Bond energy and ozone Watch

DanielDaniels
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Now clearly, when we draw the Lewis structure of ozone, we find out that it is one O=O and one O-O.

So I was wondering why we can't simple add the two bond enthalpies (from databook) together and divide them by two.

My explanation is that ozone have resonance structures and what we draw on paper is not really the actual structure so that's why we have to calculate the total bond energy using the reaction enthalpy and the bond energy of the reactants, and then divide the answer by two.

Is my explanation/understanding correct (or at least close enough) ?
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charco
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(Original post by Daniel Atieh)
Now clearly, when we draw the Lewis structure of ozone, we find out that it is one O=O and one O-O.

So I was wondering why we can't simple add the two bond enthalpies (from databook) together and divide them by two.

My explanation is that ozone have resonance structures and what we draw on paper is not really the actual structure so that's why we have to calculate the total bond energy using the reaction enthalpy and the bond energy of the reactants, and then divide the answer by two.

Is my explanation/understanding correct (or at least close enough) ?
Yes, pretty close. The simple bonding picture of shared pairs of electrons breaks down when resonance forms are possible.

Bonding in ozone

In reality, the ozone molecule is actually a three centre system with molecular orbitals holding the centres together.

This makes common sense when you consider the structure of an atom with one positive centre and regions of space where electrons may be found outside the nucleus.

Why should a molecule be any different?
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DanielDaniels
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(Original post by charco)
Yes, pretty close. The simple bonding picture of shared pairs of electrons breaks down when resonance forms are possible.

Bonding in ozone

In reality, the ozone molecule is actually a three centre system with molecular orbitals holding the centres together.

This makes common sense when you consider the structure of an atom with one positive centre and regions of space where electrons may be found outside the nucleus.

Why should a molecule be any different?
Brilliant!

This question made me really think about it. I honestly have a weak/vague ideas about it and I will try my best to answer it.

The simplest answer I can throw (without knowing exactly why) is to increase stability. First of all, we know ozone got resonance structures because of the measured O-O bond length in ozone is actually less than the one we know.
Now first approach I can say that formal charge distribution is preferred when the electrons are actually delocalised. Secondly, it might relate to hybridisation (Weak at this topic), as when it is having its resonance structure, all oxygen atoms have sp2 hybridisation. I can't actually relate hybridisation to stability. I have mixed things up :afraid:
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charco
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(Original post by Daniel Atieh)
Brilliant!

This question made me really think about it. I honestly have a weak/vague ideas about it and I will try my best to answer it.

The simplest answer I can throw (without knowing exactly why) is to increase stability. First of all, we know ozone got resonance structures because of the measured O-O bond length in ozone is actually less than the one we know.
Now first approach I can say that formal charge distribution is preferred when the electrons are actually delocalised. Secondly, it might relate to hybridisation (Weak at this topic), as when it is having its resonance structure, all oxygen atoms have sp2 hybridisation. I can't actually relate hybridisation to stability. I have mixed things up :afraid:
Anything that spreads charge out stabilises structures.

Hybridisation is just an empirical way of explaining the geometry of bonds in molecules. It is a useful tool for this purpose and also allows you to see where 'p' orbitals can overlap to form molecular pi orbitals.

Resonance is a way that you can fit alternative Lewis structures into the 'overall' picture. Once again, it is a useful tool for approaching delocalisation.

Delocalisation is the start of the molecular orbital (MO) approach which is the sophisticated way of explaining the bonding within multicentre particles (molecules and polyatomic ions).

MO theory is rather too complex for treatment at 'A' level, so we tend to stop at delocalisation and just mention the molecular orbitals that are formed (pi & sigma) rather than a formal treatment.
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DanielDaniels
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(Original post by charco)
Anything that spreads charge out stabilises structures.

Hybridisation is just an empirical way of explaining the geometry of bonds in molecules. It is a useful tool for this purpose and also allows you to see where 'p' orbitals can overlap to form molecular pi orbitals.

Resonance is a way that you can fit alternative Lewis structures into the 'overall' picture. Once again, it is a useful tool for approaching delocalisation.




Delocalisation is the start of the molecular orbital (MO) approach which is the sophisticated way of explaining the bonding within multicentre particles (molecules and polyatomic ions).

MO theory is rather too complex for treatment at 'A' level, so we tend to stop at delocalisation and just mention the molecular orbitals that are formed (pi & sigma) rather than a formal treatment.
Thanks a bunch! Really useful information.

Now I want to know how spreading of charges gives rise to stabilisation? What's exactly happening? What would happen to the molecule if there was no delocalisation?
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charco
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(Original post by Daniel Atieh)
Thanks a bunch! Really useful information.

Now I want to know how spreading of charges gives rise to stabilisation? What's exactly happening? What would happen to the molecule if there was no delocalisation?
Concentrated charge foci produce regions of the particle that can be attacked by suitable reagents. That is they are more likely to react.
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DanielDaniels
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(Original post by charco)
Concentrated charge foci produce regions of the particle that can be attacked by suitable reagents. That is they are more likely to react.
Thank you once again.
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