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Investigating the Kinetics of the Reaction Between Iodide Ions and Peroxodisulphate (vi) Ions (entire Plan)

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Investigating the Kinetics of the reaction between Iodide ions and Peroxodisulphate (VI) ions

By the use of an Iodine clock reaction I hope to obtain the length of time taken for Iodine ions (in potassium iodide) to react fully with Peroxodisulphate ions (in potassium Peroxodisulphate). I will do three sets of experiments changing first the concentration of iodide ions, then the concentration of Peroxodisulphate ions and finally the temperature of the solution in which the reaction is taking place. From these results, I hope to draw conclusions as to the effects of these changes to the environment of the reaction on the rate and also determine the order of the reaction and the activation enthalpy.

Background information

The rate of a reaction is determined by a number of factors. These include: pressure, temperature, concentration of reactants, surface area of reactants, presence of a catalyst and radiation.

The effect of these factors can be explained using collision theory. Reactions occur when the reactant particles collide, provided the colliding particles have enough energy for the reaction to take place. As the molecules approach their electron clouds repel. This requires energy - the minimum amount of which is called the 'activation enthalpy' - and comes from translational, vibrational, and rotational energy of each molecule. If there is enough energy available, this repulsion is overcome and the molecules get close enough for attractions between the molecules to cause a rearrangement of bonds and therefore an 'effective' reaction has taken place. The more collisions of particles with kinetic energy over the activation enthalpy that occur, the faster the overall reaction. During this investigation I am focusing on the effect of temperature and concentration while aiming to maintain other rate determining factors at a constant level in order to ensure reliable results.

Effect of concentration

Taking the collision theory into account the effect of concentration is simple in that the more particles of the reactants there are in the same area of space the more likely the collisions and therefore the faster the overall reaction. The following equation has been determined through experimentation showing that the rate of a reaction depends on concentration of reactants A:

Rate [A]n

Where n is a constant called the order of the reaction. This tells us the exact dependence of a rate of a reaction on the concentration. From this relationship the rate equation is obtained:

Rate = k[A]n

Where k is a constant of proportionality called the rate constant.

Effect of Temperature

A basic law of physical chemistry is that an increase in temperature causes an increase in the rate of any reaction. As the collision theory states, for a reaction to take place the particles need to collide. If the temperature is increased, each particle has greater kinetic energy transferred from the heat energy, and therefore is moving faster (the average speed of molecules is proportional to the square root of the absolute temperature.) The faster the particles are moving, the more likely they are to collide and therefore the faster the reaction. Also, the more energy transferred to each particle due to increased temperature the more likely it is to surmount the activation enthalpy and again the higher the number of effective collisions. As a general rule, the rate of a reaction doubles for every increase of 10K in temperature.

The diagram below demonstrates the effect of temperature on the rate of a reaction. Despite the initial increase in the energy of particles of a lower temperature, one can see that those at a higher temperature eventually surpass and lead to an overall higher amount of particles with energy higher than the activation enthalpy and therefore a greater number of effective collisions.

The exact relationship between temperature and rate of reaction was first proposed by a Swedish chemist called Arrhenius in 1889, most famous for his self-named equation that stated:

k = Ae -Ea/RT

where k is the rate constant for the reaction; A is a constant for the reaction; Ea is the activation enthalpy; R is the gas constant and T is the temperature in kelvin. In terms of log to the base 10 this is:

log k = log A -Ea/ 2.303 RT

Reaction between Iodine ions and peroxodisulphate ions

S2O82-(aq) + 2I-(aq)  2SO42-(aq) + I2(aq)

In order to make the reaction clearer, during my experiment I will add starch and a small known amount of sodium thiosulphate (to act as a queching agent). The thiosulphate ions turn iodine back to iodine ions:

2S2O32-(aq) + I2(aq)  S4O62-(aq) + 2I-(aq)

Which means that no starch-iodine colour will appear until all the thiosulphate has been used up. The amount of time taken for this occur (and the reaction to suddenly turn blue) is the same amount of time for the reaction to produce the equvilant amount of Iodine.


(For making up solutions)

weighing boats


Beaker (150cm3)

3 Volumetric flasks (250cm3)

Distilled water

Glass rod

(for concentraion and temperature change experiments)

4 thermometers (0-110Ñ"C)

A large number of boiling tubes (roughly 50 depending on repeats)

5 Burettes with funnels for filling

5 Clamp stands (for burrettes)


(for temperature change only)

Two large beakers (400cm3)




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