Electronegativity and Bond Strength

[justadream]

TBR says that C-I bonds are weaker than C-O bonds.

But I thought that the greater the difference in EN between the two atoms, the weaker the bond?

C-I have essentially no difference in EN whereas C-O bonds have a nonzero difference.

EN values from:

http://www.thecatalyst.org/electabl.html

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[type12]

You can safely assume the "greater the difference, the stronger the bond" on the MCAT. This is due to low electronegative elements "wanting" electrons less than high electronegative elements. Basically, opposites attract, and if a positive sodium wants to hang out with a negative fluoride, then it'll be stronger bond than something that doesn't have a difference.

 

[Teleologist]

You can safely assume the "greater the difference, the stronger the bond" on the MCAT.

Really?

Na: EN of 0.9
Cl: EN of 3.0
Difference: 2.1

NaCl's lattice energy: 787 kJ/mole.

N: EN of 3.0
N: EN of 3.0
Difference: 0

N2's bond dissociation enthalpy: 945 kJ/mole.

 

[type12]

Really?

Na: EN of 0.9
Cl: EN of 3.0
Difference: 2.1

NaCl's lattice energy: 787 kJ/mole.

N: EN of 3.0
N: EN of 3.0
Difference: 0

N2's bond dissociation enthalpy: 945 kJ/mole.

Yes, really. Please read more carefully (specifically the last three words you quoted lol). Better yet, please pay attention to which forum you're on.

Please feel free to ask me any other questions. I can tell you're having a lot of difficulty with concepts.

 

[Teleologist]

Yes, really. Please read more carefully (specifically the last three words you quoted lol). Better yet, please pay attention to which forum you're on.

Please feel free to ask me any other questions. I can tell you're having a lot of difficulty with concepts.

So we won't see NaCl on the MCAT?

 

[type12]

So we won't see NaCl on the MCAT?

Please read more carefully. Or, at the very least, quote me where I said that, or STFU.

If you do feel like arguing with yourself, go ahead, but don't pretend like I said something I didn't again, lol.

 

[Teleologist]

Please read more carefully. Or, at the very least, quote me where I said that, or STFU.

If you do feel like arguing with yourself, go ahead, but don't pretend like I said something I didn't again, lol.

Please clarify what you mean by on the MCAT. Even if you think about bond strength and electronegativity differences in the world of covalent compounds exclusively, consider N2 and HF. BDE of N2: >900 kJ/mol. EN difference: 0

HF: 565 kJ/mol. EN difference: >0.

So, how's your rule holding up?

 

[type12]

Please clarify what you mean by on the MCAT. Even if you think about bond strength and electronegativity differences in the world of covalent compounds exclusively, consider N2 and HF. BDE of N2: >900 kJ/mol. EN difference: 0

HF: 565 kJ/mol. EN difference: >0. So, how's your rule holding up?

My rule? Who said it's a rule? I think you should use a tutor to work on your reading and reasoning skills. I asked you a simple question: please quote where I said NaCl will not be on the MCAT, or STFU. If can come to terms with the fact that you are using strawman arguments to protect yourself from being wrong, then we can move forward. Can you say that? Say you were wrong? Otherwise, me explaining a simple statement will get us nowhere.

While I sympathize with your difficulties in understanding a very simple statement regarding comparisons on the MCAT, I cannot help you here.

 

[Teleologist]

My rule? Who said it's a rule? I think you should use a tutor to work on your reading and reasoning skills. I asked you a simple question: please quote where I said NaCl will not be on the MCAT, or STFU. If can come to terms with the fact that you are using strawman arguments to protect yourself from being wrong, then we can move forward. Can you say that? Say you were wrong? Otherwise, me explaining a simple statement will get us nowhere.

While I sympathize with your difficulties in understanding a very simple statement regarding comparisons on the MCAT, I cannot help you here.

