Double Covalent vs. Ionic Bonds
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.
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
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.
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."
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Update: June 2012