Zn/Mg with HCl, how do i explain different activation energies? (Nuffield) Watch

zabakuz
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Ive found the activation energies now how do i theoretically justify that Zn was larger than Mg? Is it simply that Mg reacts faster? If so how do i explain why it does...?

Please help.
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vinny221
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hmm I'd say the ionisation energies of each metal. Mg has a 1st IE of 7.642eV and a 2nd of 15.03eV. Zn has a 1st IE of 9.393eV and a 2nd eV of 17.96 eV. Mg thus forms electrostatic bonds with HCl more readily. You could talk a little bit about effects of shielding and nuclear charge etc. MgCl2 is much more stable than ZnCl2 due to having a much more negative enthalpy of formation.

I'm sure there's more to this question but this is all I can think of off the top of my head!
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zabakuz
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Oh yeah, of course.

Thanks so much!
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zabakuz
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Thanks for the help, sorry could i just ask where you got those values from? (i need to do a bibliography)

Also i dont understand why Zinc is more than Mg, they both only have two outer electrons and zinc's outer shell is further from the nucleus than magnesium's...?
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charco
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the activation energy is the energy required to overcome the energy barrier to reaction. This is usually the energy needed to break bonds so that new bonds can then be formed. You must look at the bonds being broken.
In a metal the bonding is metallic and a function of the number of delocalised electrons and the charge density on the metal ion. For a metal to react the electrons must also be removed in the process - this is the sum of the first and the second ionisation energies.
The energy barrier then will be a sum of the interatomic forces within the metal plus the 1st and 2nd ionisation energies.
Compare the values of all of these factors for zinc and magnesium to explain the difference.
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vinny221
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I got the values from Shriver and Atkins Inorganic Chemistry ed 2
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vinny221
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(Original post by zabakuz)
Thanks for the help, sorry could i just ask where you got those values from? (i need to do a bibliography)

Also i dont understand why Zinc is more than Mg, they both only have two outer electrons and zinc's outer shell is further from the nucleus than magnesium's...?
Zinc is right across the period hence outer electrons will experience a much greater nuclear charge than the likes of Mg.
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zabakuz
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Oh ok thanks.
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