[Astrokel]

hey,

today my tutor went through a question on the graph of period 3 chlorides boiling point. From the graph, it shows magnesium chloride has lower boiling point than sodium chloride, which does not seem to agree on the theoretical due to larger charge of magnesium ion, which have a greater ionic attraction? I understand about aluminium chloride because of aluminium ion high polarizing power resulting in lower boilding point but what about magensium ion? My teacher told me the explanation is out of syllabus and said there are other effects beside lattice energy, but he did not tell me what was it. Is it entropy in action or something else? Any hints would be good enough. Thanks!

[nj_bartel]

1) magnesium is more electronegative than sodium, so nacl bond is stronger than mgcl bond
2) the dipole of nacl shifts entirely in one direction, whereas there is cancellation between the dipole moments of each mgcl bond.

that's what comes to mind offhand; not entirely positive

[Astrokel]

thanks for the reply! i don't entirely get point 2, so any magnesium ionic compound would have lower boiling point than sodium ionic of the same anion?

[Borek]

I would think that at boiling dipole moments are completely meaningless; what you have is a soup of ions, not molecules.

[nj_bartel]

But that soup of cations and anions is still attracted to one another to some degree, is it not?  So my first point would still stand?

I believe you're right countering my point about the dipoles though.

[Borek]

Honestly, I am not sure what to think about the problem. Too many competing processes taking place at the same time. Perhaps I will find a moment to browse my books later, but don't hold your breath.

[Astrokel]

hello Borek and nj_bartel

thanks for the helps! so i presume #1 point by nj_bartel is only influencing the boiling and melting point to a small extent? I just googled bit and the chemguide author also have no idea on this reason, http://www.chemguide.co.uk/inorganic/period3/chlorides.html, but anyway i appreciated your replies. 

[nj_bartel]

Yes, I would say it would only be to a small extent - there's only a small electronegativity difference in the first place.  What's the difference in boiling/melting points?

[Astrokel]

hi,

i quoted this from wiki,

For magnesium chloride,
Melting point 714 °C (987 K)
Boiling point 1412 °C (1685 K)

For sodium chloride,
Melting point 801 °C
Boiling point 1465 °C (1738 K)

The difference in boiling point is not as great as melting point, does efficient packing affects melting point because i read from chemguide it says Magnesium chloride has different structure than sodium chloride?
 

[Borek]

what you have is a soup of ions, not molecules.

It is more complicated than that. AlCl₃ at some point during heating becomes molecular.