Name: Christina J.
Date: Saturday, November 16, 2002
1) Why is zinc more reactive than copper (in terms of
displacement in a redox reaction)? Should not its full valence orbitals
make it somewhat stable?
2) Why is copper more reactive than silver? Since silver atoms are larger
and their valence electrons are further from the nucleus, should they not
be easier to remove than copper's?
1) I attribute this to the fact that a zinc atom is bigger than a
copper atom, and therefore zinc's two 4s electrons are easier to remove
than copper's single 4s electron and a second 3d electron.
2) I quote from F.A. Cotton, Advanced Inorganic Chemistry, 5th ed.,
p. 937 (1988), a "bible" of sorts for the subject:
"Like copper, silver and gold have a single s electron outside a
completed d shell, but in spite of the similarity in electronic structures
and ionization potentials there are few resemblances between Ag, Au and Cu.
There are no simple explanations for many of the differences although
some of the differences between Ag and Au may be traced to relativistic
effects on the 6s electrons of the latter."
Cotton also goes on to point out that the IB and IIB "noble metals"
are generally very weird and have trends in reactivity that are opposite
those of the IA and IIA metals.
That said, I will take what may be characterized as a wild flying guess...
Here the size argument would not hold because (as you correctly
point out) copper is smaller than silver. Moreover they are
isoelectronic (have the same configuration). Therefore the only
thing we can really attribute the observed trend to is the increase
in atomic number. Silver and gold are actually about the same
size, but gold is much more inert than silver, so the only
explanation is that the increase in Z is the most important effect.
This is not the case in the group IA, IIA and VIIA elements, where the
increase in size is so much more important than the increase in
Z that the opposite trend is observed.
When comparing the energetics of formation of atom, molecules, and
especially ions, one must be careful to distinguish reactions in the gas
phase, and reactions in aqueous solution. In aqueous solution solvation
reactions frequently dominate the energetics of the reaction. Zn(+2) and
Cu(+2) are classic examples. In the gas phase the energy of the reactions:
M ---> M(+2) for M=Zn and M=Cu is
+665 kcal/mol and 730 kcal/mol -- not very different ~ 10% increase.
In aqueous solution, the relative energy of formation for Maq --->
Maq(+2) is about +15.5 kcal/mol and -37 kcal/mol respectively -- a
difference of ~ 52.5 kcal/mol in favor of zinc.
It is almost impossible to compare gas phase data and solution data --
and there is no good simple model for correlating the energetics of
reactions in solution. Another example is the dissociation of weak organic
carboxylic acids-- R-CO2H ---> H(+1) + R-CO2(-1). For this large class of
reactions the heat of reaction IN AQUEOUS SOLUTION is 0 +/- 1 kcal/mol. Yet
this class of acids vary in their ionization over several powers of 10. The
"explanations" you find in texts involving "resonance stabilization",
"inductive effects" etc. are bogus.
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Update: June 2012