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Graphite reacts with potassium to produce a compound with the empirical formula $\ce{KC8}$. Of the following, which is the best description of this compound's structure?
(a) $\ce{K+}$ ions close packed with polyhedral $\ce{C8-}$ ions;
(b) $\ce{K-}$ ions close packed with polyhedral $\ce{C8+}$ ions;
(c) $\ce{K+}$ ions packed with $\ce{C2^2-}$ ions;
(d) Negatively charged hexagonal carbon layers with intercalated $\ce{K+}$ ions between them;
(e) An expanded diamond lattice with $\ce{K+}$ ions in tetrahedral holes.

The correct answer is (d). I'm not very strong in inorganic chemistry, so I'm unsure if there is a way to deduce this answer.

The only thing I'm sure of is that it's not (b), since it seems very unlikely that potassium would have a negative charge. But I don't know why (a), (c), or (e) are all false (and why (d) is true).

Any help would be greatly appreciated.

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  • $\begingroup$ Do you know the structure of graphite? Knowing this it would be rather obvious $\endgroup$
    – Mithoron
    Commented Sep 10, 2015 at 0:02

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This question is kind of "illogical", in that the answer can't quite be derived by logic alone; you either know it, or you don't. Well, of course you are right about (b). Also, it's not (c) either, because if we balance the charges to get a neutral compound, it would be $\ce{K_2C_2}$, and that's not what we want (though this thing does exist, too). Then what?

Graphite is known for its ability to intercalate pretty much anything, as its layers are held together only weakly. Diamond is not like that. Also, changing from graphite to anything else would require some rearrangement of covalent bonds, and that's not going to be easy (think of the harsh conditions needed to make diamond from graphite). So (d) is quite likely, while (a) and (e) are not. That's just about it.

All that being said, carbon is a surprisingly versatile element (think of organic chemistry), so I'd rather avoid making too general conclusions. Suppose someone brings you a sample; you analyze it and see that it contains just K and C, in molar ratio 1:10, so you may say it's $\ce{KC}_{10}$. What's the structure of it? The problem is almost similar to your original question, except they don't tell you they obtained it from graphite, but it looks kinda like graphite anyway (a fine black powder), so shouldn't the logic be pretty much the same? What would you expect? Another graphite intercalation compound like your (d), only having slightly less potassium? Yeah, sure.

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Graphite is the most stable form of carbon, and forms as sheets of hexagonal carbon networks, with sheets that are held together by vanderwaal forces. One set of electrons from each carbon is delocalized to the plane between sheets. Because of this delocalization, it is able to stabilize a negative charge relatively easily (apparently an extra electron for every 8 carbons).

The others are incorrect, because they are forcing the carbon to adopt a less stable form, which is energetically unfavorable.

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  • $\begingroup$ Nearly all organic chemistry is energetically unfavorable in the same sense, so what? Diamond, for that matter, is indeed less stable; does that make it non-existent? $\endgroup$
    – Ivan Neretin
    Commented Sep 10, 2015 at 7:36

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