Investigating the rate of reaction between peroxydisulphate(VI) ions and iodide ions

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In this investigation I will be looking into:

  • The effect of changing the concentration of reactants upon the rate of reaction. I can calculate the order of the reaction with respect to the variable reactant (potassium iodide ions), and then I can use the known orders of other reactants to obtain the rate equation. From the rate equation I can then work out the overall order of reaction.

  • The effect of changing temperature upon the rate of reaction. I can find the rate of reaction for different temperatures and substitute these into the rate equation, allowing me to work out a value of k for each temperature. These k can be used to draw graphs from which I can calculate the activation enthalpy.

Section 1 - Theory

        The iodine clock reaction is a two stage reaction. In the first (R1), peroxydisulphate (VI) ions react with iodide ions to form sulphate (V) ions and iodine:

Both reactants and the sulphate ions are colourless. To measure the initial rate of this reaction, the colour of the iodine produced can be followed. The colour change can be detected more clearly by adding starch to the mixture. This causes a blue-black solution to form in the presence of iodine.

A way to measure the initial rate of this reaction is to time how long it takes for the reaction to produce a small, fixed amount of iodine. The time taken can be enhanced by adding thiosulphate ions to the reaction mixture at the beginning. These thiosulphate ions turn iodine back to iodide ions:

        

This means that no starch-iodine will appear until all the thiosulphate has been used up. What will be seen is a colourless solution that after a certain amount of time turns a vivid blue/back colour. By measuring the length of time taken for this colour to appear, you will know how long it took to produce the equivalent amount of iodine


Rate

        Rate of reaction is concerned with how quickly a reaction reaches a certain point. The rate of reaction can be observed either by how quickly the reactants are used up or by how quickly the products are forming. The rate of reaction can be affected by several conditions

  • Surface area of a solid
  • Temperature
  • Concentration of reactants
  • Pressure of a gas
  • Presence of a catalyst

In this investigation, I will be focusing on changing the temperature, and the concentration of potassium iodide ions.


Concentration

Concentration is a measure of the number of molecules in a substance of a given volume. By increasing the concentration of one or more of the reactants, there is an increase in both the likelihood of collisions between reactant particles and the frequency by which reactant particles will collide at any given time and volume. Therefore, an increase in concentration of one or more reactants causes products to be formed faster, increasing the rate of reaction.

The rate equation

A rate equation can be written for any reaction, as long as an experiment is carried out to find how the rate depends on the concentration of the reactants.

For the general reaction in which A and B refer to the reactants:

The rate equation is:

n = order of reaction with respect to reactant A

m = order of reaction with respect to reactant B

The overall order of reaction is calculated by adding together the order of reaction for each reactant. In the case of the equation above, the overall order of reaction would be expressed as m + n.

The order of reaction for a specific reactant is related the how its concentration affects the rate of reaction. When plotting the graph of rate against concentration, each order of reaction will show a specific trend. Figure 1.1 represents these graphs for zero, first and second order reactions. The shape of this graph can therefore be used to calculate the order of reaction of potassium iodide ions in this investigation.

The general equation for the rate of a reaction is:

        Therefore, it can be said that increasing the concentration of one or more reactants will cause an increase in the rate of reaction.

In my investigation, I will be varying the concentrations of iodide ions from the first reaction (R1):

        By changing the concentrations of iodide ions, the likelihood of collisions between iodide ions and peroxydisulphate (VI) ions is varied. With a higher concentration of iodide ions, more collisions occur over a given time and volume, leading to a higher rate of iodine production. This iodine is then used in the second reaction (R2).


Temperature

Having a reaction at higher temperatures introduces more energy into the system. This increases the average kinetic energy of the reacting particles, which in turn increases the likelihood of collisions between reacting particles. Kinetic energy is given by the formula:

The kinetic energy of a point object is proportional to its absolute temperature:

Mass will be constant within a particular species, meaning that the kinetic energy is only dependent on the velocity at which the particles are moving. From this, it can be said that the average velocity of the particle is proportional to the square root of its absolute temperature:

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So:

Building on from this, I will now work out how much more frequently particles will collide with an increase in temperature from 320K to 330K:

So, a temperature change from 320K to 330K causes the average velocity of the molecules increases by a factor of 1.016, about 1.6%. The rate actually increases by much more than this, sometimes by 200% to 300%. Therefore, this demonstrates that temperature not only has an effect on the number of collisions, but also with how much energy they collide.


Maxwell-Boltzmann:

“As the ...

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