spot plate A to B, the pipette might have accidentally touched Solution B. This would have thrown off the accuracy of the concentration of Solution A. As always, measuring short time intervals prove difficult and times may have been inexact. Nonetheless, the goal of the experiment was met. A dramatic colour change enabled us to investigate the effect of concentration on rate of reaction and figure out that when concentration of reactants and rate of reaction increase proportionally, the order of reaction is 1. ...read more.

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