Practical investigations
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I have calculated the activation energy for reactions of HCl and magnesium, and zinc with HCl, and found the activation energies to be different (14KJ for magnesium and 32KJ for zinc). I thought that, because the H-Cl bond is the only bond, breaking the metal should have no affect on the activation energy, because the activation energy only depends upon the breaking of bonds, which is the same in both reactions
Ulex suggests
Your results seem quite reasonable, but there are two ways in which I think your analysis of the situation is at fault.
Firstly, a solution of ‘HCl’ is not really this at all. The solution contains virtually no molecules HCl. They have all been converted by the water present into hydrated protons, H+(aq) and chloride ions, Cl-(aq). The reaction with both metals involves the conversion of the hydrated protons into hydrogen molecules. It is this which is the same in both reactions.
Secondly, the metals undergo change from a lattice of metal atoms to a solution containing metal ions. The energy required to break apart the lattice is likely to be a major contributor to the activation energy and will be different for the two metals.
In the light of this, your results seem quite reasonable.
You might be interested to see the list of results obtained for the magnesium reaction by other students.
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updated: 19 January 2006
