[killmyentourage]

Q.

deltaHf of CaCl2 = -796 kJ/mol
...FeCl2 = -342
...CuCl2 = -220

Rationalize why these values become less exothermic across rows.

I believe this has something to do with the electron configuration/size of the atoms but I can't pinpoint the exact nature of this question. I know that the CaCl2 formation is so exothermic because all the atom's orbitals are filled. Does anyone have any ideas?

[Cavillus]

Q.

deltaHf of CaCl2 = -796 kJ/mol
...FeCl2 = -342
...CuCl2 = -220

Rationalize why these values become less exothermic across rows.

I believe this has something to do with the electron configuration/size of the atoms but I can't pinpoint the exact nature of this question. I know that the CaCl2 formation is so exothermic because all the atom's orbitals are filled. Does anyone have any ideas?

Well, IMHO you should consider two factors:

1. The electronegativity of the metal, so his willingness to give away his electrons;
2. The electronic structure of the resulting ion.

So, it is clear that CaCl₂ is the most stable one. Ca has the lower electronegativity, and Ca²⁺ has a closed electronic structure.

Explaining the difference between Fe and Cu is a little trickier, because the electronegativity difference is negligible.
We can look, however, at the electronic structure:

Fe: 4s²3d⁶
Cu: 4s¹3d¹⁰

So, their ions have this structures:

Fe²⁺: 4s¹3d⁵
Cu²⁺: 4s²3d⁷

The iron ion structure is called "semi-closed orbitals structure" and it is more stable than other structures with more or less than half of the total electrons in there.

This, in my opinion, could explain why FeCl₂ is more stable.

[killmyentourage]

Thank you. I wrote something along those lines but I'll be sure to update once my professor posts the correct answers.