Bro just clarify to me this statement in the light of N2 and HF. I genuinely don't understand this. It's hard. Yes I used a strawman before. Now can you explain this?

You can safely assume the "greater the difference, the stronger the bond" on the MCAT.

 

[type12]

Bro just clarify to me this statement in the light of N2 and HF. I genuinely don't understand this. It's hard.

Bro, I'm trying to help you. You first need to come to terms with the fact that you are wrong. Otherwise, you'll deflect and start saying I said things I didn't say again.

I got your back, and you're welcome to ask me for any help, but my time is limited.

 

[Teleologist]

Okay lol this is going nowhere; OP: there are both electronic and steric factors to consider when assessing bond strength. Even if we limit our consideration to simple cases - such as the following compounds in which we're only talking about single bonds and one of the atoms is the same on both compounds (such as the case you raise in your original post), the other guy's so-called "safe" assumption is a dangerous misunderstanding.

H2: EN difference of 0. BDE of 436 kJ/mole.
HBr: EN difference > 0. BDE of 365 kJ/mole.

As we can see, as EN goes up, the BDE doesn't exactly follow suit. The reason is intuitive: bond length also plays a role. And thus the other guy falls into his very own trap when he raises Coulomb's law.

This is due to low electronegative elements "wanting" electrons less than high electronegative elements. Basically, opposites attract, and if a positive sodium wants to hang out with a negative fluoride, then it'll be stronger bond than something that doesn't have a difference.

Opposites attract refers to Coulomb's law. That's all fine and dainty to state that opposites attract even if we all know. But he forgets the other part of Coulomb's law: the force of attraction falls off dramatically with distance (bond length).

And yes Coulomb's law is on the MCAT:

https://www.aamc.org/students/download/85562/data/ps_topics.pdf

 

[type12]

Okay, I'm wrong about the fact that N2 has a stronger bond than HF?

----

Okay lol this is going nowhere;

Agreed, this is going nowhere. You again misconstrued my point to protect your ego. I never even said anything about N2 or HF, so now you have to either show where I said anything about NaCl not being on the MCAT, or I said you're wrong about N2/HF... OR STFU.

I cannot help you, I'm sorry. Get a tutor.

 

[Teleologist]

This is due to low electronegative elements "wanting" electrons less than high electronegative elements. Basically, opposites attract, and if a positive sodium wants to hang out with a negative fluoride, then it'll be stronger bond than something that doesn't have a difference.

Agreed, this is going nowhere. You again misconstrued my point to protect your ego. I never even said anything about N2 or HF, so now you have to either show where I said anything about NaCl not being on the MCAT, or I said you're wrong about N2/HF... OR STFU.

I cannot help you, I'm sorry. Get a tutor.

Bruh if you want to tell me where I went wrong that'd be cool, or if you don't want to, I'll understand why not. Seems like you have a lot of time for telling me to STFU but not a lot of time for clarifying chemistry. If you want to show me I'm wrong I'd be cool with that.

 

[type12]

Bruh if you want to tell me where I went wrong that'd be cool, or if you don't want to, I'll understand why not.

I'm trying to, but you gotta really help me. I totes know why you don't want to, because you'll have to come to terms being wrong.

Okay, my last attempt to aid you with your reasoning difficulties (though you should make a thread so others can help you instead of continuing your attempts to derail the thread):

Can you help me to help you?

So we won't see NaCl on the MCAT?
...
Okay, I'm wrong about the fact that N2 has a stronger bond than HF?
...
But he forgets the other part of Coulomb's law: the force of attraction falls off dramatically with distance (bond length).

Can you show me where I said this? Can you admit you have difficulties with reasoning, or, to save time, admit you're wrong? I mean, even your understanding of bond length is flawed: electronegativity effects bond length, not the other way around, lol, so I didn't forget, nor did I say Coulomb's law is not on the MCAT.

So, first, show me where I said those things, or admit fault?

