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Name: Rich
Status: N/A
Grade: educator
Location: MD
Country: N/A
Date: 9/25/2005


Question:
I need some clarification please. Is a double covalent bond stronger than a ionic bond? Does the answer solely pertain to the substances that are bonded to one another? MCAT review states that ionic bonds are stronger, but AP books state that double covalent bonds are stronger.


Replies:
With due respect to both MCAT or AP, the question is an "apples" vs. "oranges" comparison unless the physical state(s) of reactants and products are specified, and as you point out, the substances are specified. Comparing:

MX(solid) ---> M(+1)(gas) + X(-1)(gas) is very different than MX(gas) --->

M(0)(gas) + X(0)(gas) or:

MX(gas) ---> M(+1)(gas) + X(-1)(gas): or MX(solid) ---> M(solid) + X(solid):

and so on: all are, in a sense a measure of the strength of the (MX) bond, but different things are being compared. It is difficult to defend, and embarrassing to know, how such questions make it past the editors.

Vince Calder


There are several problems with this:

1) Although we can say that there could be a practical qualitative difference between an "ionic" bond and "covalent" bond in that molecule with ionic bonds tend to ionize in water. However, while certain ionic molecules completely dissociate in water, others form an equilibrium between the molecule and its ions - which does beg the question as to what point will the ionization equilibrium constant be so low as to not consider such a molecule ionic.

Moreover, the high school (and some college) textbook definition of ionic bond as a transfer of electrons as opposed to a sharing of electrons is a formalism. It begs the question as to what point is the ionization potential/electron affinity/electronegativity difference is small enough so that a full transfer can not occur and a bond becomes covalent. From such a formalism, it would appear that the ionic to covalent bond range is a continuum and not two qualitatively different types of bonds.

2) Covalent bond measurements are averages. While we can report the covalent bond strength of a molecule such as N2 or O2, there are many bond energies for C=C because that bond energy is dependent on the substituents on the individual C's. This means that the reported values are averages.

3) To further muddy the waters, while some trends in ionic bond strength may be observed. L-F: 1017kJ/mol, Li-Cl: 828kJ/mol, L-Br: 787kJ/mol seems to follow the electronegativity difference trends (3.0, 2.8, 1.8 respectively). But then, throw in the Mg-O bond, electronegativity difference 2.3, bond energy 3890kJ/mol and the trend fails.

Moreover, while we would consider Na-Br: electronegativity difference 1.9, bond energy 686kJ/mol as an ionic bond, we don't consider the H-F bond, electronegativity difference 2.9, bond energy 568kJ/mol as ionic. This goes back to point #1.

4) Finally, in trying to make a sweeping generalization such as *all* ionic bonds are stronger than *all* covalent double bonds would state that -say- the weakest ionic bond is still stronger than the strongest double bond. And that simply can not be said. Likewise the reverse situation.

It seems then that terms such as ionic bond and covalent bond have risen to common usage because it does make communication (and conceptualization) simpler, but it is an agreed upon formalism (and okay to use between scientists who know the ramifications of the formalism) but it does not survive closer scrutiny. If you try to split hairs as to which is stronger, etc. then the formalism fails.

Greg (Roberto Gregorius)


There is no simple answer to that question...but here are some melting and boiling points:

Name      type of solid   melting point, C   boiling point, C

Sodium         metallic           98                   883
Tungsten       metallic         3410                  5660
CsCl           ionic             645                  1290
NaCl           ionic             801                  1413
MgO            ionic            2800                  3600
SiO2 (quartz)  covalent         1610                  2230
C (diamond)    covalent         3550                  4827

Depending on which examples you choose you can come to whichever conclusion you wish. For example, NaCl (ionic) has lower transition temperatures than SiO2 (covalent), but MgO has higher transition temperature.

These data are for solids. You can also compare the lattice energy per mole of an ionic solid to the bond energy per mole for typical covalent bonds. But are we comparing apples and oranges? (yes)

Anyway, for those who do not mind a little intellectual fruit salad, the H-F bond is the highest energy covalent single bond I know of, at 565 kj/mole. Double bonds tend to be on the order of 600-800 kj/mole. The lattice energy of NaCl is 786 kj/mole which of course is similar to the energy of a covalent double bond. However, the lattice energy does not refer to a single Na---Cl bond but to the net energy of *all of the Na-Cl attractions and all of the Na-Na and Cl-Cl repulsions in the crystal. Therefore, we are comparing two different things.

I guess in the end I disagree with both of these "authorities."

Dr. Topper



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