The Harcourt Essen Experiment
The aim of this investigation is to: 1) find the rate equation for the reaction between hydrogen peroxide, potassium iodide and sulphuric acid by using the iodine stop clock method and plotting graphs of 1/time against concentration for each variable. Then to find the activation energy by carrying out the experiment at different temperatures using constant amounts of each reactant and then by plotting a graph of in 1/t against I/T, 3) to deduce as much information about the mechanism as possible from the rate equation.
The first experiments investigate the order of reaction with respect to the reactants; hydrogen peroxide, potassium iodide and sulphuric acid by varying the concentrations and plotting them against 1/time. An initial rate technique is used in this experiment so ‘the rate of reaction is inversely proportional to time.’ To find the order of reaction in respect to the reactants, 1/time is plotted against the concentration of Hydrogen Peroxide using the equation:
1/t = k(a) ͯ
t = time, a = volume of reactant, k is a constant of proportionality; x is the order of reaction. Because k is a constant of proportionality 1/t is directly proportional to the rate of reactant. Then to find out the order of reaction in a catalysed system the volume of ammonia molbydate is varied and the concentration of the other reactants kept the same. Thirdly to investigate the activation energies, the concentrations are kept the same and the temperature is varied.
The Harcourt Essen experiment consists of two reactions:
2H+ (aq) + H202(aq) + 2I-(aq) 2H2O(l) + I2(aq)
This is the first reaction in the Harcourt Essen experiment. The iodine is oxidised to produce I2 when reacted with the Hydrogen Peroxide. Then a further reaction takes place between the sodium thiosulphate (VI) and iodine as shown in the reaction below. Any unreacted iodine turns blue because of the starch indicator signalling the end point of the experiment.
I2(aq) + 2S2O3 2+(aq) 2I-(aq) + S4O6 2- (aq)
Hydrogen peroxide decomposes readily as shown in this equation
2H2O2 2H2O + O2 which is why it must be made up just before use in order to stop it reacting with the air and contaminating the experiment. The concentration of hydrogen peroxide is described as 10volume. This means that for everyone one volume of hydrogen peroxide that decomposes 10 volume of oxygen is given off.
The structure of Hydrogen peroxide makes it a good oxidising agent because of the lone pair of electrons which is why it reacts with the iodine in the first reaction.
Theory of kinetics
The first experiment investigates the order of reaction with respect to Hydrogen peroxide in an uncatalysed system. Firstly solution one is made up into a 300cm beaker containing 20cm of 1 percent starch solution, 10cm of 0.1M aqueous potassium iodide, 10cm 1M aqueous sulphuric acid, 5cm 0.1M aqueous...