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chem question

Lattice enthalpy values can be obtained from Born-Haber cycles and by calculations based on a perfect ionic model. Which compound shows the greatest percentage difference between these two values? (1) A CsF B CsI C LiF D LiI
the answer is D but am not sure why
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Original post by sed.sa
Lattice enthalpy values can be obtained from Born-Haber cycles and by calculations based on a perfect ionic model. Which compound shows the greatest percentage difference between these two values? (1) A CsF B CsI C LiF D LiI
the answer is D but am not sure why

It is about polarising power and polarisability.

The metal ion gives away an e-, but since it is +ve it will attract that e- back - the smaller it is, the more attractive to e- it is.

The non-metal gains... -ve... struggles to hold onto e- (weakly attracts e-)... blah blah blah.
Original post by sed.sa
Lattice enthalpy values can be obtained from Born-Haber cycles and by calculations based on a perfect ionic model. Which compound shows the greatest percentage difference between these two values? (1) A CsF B CsI C LiF D LiI
the answer is D but am not sure why

At A level, the biggest differences between the theoretical and experimental lattice enthalpies are always seen in cases where the cation is small and polarising and the anion is large and polarisable. Remember the perfect ionic model used to estimate lattice enthalpies works under the assumption that there is no polarisation and so it fails to account for the bonding being strengthened by covalent character (indicated by having an experimental lattice enthalpy with a magnitude greater than that of the theoretical).

Li^+ is a small and very polarising cation and I^- is a large and very polarisable anion, so D would be the correct answer. You can convince yourself of this by locating both lithium and iodine on the periodic table and taking note of the periods they are on.

This means in reality, the lithium ions will significantly polarise the iodide ions and so the bonding will not be quite as described by the perfect ionic model.

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