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Topic: Born-Haber cyrcle  (Read 5531 times)

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Offline Rutherford

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Born-Haber cyrcle
« on: July 31, 2012, 05:41:09 AM »
http://portal.acs.org/portal/PublicWebSite/education/students/highschool/olympiad/pastexams/CNBP_028208
It is 3.b).
The EA confuses me in the note it says the a positive EA1 is exothermic, why is it positive then? How do I know if I should use LE as positive or negative?

Offline Dan

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Re: Born-Haber cyrcle
« Reply #1 on: July 31, 2012, 06:53:13 PM »
It is discussed here: http://en.wikipedia.org/wiki/Electron_affinity

Briefly, EA is defined as the amount of energy released in the process of adding an electron. A more positive value for EA translates to a larger release of energy (or a more negative ΔE).

It is an annoying convention that frequently causes confusion. I expect is probably this way for historical reasons.
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Offline Rutherford

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Re: Born-Haber cyrcle
« Reply #2 on: August 01, 2012, 04:26:15 AM »
Didn't understand well, but I suppose that I should use the sign that is given in such problems. And the lattice enthalpy is confusing me, too. I don't know is it the lattice formation enthalpy or the lattice dissociation enthalpy. I think that those things should be mentioned.

Offline Dan

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Re: Born-Haber cyrcle
« Reply #3 on: August 01, 2012, 04:56:43 AM »
And the lattice enthalpy is confusing me, too. I don't know is it the lattice formation enthalpy or the lattice dissociation enthalpy. I think that those things should be mentioned.

Yes, these things should also be specified - see for example http://www.chemguide.co.uk/physical/energetics/lattice.html

In cases such as the one you have, you should specify your answer as formation or dissociation enthalpy as appropriate. I always work in formation enthalpy unless specifically asked for dissociation enthalpy.
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Offline bossza500

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Re: Born-Haber cyrcle
« Reply #4 on: August 03, 2012, 11:36:32 PM »
Why EA is positive??
   EA is the energy that released when adding electron. If that atom is metal atom, we know that metal is a good reducer .They don't want to be added electrons.
But if we want to adding electron to metal atom, we must force them by put energy into metal atom. I think it will be positive. 

Offline Rutherford

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Re: Born-Haber cyrcle
« Reply #5 on: August 04, 2012, 04:03:46 AM »
Oxygen is a nonmetal.

Offline bossza500

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Re: Born-Haber cyrcle
« Reply #6 on: August 04, 2012, 07:52:28 AM »
EA 2 of oxygen positive?
  it's hard to add electron to the place that has a lot of electron density.

Offline Rutherford

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Re: Born-Haber cyrcle
« Reply #7 on: August 04, 2012, 08:28:42 AM »
Man, EA1 is positive.

Offline Rutherford

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Re: Born-Haber cyrcle
« Reply #8 on: August 04, 2012, 09:32:35 AM »
I just thought I understood it, but it is really complicated. I have read from many sites, but couldn't find a common patern to explain it. On many sites it is like bossza wrote, EA1 negative, EA2 positive, but on the USNCO test it is written opposite, and as a note it says for the first EA: "A positive EA value is exothermic", but in calculations the positive value is still used. It shouldn't be a mistake when so many people took part in making those problems, but I couldn't a good explanation. Really complicated.

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