Born-Haber cycles chem revision please :)

Does anyone have any worksheets or know where I can get some with answers for Born Haber cycles, I take OCR A Alevel chem and have been struggling on making them, I can do the lattice enthalpy equation quite easily but drawing them is a bit iffy, I'd like some practice. Any help is appreciated, thank you

Reply 1

TypicalNerd

Original post

by LisaPerera

I can do better than that. If I name an ionic compound and ask you to draw out the Born-Haber cycle, I can always give you feedback on the cycle in question.

If you are happy with that idea, let’s start with aluminium oxide (Al2O3), as I think it’s quite a nice, instructive example.

Reply 2

LisaPerera

(upward arrow) lattice enthalpy of aluminium oxide (cycle to Al2O3/ bottom of this message (it's supposed to be in the cycle form, from the electron affinities thats when its on the right hadn't side of the cycle, they look like downward arrows to the Al2O3 at bottom of cycle, if that makes sense, I'll try to include an image)
2Al^3+ + 3O^2-
(upward arrow) electron affinities of Oxygen x2
2Al^3+ (g)+ 6e- +3O (g)
(upward arrow) atomisation of Al x2
Al^3+ (g) + 3e- + 3O (g)
(upward arrow) ionisation of Al x3
Al (g) + 3O (g)
(upward arrow) atomisation of Aluminium
Al (s) + 3O (g)
(upward arrow) atomisation of oxygen x3
Al (s) + O (g)
(upward arrow) atomisation of oxygen
Al (s) +1/2 O2 (g)
(downward arrow) enthalpy of formation of aluminium oxide
Al2O3 (aq)

I've included an image of a basic template this would be on, next time I'll try draw it on my iPad and send a picture over if student room lets me (maybe ill try a link even), anyways I hope this makes sense, thanks you for helping me. I really appreciate it. Please let me know the feedback. :smile:

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Reply 3

t.s.r.

Alpha glucose and beta glucose. Cheers.

Reply 4

LisaPerera

Original post

by t.s.r.

Alpha glucose and beta glucose. Cheers.

Could you explain please?

Reply 5

TypicalNerd

Original post

by LisaPerera

(upward arrow) lattice enthalpy of aluminium oxide (cycle to Al2O3/ bottom of this message (it's supposed to be in the cycle form, from the electron affinities thats when its on the right hadn't side of the cycle, they look like downward arrows to the Al2O3 at bottom of cycle, if that makes sense, I'll try to include an image)
2Al^3+ + 3O^2-
(upward arrow) electron affinities of Oxygen x2
2Al^3+ (g)+ 6e- +3O (g)
(upward arrow) atomisation of Al x2
Al^3+ (g) + 3e- + 3O (g)
(upward arrow) ionisation of Al x3
Al (g) + 3O (g)
(upward arrow) atomisation of Aluminium
Al (s) + 3O (g)
(upward arrow) atomisation of oxygen x3
Al (s) + O (g)
(upward arrow) atomisation of oxygen
Al (s) +1/2 O2 (g)
(downward arrow) enthalpy of formation of aluminium oxide
Al2O3 (aq)
I've included an image of a basic template this would be on, next time I'll try draw it on my iPad and send a picture over if student room lets me (maybe ill try a link even), anyways I hope this makes sense, thanks you for helping me. I really appreciate it. Please let me know the feedback. :smile:

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The image hasn’t yet loaded, so I’ll interpret the written attempt.

For starters, with a Born-Haber cycle, you are forming a solid compound - not an aqueous one. There are Hess cycle questions that involve the formation of a solution, but this wouldn’t be one of them (these involve hydration enthalpies).

I’d agree that Al2O3 belongs on the lowest level of the cycle, but it should be solid and not aqueous.

I’ll split my comments into the right hand side and the left hand side individually:

Left hand side:

It should be 2Al(s) and not just Al(s) and you need 3/2 (or 1.5) O2(g) and not 1/2 O2(g). After all, you need 3 oxygen atoms and not just one. The level with the elements in their standard states must show all the atoms you use and not just some of them. In not showing the correct numbers of elements to begin with, you end up doing the wrong numbers of ionisations and atomisations, which completely throws off any calculations you might do with the Born-Haber cycle.

I would agree 2Al(s) and 1.5O2(g) should be above Al2O3(s) with a downward arrow connecting their two levels.

I would also make it clear you need to atomise Al first before you ionise it. By definition, ionisation energies are only measured for gaseous atoms and not the solid element. I would also stress that the three ionisations of aluminium do not have the same ionisation energy value associated with them, so you need twice the first ionisation energy, twice the second ionisation energy and twice the third ionisation energy. I suspect you understand this and this is just an incorrect description given as a label, but examiners will interpret this the wrong way and mark you down.

Right hand side:

Each oxygen undergoes two different electron affinities (the first is exothermic and the second is endothermic) - it’s not a single value you multiply just twice, similar to my comments about the ionisation energies of aluminium. Because there are three oxygens, you actually need three times the first electron affinity and three times the second electron affinity.

The arrow between Al2O3(s) and 2Al^3+(g) and 3O^2-(g) should point downward towards the bottom of the cycle rather than upward. This is because OCR defines lattice enthalpy as the enthalpy change when gaseous ions combine to give 1 mol of solid ionic lattice. As such, the arrow should point downward to match this.

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