Practical investigations
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I have just finished working out the activation energies for reactions between HCl + Mg and CH3COOH + Mg. The values I found were 40.7 kJ/mol for HCl and 42.1 kJ/mol for CH3COOH. All of my results for my experiments suggest that the activation energy for HCl should be much lower than that for CH3COOH.
I used the method where 1/time taken to collect the gas is propotional to the rate constant. I got in the region of 20 s for 1.0 M HCl at 278 K and in the region of 140 s for 2.0 M CH3COOH at 278 K. I've double checked all of my calculations so I'm very confused. Do my results seem right or way off the mark? I've looked at what other people have posted but this is no help. Do you have a definite answer for the activation energy for my 2 reactions?
Ulex writes
I would say that your results are really quite reasonable. One would expect the activation energy for the reaction of the weak acid to be higher than the one for the strong acid and a few kJ mol-1 is reasonable. Bearing in mind that the Arrhenius equation is a logarithmic relationship, a small difference in activation energy corresponds to a very small difference in ln k, but to a large difference in k itself and therefore to a large difference in rates for similar reactant concentrations.
You have actually done rather well to identify the difference in activation energies so well – most sets of experiments of this kind are insufficiently accurate to identify such a small difference.
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updated: 11 November 2007
