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Investigating The Rate Of Reaction Between Peroxydisulphate (Vi) Ions And Iodide Ions

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The aims of this investigation are:

1. To find the rate equation of the reaction of hydrogen peroxide and iodide ions. This will be achieved by using an iodine clock method and colorimetric analysis.

2. Draw a graph of rate against concentration for each reactant (Hydrogen peroxide, potassium iodide and H+ ions).

3. Finding the order for each reactant

4. Finding the rate-determining step.

5. Proposing a mechanism for the reaction.

6. Using Arrhenius' equation to find the activation enthalpy.


The basic reaction for this can be illustrated with the following equation:

3I-(aq) + H2O2(aq) + 2H+(aq) → I3-(aq) + 2H2O(aq) (1)

The half equations for this reaction can be written as follows:

3I-  I3- + 2e-

H2O2 + 2H+ + 2e-  2H2O

This reaction demonstrates that iodide ions are oxidised by hydrogen peroxide to tri-iodide ions.

This is stage one of a sequence of reactions, which continues below:

I3¬-(aq) + 2S2O32-(aq) ¬ 3I-(aq) + S4O6-(aq) (2)

This shows that the tri-iodide ions are reduced back to iodide ions by the thiosulphate ions. Thus, the iodine that is formed in reaction (1) is immediately transformed into iodide ion and we do not see the blue-black colour of the starch-iodide complex until all of the thiosulphate ion has reacted with I 2(aq) and is exhausted.

I3- (aq) + starch  Starch-I5- complex + I-(aq)

Once the thiosulphate ion has been exhausted, the tri-iodide ion can react with the starch, forming the Starch-I5- complex, giving the blue-black colour. When this occurs, we will then know the amount of hydrogen peroxide that has reacted and the time it took to react.

These equations will, thus, enable the slow step (rate-determining step) to be determined, which is another aim of this experiment.

Though details of the starch and iodine reaction are not yet fully known, it is thought that iodine fits inside the coils of amylose. The transfer of charge between the iodine and the starch and the spacing between the energy levels in the complex formed corresponds to the absorption spectrum, and so, the complementary colour, a blue-black solution, is observed.

Factors that affect the rates of reaction ,

There are many factors that affect the rate of a reaction. These include surface area, concentration difference, presence of a catalyst, pressure and temperature.

Affect of surface area on the rate of reaction

Surface area can also affect the rate of reaction. A reaction will happen more quicker if the solid is finely divided into a powder, rather than a lump of the same mass. This is because a reaction can only occur if the particles taking part in the reaction collide. A larger surface area provides a higher likelihood of collisions (and thus, a reaction) to take place. One example is called the "Bread and Butter Theory." If you take a loaf of bread and cut it into slices it, you have more surfaces to spread butter onto. Taking a more practical example,

In the above example, reactant 1 can get to the outer atoms of reactant 2, but not the central atoms. This has a small surface area. However, if the surface area is increased,

Reactant 1 can get to all the atoms of reactant 2.

Increasing the number of collisions per second increases the rate of reaction.

Affect of concentration on the rate of reaction

For many reactions, including this one, increasing the concentration of the reactants increases the rate of the reaction. This is because, for a reaction to occur, a collision must take place first. Increasing the concentration of the reactants will increase the frequency of the collisions between two reactants, as there are a higher number of reactants to collide with. From a probabilistic point of view, if there are a higher number of reactants (i.e. a higher concentration), the chance of a collision, and therefore, a reaction to take place, increases. For example, if we have the following situation:

supposing fixed positions and an equal probability of being hit, the probability of a green particle hitting a red particle is 1/3. If we increase the number of red particles to 2, the probability now of a green particle hitting a red particle is Ð..., which is thus, an increase by 1/6.

Although the temperature is being kept constant, however, the kinetic theory is applicable. This is because the molecules involved in the reaction have a range of energy levels. When colliding molecules have sufficient energy, a reaction takes place. If they do not, then a reaction cannot take place. This is because the temperature that is being measured is only the average temperature (and thus, kinetic energy, because T α Ek) of the substance. It is impossible for the kinetic energy of every atom in the substance to be the same, and so, the temperature is an average. The reason for this is that each molecule has a certain amount of kinetic energy and once it collides (perfectly elastically) with another molecule, it transfers its kinetic energy to the molecule it collided with, giving it a higher kinetic energy than the initial molecule. In most cases, when you increase the concentration, the rate of reaction also increases. In certain multi-step reactions, however, the reaction happens in a series of small steps. Suppose the reaction happens as so:

The speed at which A splits into X and Y dictates the rate of the reaction. This is also known as the rate-determining step. If you increase the concentration of A, the chances of the first step happening also increase, due to the increase in the number of molecules of A. Increasing the concentration of B undoubtedly speeds up the second step, but makes little difference to the overall rate.

Affect of



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