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Name: Christina J.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: Saturday, November 16, 2002


Question:
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?


Replies:
Hi...

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.

prof. topper


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.

Vince Calder



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