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Name: Bernadette
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 2001 - 2002


Question:
Strength of Covalent Bond vs Ionic Bond Apparently, the AP Bio exam "says" covalent bonds are stronger than ionic bonds. When I check bond energies, they seem to be in the same range. Is one stronger?


Replies:
Alas! Ideally, the term "bond energy" should probably be deleted from the chemical lexicon. Like a lot of terms in chemistry, the closer you look at the definition of the term the more you have a list caveats, exceptions, and footnotes.

First, there is no such thing as a purely covalent, or purely ionic bond. There is a continuum of covalent and ionic character that we can assign to any bond.

Second, it depends upon the the products of dissociation. For example, the energy required to dissociate NaCl ----> Na+ + Cl- [all in the gas phase] is about 1.5 eV greater than the energy required to dissociate it into neutral atoms. So the "covalent" NaCl bond is weaker than the "ionic" NaCl bond.

Third, bond energies are useful when comparing similar classes of substances. Since some covalent bonds form multiple bonds, e.g. N2, and O2, they are stronger than F2 which only forms a single bond, but is that a fair comparison. I do not think so.

Ambiguous questions that require ambiguous answers should not find their way onto exams such as AP's, because the more astute student is likely to see the flaws and give the "incorrect" answer.

I agree with you, there "bond energies" of what we loosely call ionic and covalent bonds overlap, depending upon what example you choose to examine.

Vince Calder



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