 

[Teleologist]

I mean, even your understanding of bond length is flawed: electronegativity effects bond length, not the other way around

That's interesting because I've re-read my post and I don't see where I claimed that.

Also regarding NaCl ... I was asking you a question about whether it would be on the exam. I don't see how you misconstrued a question as a statement.

So we won't see NaCl on the MCAT?

Please read more carefully. Or, at the very least, quote me where I said that, or STFU.

If you do feel like arguing with yourself, go ahead, but don't pretend like I said something I didn't again, lol.

 

[type12]

That's interesting because I've re-read my post and I don't see where I claimed that.

No worries, bro, you didn't claim that. It was a trap to see if you are incapable of reasoning, or you are using strawmen to protect your ego. So, your difficulty in comprehension stems not from a fundamental inability, but simply your fear of being wrong. Your ability to catch it when you are given a taste of your own medicine shows this.

Also regarding NaCl ... I was asking you a question about whether it would be on the exam. I don't see how you misconstrued a question as a statement.

Okay, you can play ignorant to protect your ego. What about the other? I never said you were wrong about HF or N2, now did I? And also this:

So, how's your rule holding up?

What rule would that be?

 

[The Brown Knight]

TBR says that C-I bonds are weaker than C-O bonds.

But I thought that the greater the difference in EN between the two atoms, the weaker the bond?

C-I have essentially no difference in EN whereas C-O bonds have a nonzero difference.

EN values from:

http://www.thecatalyst.org/electabl.html

C-I is weaker than C-O due to size - Iodine is gigantic compared to oxygen

Bond dissociation energy is determined by bond strength --> bond strength is determined by a variety of factors; in general:

1) bond strength of homonuclear diatomic molecules: e.g. H2, N2, O2, F2, Br2, etc.; these have no electronegativity difference, so a more comprehensive definition requires construction of molecular orbital diagrams and then using bond order equation [(# bonding electrons - # anti bonding electrons)/2]; in simpler terms, you basically construct the best possible Lewis diagram with the available valence electrons

2) bond strength of heteronuclear diatomic molecules: e.g. carbon monoxide vs. cyanide, H-F vs. H-Cl vs. H-Br, etc.; here electronegativity MATTERS - in GENERAL, the greater the bond polarity, the stronger the bond; ATOMIC SIZE also matters - larger atoms make weaker bonds

3) bond strengths in more complex molecules (like organic molecules): you have to look at factors IN ADDITION to bond polarity and atomic size; e.g. inductive effects (halogens are withdrawing groups vs. methyls are donating groups), resonance stabilization (you could get ~1.5 bond order for C-O bond in carbonate), steric effects, etc. -- for these kind of molecules they would most likely provide you a table with values, so you wouldn't need to know specific rules

4) ionic solids: for MCAT purposes, use only Coulomb's Law; hard ions (small, highly charged ions) will have greater lattice energies

 

[Teleologist]

C-I is weaker than C-O due to size - Iodine is gigantic compared to oxygen

Bond dissociation energy is determined by bond strength --> bond strength is determined by a variety of factors; in general:

1) bond strength of homonuclear diatomic molecules: e.g. H2, N2, O2, F2, Br2, etc.; these have no electronegativity difference, so a more comprehensive definition requires construction of molecular orbital diagrams and then using bond order equation [(# bonding electrons - # anti bonding electrons)/2]; in simpler terms, you basically construct the best possible Lewis diagram with the available valence electrons

2) bond strength of heteronuclear diatomic molecules: e.g. carbon monoxide vs. cyanide, H-F vs. H-Cl vs. H-Br, etc.; here electronegativity MATTERS - in GENERAL, the greater the bond polarity, the stronger the bond; ATOMIC SIZE also matters - larger atoms make weaker bonds

3) bond strengths in more complex molecules (like organic molecules): you have to look at factors IN ADDITION to bond polarity and atomic size; e.g. inductive effects (halogens are withdrawing groups vs. methyls are donating groups), resonance stabilization (you could get ~1.5 bond order for C-O bond in carbonate), steric effects, etc. -- for these kind of molecules they would most likely provide you a table with values, so you wouldn't need to know specific rules

4) ionic solids: for MCAT purposes, use only Coulomb's Law; hard ions (small, highly charged ions) will have greater lattice energies

All fine and dandy, but are you not going to mention multiple bonding character? You touch on it by mentioning bond order. But when you talk about heteronuclear diatomics ... bond order is perhaps the most important factor.

