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born-haber cycles

i don't have a clue how to construct one, and as they are worth about 9-10 marks i think i should learn. Does neone know how to construct one, like general method. If u could let me know it would be great

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Reply 1
Avatar for halc
halc
Well they're very hard to show on a forum...

But what they do is show the relative energy levels of the intermediates in the formation of an ionic solid, usually things like NaCl, NaBr, KCl, KBr, Na2O, etc.

Really I'd need to draw this but, in general, when making an ionic solid from its elements in standard states, under standard conditions

e.g. Na (s) + 1/2 Cl2 (g) ---> NaCl (s)

You have a number of intermediates... firstly each of the reactants needs to be sublimed/atomised - the metallic/covalent bonds need to be broken to form gaseous atoms of each element: e.g 1/2 Cl2 (g) ---> Cl (g)
This is an endothermic process (requires energy to break the covalent bonds) and so you need to draw it at a higher energy level (above) the starting reactants

Then they need to be ionised, Na loses an electron and Cl gains it, this is shown in 2 steps:

(First) Ionisation Na (g) ---> Na (g) + e- (endothermic)
(First) Electron Affinity Cl (g) + e- ---> Cl- (g) (exothermic, further affinities are endothermic)

So you now have your two ionic species, Na+ and Cl-, which combine to give you your final product, NaCl in the final step:

Lattice Enthalpy of Formation Na+ (g) + Cl- (g) ---> NaCl (exothermic)

Don't know how helpful this will be all in text, so here's a google image of a cycle to show you how it's arranged (it's for NaF):
http://stage.cchem.berkeley.edu/chem1b/old_stuff/spr1ng00/assignments/ps13/ps13out1.gif
Reply 2
Avatar for halc
halc
http://jbfpc2.ycp.edu/gchem/ch09_bh.jpg quite good too

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