Rates of reaction - kinetics (A2)
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I am doing the iodine clock reaction with starch and potassium iodide. I have varied the temperature but kept everything else constant. How do I work out the activation enthalpy for this reaction?
Jenkin writes
You should be able to find general information about this (not necessarily concerning this particular reaction) in most standard A-level textbooks. It is clearly described in the Nuffield Students’ Book, section 11.4. But here is the usual way forward.
The relationship between rate and activation energy is shown by the Arrhenius equation (Arrhenius, 1859-1927, was one of the great 19th century physical chemists. Himself Swedish, he worked in Germany before returning to Sweden, where he was awarded the Nobel Prize for Chemistry in 1903).
The equation, in its most useful, logarithmic, form is
lnk = constant – EA/R x (1/T)
where k is the rate constant,
EA is the activation energy,
R is the gas constant, 8.31 J K-1 mol-1
T is the temperature in K.
So if you plot lnk, or more easily, ln(rate), on the vertical axis against 1/T on the horizontal axis, you should get a straight line of slope –EA/R.
In your case, you will have values of times of reaction at different temperatures. Convert the times (in seconds) into rates by calculating their reciprocals (1/t). Then find ln (the natural logarithm) of each rate. Also find the reciprocal of each temperature (1/T). Then plot your graph, measure the slope of the line, sticking to the units you have used for 1/T (ln rate will have no units). The line will have a negative slope which you now multiply by -8.31 to get your value for EA. You will need to divide it by 1000 to get it in kJ mol-1.
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updated: 24 April 2006
