Rates of reaction - kinetics (A2)
Read our general notes on Risk Assessment
What I am actually doing is investigating a rates reaction (kinetics) and also taking into account the activation energy of the reaction.
My method is to mix phenol and potassium bromide/potassium bromate solution in one test-tube (also add a few drops of methyl red), warm it to a variety of temperatures, then add sulphuric acid which has been warmed to the same temperature. Then I will mix the two test-tubes contents, and measure the time taken for the red colour to disappear. This will give me the initial rate of the reaction.
The simple mechanism of this reaction is that bromine is produced, reacts with phenol to give tribromophenol, then when all phenol is ‘used up’, the next excess of bromine molecules will oxidise the methyl red so the colour is bleached.
My results show that the order of reaction wrt sulphuric acid is 2, and wrt bromide/bromate 1. But I have no real trend for the order wrt phenol, can you see why?
However, my main problem is that after extensive research, I cannot find an alternative method to do the same investigation. I think an alternative method could be a titration, however, I cannot find out any information about this so I am not sure exactly what i will need to prepare and how to stop the reaction or vary temperature.
090208
Ulex replies
Thank you for getting back to me about this. I am intrigued by the method but still very puzzled about what is going on. You say that the time taken for the indicator to be bleached is a measure of the initial rate, but of what reaction? You have three reactions going on here! They are as follows:
Bromide + bromate + acid -> bromine
Bromine + phenol -> tribromophenol
Bromine + methyl red -> colourless products
The method is a very clever one, which must be examining the rate of the first of these reactions. It works because the second and third reactions are instantaneous. This is why you are not getting sensible results by varying the phenol concentration.
Your results for the sulphuric acid are valid, but the results for bromide and bromate need to be separated – you can’t attach meaning to an order with respect to a mixture. From previous experience, I think the reaction is first order with respect to bromide, and first order with respect to bromate, but you do need to demonstrate that this is so.
A possible alternative method for studying the bromide/bromate/ acid reaction is to prepare a mixture of potassium bromide and potassium bromate solutions, and add acid to start the reaction. Bromine is produced. Then, at measured time intervals, remove portions of the reaction mixture with a pipette, stop the reaction by running the pipette-load into sodium hydrogencarbonate solution which neutralises the acid, add potassium iodide, and titrate the liberated iodine with sodium thiosulphate. You would have to work out some suitable concentrations and volumes but it is a possibility.
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 Rates of reaction - kinetics (A2)
Rate this page or react
Share your views on this page, 1 ratings so far
, rated at 
updated: 09 February 2008
