Born Haber Cycles Watch

krishthakrar
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How do Born Haber cycles give experimental values for lattice energies when they involve using ionisation energy and electron affinity values which are theoretical? Is it the enthalpy of formation that causes it to be 'experimental'? Thanks
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krishthakrar
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(Original post by EierVonSatan)
Lattice enthalpy is the ionic equivalent of covalent bond energy. In covalently bonded molecules it's fairly easy to define a single bond - but this isn't possible in an ionic lattice where every positive ion is being attracted to every negative ion from all directions (and vice versa). So we can't isolate a single bond between one anion and one cation - so we just worry about the attractive energy of a given amount.
Hello again I was a bit confused about something so I thought I would ask you again since you seem to know quite a lot about chemistry! I'm a bit confused about why the lattice energy values found from Born Haber cycles are 'experimental' since they take into account electron affinity and ionisation energy which are 'theoretical' values for ions? Is it the value of standard enthalpy of formation that makes the value of lattice energy calculated from Born Haber cycles 'experimental'? Thanks!
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krishthakrar
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(Original post by EierVonSatan)
Born Haber cycles give a theoretical value for lattice enthalpies, using experimentally derived values. The reason why there are often discrepancies between the theoretical and experimental values, is because the model assumes a purely ionic system .
But in all my textbooks it says that the Born Haber Cycle gives experimental enthalpy values and theoretical way of calculating lattice energy is by doinf calculations based ont he purely ionic model
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krishthakrar
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(Original post by EierVonSatan)
I'm afraid you're not making much sense to me.

If you are calculating a value from a theoretical model like the Born-Haber cycle, this gives you a theoretical value. The experimental value (one that is measured experimentally) may agree or disagree with that theoretical value. If it does, you have a good model, if it doesn't the model doesn't work well for that substance.
That can't be right because according to all my textbooks, my teacher and various website, the Born Haber cycles gives an experimental value for the lattice energy. I know that it is experimental, I just don't understand how because ionisation energy and electron affinity values are theoretical values.
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EierVonSatan
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(Original post by krishthakrar)
That can't be right because according to all my textbooks, my teacher and various website, the Born Haber cycles gives an experimental value for the lattice energy. I know that it is experimental, I just don't understand how because ionisation energy and electron affinity values are theoretical values.
I guess I'm taking the two terms a bit too literally.

The values calculated from the Born-Haber cycle could be considered to be ''indirect'' experimental values if all of the other values are experimental. Ionisation energies are fairly straight forward to measure. You can also measure electron affinities, it's just a real pain to do so.
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krishthakrar
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(Original post by EierVonSatan)
I guess I'm taking the two terms a bit too literally.

The values calculated from the Born-Haber cycle could be considered to be ''indirect'' experimental values if all of the other values are experimental. Ionisation energies are fairly straight forward to measure. You can also measure electron affinities, it's just a real pain to do so.
Oh okay I think I understand now; ionisation, electron affinity and atomisation energies are theoretical but the standard enthalpy of formation is not which is what causes the discrepancy between the experimental values (ie born haber cycles) and theoretical values.
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Borek
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(Original post by krishthakrar)
ionisation, electron affinity and atomisation energies are theoretical
No idea what you are trying to say here - all these are determined experimentally.

What makes them "theoretical"?
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krishthakrar
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(Original post by Borek)
No idea what you are trying to say here - all these are determined experimentally.

What makes them "theoretical"?
Sorry "theoretical" is the wrong word, I myself am confused. I don't understand why the values for lattice energy obtained from Born-Haber cycles are different to those obtained "theoretically" eg. from electrostatics. What part of the Born Haber cycle makes the value "theoretical"? It can't be the ionisation energy or electron affinity as the values for those do not take into account the "degree of covalency" in the bonds. So what causes the discrepancies between the two then?
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Borek
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You start with the ions and combine them to create the lattice. This process produces energy. If ions were just point charges, that would be the only process present, and the energy produced would be exactly the lattice energy. However, when ions combine they create a compound, which is in part covalent, Thus the energy produced can be split into two parts (or two stages of the ion combination) - one is responsible for the creation of the covalent bond, the other is responsible for the creation of lattice from the product of the first stage. We are not able to measure these things separately, so the lattice energy calculated from the BH cycle is overestimated by the first part.
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krishthakrar
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(Original post by Borek)
You start with the ions and combine them to create the lattice. This process produces energy. If ions were just point charges, that would be the only process present, and the energy produced would be exactly the lattice energy. However, when ions combine they create a compound, which is in part covalent, Thus the energy produced can be split into two parts (or two stages of the ion combination) - one is responsible for the creation of the covalent bond, the other is responsible for the creation of lattice from the product of the first stage. We are not able to measure these things separately, so the lattice energy calculated from the BH cycle is overestimated by the first part.
What you're saying is that the BH cycle gives a value that assumes the compound is purely ionic, but the value the BH cycle gives DOES take into account the degree of covalency in the bond?

EDIT: just to clarify, do ionisation energy and electron affinity values assume that the atoms that form pure ions, or do they take into account the degree of covalency the bond might have?
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Borek
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(Original post by krishthakrar)
What you're saying is that the BH cycle gives a value that assumes the compound is purely ionic, but the value the BH cycle gives DOES take into account the degree of covalency in the bond?
Sorry, you've lost me. A gives B but A gives C?

just to clarify, do ionisation energy and electron affinity values assume that the atoms that form pure ions, or do they take into account the degree of covalency the bond might have?
Ionisation and affinity are properties of an ion. They don't depend on the identity of the counterion - so they can't take covalent character of the salt into account.
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krishthakrar
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(Original post by Borek)
Sorry, you've lost me. A gives B but A gives C?



