[Rutherford]

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?

[Dan]

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.

[Rutherford]

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.

[Dan]

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.

[bossza500]

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. 

[Rutherford]

Oxygen is a nonmetal.

[bossza500]

EA 2 of oxygen positive?
  it's hard to add electron to the place that has a lot of electron density.

[Rutherford]

Man, EA1 is positive.

[Rutherford]

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.