Practical investigations
Read our general notes on Risk Assessment
I am currently carrying out an iodine clock reaction between peroxodisulphate and iodide ions. I have determined the rate equation (I got first order wrt both reactants), calculated the activation energy ( approx 50 kJ mol-1), and now must investigate the catalysts. I have no idea how to even plan what catalysts I am required to try. Is it just trial and error or is there a way of determining what solutions are most likely to catalyse my reaction? I understand that solutions such as Fe2+ are useful as they form Fe
Igloo says ...
In order to understand these concepts fully you need to have a knowledge and understanding of redox potentials, but even if you haven’t, perhaps my explanation below will give you some idea and encourage you to research this topic for yourself.
The reaction between peroxodisulphate and iodide ions is redox and involves two half reactions, the peroxodisulphate / sulphate ions system (which has a standard electrode potential of +2.01 volt) and the iodide ions / iodine molecules system (which has a standard redox potential of +0.54 volt). Since the first of these systems is more positive than the second, the reduction of peroxodisulphate ions drives the oxidation of iodide ions to iodine molecules, and the voltage difference (about 1.5 volt) represents the “driving force” behind the reaction.
The iron(III) / iron(II) ions system has a voltage of +0.77 volt, and this lies between that of the two mentioned above, so it is feasible for peroxodisulphate ions to oxidise iron(II) ions first, and then for the iron(III) ions formed to oxidise the iodide ions in a second step. If the activation energies for these two processes are below the activation energy for the reaction occurring without the catalyst then the rate of reaction should be increased, and since the Fe2+ ions have been regenerated we can say that they have acted as catalysts.
You could also try any other redox system for which the standard electrode potential lies between +0.54 volt and +2.01 volt. A good book of data will supply you with some ideas.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
back to Practical investigations
Rate this page or react
Share your views on this page, 40 ratings so far
, rated at 
updated: 08 February 2006
