Practical investigations
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Hello, I have read closely the web pages related to the metal-acid reaction. My question is which, in theory, is supposed to have a lower activation energy, mono or dibasic acids? I reacted magnesium with HCl and H2SO4, but found that HCl had a lower Ea than H2SO4, as I expected. However, on the page where people listed there Ea's, they all seemed to be the other way around, the H2SO4 having a lower Ea. I thought that as HCl was a stronger acid, that it should have a lower Ea, as I calculated, but this seems different to others results. I checked that they used the same metal and similar concentrations. The values themselves seem OK, but I was just wondering if you could tell me which one should be higher? Thank you.
Ulex writes
I agree with you that the value with hydrochloric acid might be expected to be fractionally geater than with sulphuric acid. I have two reservations about this, however. Firstly, I doubt if the difference would be great enough to be reliably discernable by rate/temperature measurements of the kind which you performed. To put it another way, the difference is likely to lie within the range of possible/probable errors in the experiments.
The wide scatter of student results, even for the same acid, is surely evidence of this. If you have access to the Nuffield Book of Data, look at Table 5.7 on p113. Enthalpies of neutralisation give a quantitative measure of the difference between ease of ionisation of strong and weak acids. The difference in values between hydrochloric acid(strong) and ethanoic acid(weak) is less than 2 kJ mol-1. The difference is much less between two relatively strong acids.
The second point is rather more subtle. You are assuming a connection between the ability of acid molecules to dissociate in solution (their strength) and the rate of reaction - that is, you are assuming that the reaction is exclusively between metal atoms and 'free' hydrogen ions. Furthermore, you are assuming that this interaction is the rate-determining step of the mechanism of reaction. Your reading of the evidence gathered so far might convince you that neither of these things is necessarily true.
There is little doubt that the metal/acid reaction is a great deal more complex than it seems!
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updated: 02 April 2007
