Rates of reaction - kinetics (A2)
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What is balanced chemical equation between phenol and bromine? and what are the functions of phenol and methyl red in reaction between:
5KBr + KBrO3 + 3H2SO4 -> 3Br2 + 3K2SO4 + 3H2O
and can it be carried out at high temperatures such as 90°C ?
Jenkin writes
Phenol and methyl red are involved in an experiment to determine the activation energy, Ea, for the bromide/bromate reaction which has the equation you have quoted.
It can be written as the ionic equation:
BrO3- + 5 Br- + 6H+ -> 3Br2 + 3H2O
The idea is to measure the time, t, for the reaction to proceed to the same (small) extent at different temperatures. Then, a graph of ln(1/t) against 1/T should be a straight line of gradient –Ea/R (using the Arrhenius equation).
The reason for adding a small amount of phenol at the start is that it will react with the bromine formed early in the reaction. Once all the phenol has reacted the bromine now formed will bleach the methyl red so there is a rapid colour change from red to colourless and the time taken to reach this point can be recorded. If no phenol was added the reaction would be too fast to time; the phenol allows a ‘breathing space’ which is the same for every run of the experiment. The methyl red here is a substance rapidly bleached by bromine, not a normal acid-base indicator.
The equation for the reaction of phenol with bromine is
C6H5OH + 3Br2 -> C6H2(OH)Br3 + 3 HBr
(Unfortunately I can’t draw benzene rings here; the organic compound above is 2,4,6-tribromophenol. You can probably work out its formula but, if not, you should find this reaction in a standard A-level text which deals with phenol).
At 90 °C the reaction will be too fast to time; 75 or 80 °C is probably the maximum (likely to be under 10 seconds). But a range of temperatures from 25 to 75 should give plenty of points for a good graph.
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updated: 02 March 2006
