Bertybassett
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Hi, I was wondering if the born haber cycle value for lattice enthalpy is experimental or theoretical? In a mark scheme, it says that born haber allows for "additional covalent interaction"? Why is this?

Thanks
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virtualcounselor
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Lattice enthalpy can be both theoretical and experimental. However, it is only "theoretical" when it is based on the ionic model (this is because you don't need to carry out experiments for something that naturally exists - the size of the ion! Therefore comparisons are made theoretically, using factors such as ionic charge, the size of the ion and charge density)

and...

it is "experimental" when it is based on the Born-Haber cycle (which I guess is due to the fact that there are many other energy / enthalpy involved in the cycle so the values obtained must be accurate and not an estimation or so)

Whether you are doing CIE AS and A Level Chemistry or not (which includes other exam boards), I'd suggest you to have a look at the textbook or even possibly download it using www.gceguide.com


Later on, have a look at Chapter 19 which starts on page 258 up until 269 *where there will be a summarised end*

PS: I just finished my A Levels this June (Business, Economics and Chemistry) so if you need any help, feel free to message me
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Bertybassett
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(Original post by virtualcounselor)
Lattice enthalpy can be both theoretical and experimental. However, it is only "theoretical" when it is based on the ionic model (this is because you don't need to carry out experiments for something that naturally exists - the size of the ion! Therefore comparisons are made theoretically, using factors such as ionic charge, the size of the ion and charge density)

and...

it is "experimental" when it is based on the Born-Haber cycle (which I guess is due to the fact that there are many other energy / enthalpy involved in the cycle so the values obtained must be accurate and not an estimation or so)

Whether you are doing CIE AS and A Level Chemistry or not (which includes other exam boards), I'd suggest you to have a look at the textbook or even possibly download it using www.gceguide.com


Later on, have a look at Chapter 19 which starts on page 258 up until 269 *where there will be a summarised end*

PS: I just finished my A Levels this June (Business, Economics and Chemistry) so if you need any help, feel free to message me
Hi, thanks so much for the reply! So in regards to born haber cycles, are they more accurate because they take partial covalent character within the bonding between ions into account when calculating the values? How do you work out which ion has the greater charge density e.g. between cl- and br-? many thanks
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virtualcounselor
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(Original post by Bertybassett)
Hi, thanks so much for the reply! So in regards to born haber cycles, are they more accurate because they take partial covalent character within the bonding between ions into account when calculating the values? How do you work out which ion has the greater charge density e.g. between cl- and br-? many thanks
No worries!

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charco
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The whole idea of Born-Haber is that it is based on experimental data. If this data disagrees with the pure ionic model calculations then it is assumed that the lattice has a degree of covalent character.

Implications:
What isn't clear is whether or not this covaent character is 'giant' covalent or 'simple'. In many cases the experimental value exceeds (is greater in magnitude) the theoretical, in which case the "covalent character" appears to be strengthening the bonding, as is the case of the silver halides.

However, the concept of "covalent character" is sometimes taken to mean that the bonding is weaker, as in the case of tin(II) chloride, and given as a reason for the rather low melting point, 247ºC

Just another area where similar concepts are invoked with conflicting conclusions.
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