Ionisation and affinity are properties of an ion. They don't depend on the identity of the counterion - so they can't take covalent character of the salt into account.
I'm really sorry, you probably think I'm really stupid right now!

Okay, so BH cycle uses enthalpy of atomisation, ionisation energy, electron affinity and standard enthalpy of formation to calculate lattice energy. Ionisation energy and electron affinity don't take into account covalency. So which value from enthalpy of atomisation and enthalpy of formation causes the lattice energy derived from BH cycles to be different (and take into account the covalency in the compound) to the "theoretical" value? I hope I made some sense haha
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Borek
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Hint: atomisation - just like ionisation and electron affinity - is a property of an element.
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krishthakrar
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So just to clarify the value of enthalpy of formation is what results in the the theoretical lattice energy being less exothermic than the experimental lattice?

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oversizedcarrot
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(Original post by krishthakrar)
So just to clarify the value of enthalpy of formation is what results in the the theoretical lattice energy being less exothermic than the experimental lattice?
No. Think of it in this context: you do an experiment to find out the lattice enthalpy of a molecule, e.g. Al2Cl3 using a born haber cycle. You use a combination of the enthalpy of formation, atomisation, ionisation and electron affinity to calculate the lattice enthalpy. The value you get from doing this experiment is your experimental value.

There is also a theoretical value. This theoretical value is based on a model, the "perfect ionic model" which predicts a theoretical value based on the following assumptions:
- Is purely ionic, with no covalent character in bonding
- Charge density of ionic radii is evenly distributed in perfect spheres

Depending on how different your experimental value is to this theoretical value based on the perfect ionic model tells you whether your experimental value fits the perfect ionic model or doesn't.

If your experimental value is very different for lattice enthalpy, such as in the case of Aluminium chloride, this tells us that Al2Cl3 doesn't fit the perfectly ionic model. This would suggest aluminium chloride has some covalent character in bonding which distorts the charge density of electrons so is't perfectly ionic. It's this that makes the lattice enthalpy of your compound from experimental value different from the theoretical value.

On the other hand the experimental value and theoretical value could be very similar suggesting the ionic bonding in compound formed has no/very little covalent character and so is perfectly ionic with evenly spread charge density, e.g. NaCl.

Hope that explains it.
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charco
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(Original post by oversizedcarrot)
No. Think of it in this context: you do an experiment to find out the lattice enthalpy of a molecule, e.g. Al2Cl3 using a born haber cycle. You use a combination of the enthalpy of formation, atomisation, ionisation and electron affinity to calculate the lattice enthalpy. The value you get from doing this experiment is your experimental value.

There is also a theoretical value. This theoretical value is based on a model, the "perfect ionic model" which predicts a theoretical value based on the following assumptions:
- Is purely ionic, with no covalent character in bonding
- Charge density of ionic radii is evenly distributed in perfect spheres

Depending on how different your experimental value is to this theoretical value based on the perfect ionic model tells you whether your experimental value fits the perfect ionic model or doesn't.
This is good


If your experimental value is very different for lattice enthalpy, such as in the case of Aluminium chloride, this tells us that Al2Cl3 doesn't fit the perfectly ionic model. This would suggest aluminium chloride has some covalent character in bonding which distorts the charge density of electrons so is't perfectly ionic. It's this that makes the lattice enthalpy of your compound from experimental value different from the theoretical value.

On the other hand the experimental value and theoretical value could be very similar suggesting the ionic bonding in compound formed has no/very little covalent character and so is perfectly ionic with evenly spread charge density, e.g. NaCl.

Hope that explains it.
But you have not chosen a very good example. At room temperature aluminium chloride is covalent (it is Al2Cl6 and at higher temperatures it becomes AlCl3)

The classic example is silver chloride, whose experimental and theoretical lattice enthalpies are very different...
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krishthakrar
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(Original post by charco)
This is good



But you have not chosen a very good example. At room temperature aluminium chloride is covalent (it is Al2Cl6 and at higher temperatures it becomes AlCl3)

The classic example is silver chloride, whose experimental and theoretical lattice enthalpies are very different...
but why? which specific part of the BH cycle process for calculating lattice enthalpy makes it different to the theoretical one?
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oversizedcarrot
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(Original post by krishthakrar)
but why? which specific part of the BH cycle process for calculating lattice enthalpy makes it different to the theoretical one?
Lattice enthalpy = electron affinity + ionisation enthalpy + atomisation enthalpy - enthalpy of formation

It's mostly the atomisation and formation enthalpies which make the experimental different from theoretical for the reasons I said earlier.

Silver chloride for instance has some covalent character in its bonding. This reduces the amount of energy needed to form one mole of gaseous element from its compound as covalent bonding is weaker than electrostatic attraction in ionic bonding so less energy needed to break its bonds, so is less exothermic than theoretical. The same thing applies in the enthalpy of formation to make the experimental value different from the theoretical.

The key point is the covalent character in bonding is what makes the experimental different from theoretical.
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KatieeRuuuu
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(Original post by oversizedcarrot)
Lattice enthalpy = electron affinity + ionisation enthalpy + atomisation enthalpy - enthalpy of formation

It's mostly the atomisation and formation enthalpies which make the experimental different from theoretical for the reasons I said earlier.

Silver chloride for instance has some covalent character in its bonding. This reduces the amount of energy needed to form one mole of gaseous element from its compound as covalent bonding is weaker than electrostatic attraction in ionic bonding so less energy needed to break its bonds, so is less exothermic than theoretical. The same thing applies in the enthalpy of formation to make the experimental value different from the theoretical.

The key point is the covalent character in bonding is what makes the experimental different from theoretical.
Sorry to interrupt, but why is it mostly? Surely it should be entirely due to the atomisation and formation enthalpies?
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