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.
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 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 increasing the effective surface area.
6. In particular cases the rate may depend on the influence of electromagnetic radiation such as visible or ultra-violet light.
In this investigation the two factors affecting the rate of reactions that will be investigated, are concentration and temperature.
The Effect Of Concentration On The Rate Of A Reaction
In a reaction that takes place between two substances, A and B, if we were to
look at how quickly substance A is used, the rate of the reaction would be the rate of change of substance A, symbol rA. The rate of change of concentration of substance B, rB may also have been measured.
The rate of change of the concentration follows a general mathematical expression in the form:
Rate = k [A]a[B]b[C]c
This is known as the concentration rate equation where:
· The rate equation has units of mol dm-3s-1 (other units may be used).
· The square brackets denote the concentration (in mol dm-3).
· The sum of all of the indices is called the overall order of the reaction.
'k' is a constant of proportionality called the rate constant. The units of k depend on the order of the reaction and can be worked out from the rest of the rate equation.
Order of the Reaction
The kinetics of a reaction can be classified in terms of its order; these are experimentally determined quantities related to the rate expression.
Rate = k [A]a[B]b[C]c
The order with regard to a particular species (A, B or C) is equal to the power to which the concentration of this species is raised to (a, b or c)
The overall order of the reaction is equal to the sum of the powers of the concentration.
Order = a + b + c
Where the rate expression shows the reaction to be dependent on the concentration of one reactant, the concentration of the substance is raised to the power zero. Therefore, the rate is independent of the ...