Practical investigations
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My aim is to investigate the activation energy of the reaction of a monoprotic, a diprotic acid and a triprotic acid with magnesium by varying the temperature of acid
Basically I'm seeing how long it takes for the reaction to end at different temperatures. My results are:
HCl= 26.8 kJ mol-1
H2SO4= 12.5 kJ mol-1
H3PO4= 16.8 kJ mol-1
I don't understand what this means though.... what does this tell me? What am I supposed to do afterwards - just say that monoprotic acids have the highest Ea, then triprotic and then diprotics have the lowest Ea?
What does this mean then - that since the Ea is low, the reaction is faster as less energy is needed to overcome the Ea? and does the Ea have +/- sign in front?
090109
Corrie writes .....
Activation energies relate to the rate-determining step in a reaction, which you may have already proposed based on any studies of the order of these reactions you might have done as well.
The first thing you can say about an individual activation energy is to comment on its size - compared for instance to the energy of breaking a typical covalent bond, say 300 kJ mol-1. Comparing your value to this gives some clue as to the type of step that might be involved in the rate-determining step in the mechanism for the reaction - does it look as if it involves breaking covalent bond(s), reaction between ions, or some other step?
On the other hand, when you are comparing activation energies from different acids, you must decide whether they a really different WITHIN your experimental error - I doubt very much that you can claim activation energies accurate to 3 significant figures in this sort of experiment. So, are they really different, or or they more or less the same? If they are the same, is this what you would expect, and why?
On the other hand, if they really are different, can this be explained in terms of any difference in the way these acids might react? If you go to 'Practical investigations' link on this site you will find a tutorial called 'Activation energy: what answer did you get?'. Here we publish values for activation energies reported to us by A-level students for the reactions of metals with acids. There are now enough results for you to be able to see the typical values and the range of values obtained by others, and compare them with yours
It may also be helpful to look at some of the past questions and answers on this topic, which you can find via the 'Sorted FAQs on Investigations' link on the Re:act Homepage, in particular section D of Metal + Acid investigations.
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updated: 11 January 2009
