The Iodine Clock Investigation

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The Iodine Clock Investigation

Introduction

This is an investigation into the rate of a reaction and the factors

that contribute to how fast a reaction will take place. Through the

recording and analysis of raw data, this investigation also allows us

to apply generally accepted scientific rules and to test them against

results gained from accurate experimental procedures.

Aim

The aim of this experiment is to investigate the rate at which iodine

is formed when the concentration and temperature of the reactants are

varied, and to attempt to find the order and activation energy.

The Chemistry

'THE IODINE CLOCK' - This is the experiment that will be used to

investigate reaction rates, and it is a reaction between acidified

hydrogen peroxide and potassium iodide:

2H+(aq) + 2I¯ (aq) + H[-1] 2O2 (l) ÕI2 (aq) + 2H2O2 (aq)

Iodide ions are firstly oxidised by the hydrogen peroxide, as shown in

the above equation. The iodine that is then produced reacts

immediately reacts with thiosulphate ions as follows:

I2 (aq) + 2Na2S2O3 (aq) Õ 2NaI (aq) + Na2S406 (aq)

As soon as all of the thiosulphate ions have reacted with the iodine,

the excess iodine molecules react with the 2% starch solution that is

present in the reaction. This can be seen as an instant change in

colour, from a colourless solution, to a deep purple coloured

solution. This change in colour denotes the completion of the

reaction.

Factors affecting the rate of reactions:

All chemical reactions occur at a definite rate under particular

conditions. In order to increase the rate at which reactions occur,

the frequency at which reacting molecules collide must be increased.

This may be achieved in a number of ways:

1. By increasing the concentrations of reacting species.

2. By increasing the temperature.

3. By increasing the pressure (only really significant in reactions

involving gases).

4. By the use of a suitable catalyst.

5. In the case of solids, by reducing particle size and thus

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