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Fr E3 Sample Copy Essay

3577 words - 15 pages

CHEMICAL KINETICS: THE IODINE CLOCK REACTION

DARLENE M. ROXAS
NATIONAL INSTITUTE OF MOLECULAR BIOLOGY AND BIOTECHNOLOGY, COLLEGE OF SCIENCE
UNIVERSITY OF THE PHILIPPINES, DILIMAN, QUEZON CITY 1101, PHILIPPINES
DATE SUBMITTED: 10 DECEMBER 2013
DATE PERFORMED: 23 NOVEMBER 2013

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ABSTRACT

In a given chemical reaction, the reaction rates are expressed and explained through chemical kinetics. The iodine-clock reaction aims to explain concepts of reaction rate and show the effects of major factors in the reaction rates including the concentration, temperature and presence of catalyst between persulfate and iodide ion. Five ...view middle of the document...

(1)

Many of the biochemical processes proceeds spontaneously based from thermodynamics. But whether the rate of this reaction is slow or fast is described by chemical kinetics. Take for example an enzyme-catalyzed reaction C D, where C is the substrate and D is the product. Kinetics relates the concentration of the reactants to the rate of reactions. Hence, the rate law (2) is given by using the formula

(2)
where k is the rate constant and [C] is the given molar concentration of reactant C. The overall order of reaction can also be obtained using the equation (X) which is the sum of the exponents, m, of the reactants. These exponents, however, are not related to the stoichiometric coefficients; rather it is determined from the actual experiment. The order of the reaction sets up the rate law that can calculate rate constants given the molar concentrations of reactants and establish an integrated law that can relate the change of concentrations in a given period of time [2].

This experiment has been used as a visual representation involving the investigation of rate expression in chemical kinetics. The “"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. The time is controlled by the temperature and/or the concentrations of the reactants [3]. The experiment aims to understand the reaction mechanism of the iodine clock reaction and to describe the rate of reaction through measuring the rate of formation of I2 from the reaction of S2O82- and I-. The time when all 2O82- are consumed, the rate of I2 formation can be calculated.

To describe the kinetics of the iodine clock reaction, certain factors that may affect the rates of reaction were observed including the concentration of the reactions, temperature, and the presence of catalyst. In the experiment, the reaction (3) between persulfate, S2O8-2, and iodide, I- was studied.

The solutions from table 1 are prepared under controlled conditions: varying concentrations at constant temperature, varying temperatures at constant molar concentration, and presence of catalyst.

Table 1. Different runs for the effect of concentration to reaction rates
| Beaker A, mL | Beaker B (+ 3 drops of starch solution), mL |
Run | 0.2 m KI | 0.2 M KCl | 0.1 M K2S2O8 | 0.1 M K2SO4 | 4 mM Na2S2O3 |
1 | 10 | 0 | 5 | 5 | 5 |
2 | 5 | 5 | 5 | 5 | 5 |
3 | 2.5 | 7.5 | 5 | 5 | 5 |
4 | 5 | 5 | 7.5 | 2.5 | 5 |
5 | 5 | 5 | 10 | 0 | 5 |

The rate of reaction for the first condition is measured using the following rate law (4,5) according to the rate of disappearance of S2O82-/I- and appearance of I2 / SO42-.

(4)

(5)

where k is the rate constant, m and n are the orders of the two reactants obtained from the experiment using the initial rate...

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