There are plenty of more valid arguments and more sophisticated arguments as well. Such as orbital overlap.

 

[The Brown Knight]

All fine and dandy, but are you not going to mention multiple bonding character? You touch on it by mentioning bond order. But when you talk about heteronuclear diatomics ... bond order is perhaps the most important factor.

There are plenty of more valid arguments and more sophisticated arguments as well. Such as orbital overlap.

Yup Bond order should be part of #2 and first statement in #3.

You are also right about the electronegativities of the atoms impacting orbital energies in an MO diagram, but that's beyond scope of MCAT don't you think? By bringing up MO diagrams for homonuclear diatomic molecules, I was just making the OP aware what goes into determining bond strength apart from the more common trends. My list is by no means comprehensive, so feel free to add to it!

 

[type12]

Yup Bond order should be part of #2 and first statement in #3.

You are also right about the electronegativities of the atoms impacting orbital energies in an MO diagram, but that's beyond scope of MCAT don't you think? By bringing up MO diagrams for homonuclear diatomic molecules, I was just making the OP aware what goes into determining bond strength apart from the more common trends. My list is by no means comprehensive, so feel free to add to it!

Yeah, your explanation went just a little beyond the MCAT for the sake of understanding the concept, which I think is perfect. And you wrote it in a thoughtful, coherent way. It wasn't convoluted or poorly-written for the sake of trying to impress mommy with all the stuff you know, throwing concepts in carelessly and without flow or approach, but instead considered the audience.

 

[Teleologist]

Yup Bond order should be part of #2 and first statement in #3.

You are also right about the electronegativities of the atoms impacting orbital energies in an MO diagram, but that's beyond scope of MCAT don't you think? By bringing up MO diagrams for homonuclear diatomic molecules, I was just making the OP aware what goes into determining bond strength apart from the more common trends. My list is by no means comprehensive, so feel free to add to it!

It's not that hard; EN lowers energy of atomic orbitals. Which are distinct from molecular orbitals.

 

[bartzx3]

.

 

[Axes]

Really?

Na: EN of 0.9
Cl: EN of 3.0
Difference: 2.1

NaCl's lattice energy: 787 kJ/mole.

N: EN of 3.0
N: EN of 3.0
Difference: 0

N2's bond dissociation enthalpy: 945 kJ/mole.

I'm not sure that you can compare an ionic compound to a molecule in this way.

 

[kingtal0n]

C-I is weaker than C-O due to size - Iodine is gigantic compared to oxygen

4) ionic solids: for MCAT purposes, use only Coulomb's Law; hard ions (small, highly charged ions) will have greater lattice energies

I like these, and to add a teeny grain of sand to it, you might imagine the iodine as a large heavy (fluffy?) ball tethered gently to a pole by a stretchy rope. The natural motions molecules have mean that occasionally the rope will stretch and the iodine will snap back- slowly, because it is a large heavy ball.

So using the same example, an oxygen atom, who is tethered to the same pole, will also occasionally stretch its rope- but because it is lighter, it will snap back faster. This is where I point out that, without looking, you should be able to discern that the large heavy ball is going to have a lower bond frequency, where the lighter ball (oxygen) is going to stretch and snap back more often, with higher frequency, it should by all means jostle and tumble around a lot more given the same environments.

To cap this example off, imagine now we throw both balls as hard as we can to try to break that rope. Which one do you think is going to have an easier time breaking the rope?