December 03, 2021, 01:53:35 PM
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Topic: Which is generally stronger, ionic or covalent bonds?  (Read 828 times)

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Offline Jack Richards

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Which is generally stronger, ionic or covalent bonds?
« on: December 03, 2020, 01:46:16 PM »
This was a question on one of my chemistry tests and the teacher had the correct answer as being covalent bond is stronger then ionic bond. I did some research and I got conflicting answers some said covalent were stronger and some said ionic were stronger.

I think it personally makes sense that ionic bonds are stronger since they typically have a higher boiling points which I have been taught is an indicator of intermolecular bonding strength.

Most ionic bonds are almost always solid at room temperature, like NaCl. However many covalent bonds such as H2O and O2 are either liquid or gas. So wouldn't this mean that ionic bonds form stronger bonds?

Thanks, Jack 


Offline Corribus

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Re: Which is generally stronger, ionic or covalent bonds?
« Reply #1 on: December 03, 2020, 02:16:45 PM »
You can't use macroscopic properties as an indication of bond strength on a per bond basis, which we might define as the amount of energy it takes to separate two nuclei from their equilibrium bond length to infinite separation. The reason is that a property like boiling point are dependent on the amount of energy per bond and the number of bonds present across some defined volume or mass unit.

Direct comparisons on a per-bond basis are honestly difficult and not particularly meaningful, because ionic compounds tend to exist as aggregate lattices composed of networks of ionic nuclei whereas as you have noted covalent bonds usually exist in discrete molecules connected by weaker intermolecular forces. I believe the conventional general chemistry wisdom is that covalent bonds are stronger, but it's probably one of these things that needs to go away because it leads to some misleading conceptions of bonding in students. Aside from which, nonpolar covalent and ionic bonds are more like extremes along a spectrum rather than discrete and absolute things. At what point does a bonding relationship go from highly asymmetric sharing of electrons (very polar covalent) versus complete localization on one nucleus versus the other (ionic)? Bond strength depends on so many things, to boil it down to ONE class is always stronger than the other is doing a disservice to students.

You could probably argue your case on grounds such as these, but it may not be worth your time.

What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Jack Richards

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Re: Which is generally stronger, ionic or covalent bonds?
« Reply #2 on: December 03, 2020, 03:14:17 PM »
I showed your answer to my teacher who said that covalent bonding is considered the strongest because it has Hydrogen bonding which is the strongest intermolecular force. She also said ionic bond has dipole dipole forces which are less strong than H-bonding.

Does this make sense? I always thought H-bonding was a certain type of bonding that occurred in some covalent compounds, not all of them.

Offline Corribus

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Re: Which is generally stronger, ionic or covalent bonds?
« Reply #3 on: December 03, 2020, 06:17:56 PM »
I showed your answer to my teacher who said that covalent bonding is considered the strongest because it has Hydrogen bonding which is the strongest intermolecular force. She also said ionic bond has dipole dipole forces which are less strong than H-bonding.

Does this make sense? I always thought H-bonding was a certain type of bonding that occurred in some covalent compounds, not all of them.
As you have written it, this makes no sense. I don't want to undermine your teacher based on hearsay, but hydrogen bonding is present in only a small portion of covalently bonded molecules, and then only in the condensed phase. (Likewise, covalently bonded molecules also have dipole-dipole interactions.) So, to me this statement is kind of like saying humans are smarter than lizards because basketballs are round.

The only way to compare bond strength is to quantify it in some way. The most typical way is the binding energy per bond. This isn't always easy to rigorously measure, and it also depends on what phase of matter we're dealing with.

But just as an example, let's consider sodium chloride. Usually it would be found in the condensed phase, in a crystalline lattice, where you could infer the bond energy by taking the total lattice energy (energy of formation) and distributing it among the number of bonds in a unit cell. But to be fair, let's look at gaseous NaCl, one isolated NaCl unit. For gaseous NaCl, the binding energy I found to be 1.52 eV.

http://hyperphysics.phy-astr.gsu.edu/hbase/molecule/NaCl.html

Most covalent single bonds have binding energy typically in the range of 2-5 eV. These are stronger than that ionic NaCl bond. The triple bond of carbon monoxide has a binding energy of >11 eV, much stronger than the ionic bonds of a single gaseous NaCl "molecule" based on theory.

https://en.wikipedia.org/wiki/Bond-dissociation_energy

So, you can see it is quite complicated. To be frank, I never loved the idea of an "ionic bond", particularly in condensed phase. The idea of discrete bonds in organized crystal lattices is fundamentally disagreeable to me. But, it is established as dogma in the early chemistry curriculum so I guess we're stuck with it.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Jack Richards

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Re: Which is generally stronger, ionic or covalent bonds?
« Reply #4 on: December 03, 2020, 06:59:45 PM »
Thank you for your detailed replies, this is something I am quite interested in so I am grateful you put the effort in to answer my question. It really is annoying when teachers ask ambiguous questions like that on a test.

Offline Babcock_Hall

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Re: Which is generally stronger, ionic or covalent bonds?
« Reply #5 on: December 04, 2020, 09:56:08 AM »
"However many covalent bonds such as H2O and O2 are either liquid or gas. So wouldn't this mean that ionic bonds form stronger bonds?"  When solid water becomes a liquid, no covalent bonds are broken.  The same is true when liquid water becomes a gas.  In other words the phase of a substance is not really indicative of the strength of a covalent bond. 

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