Modern inorganic chemistry

76 ENERGETICS

Hence

+ Ah + Ah + Ah + Ah + Ah 5 = 2140 kJ mol"^1

The positive enthalpy of formation of NaCl 2 is so large that the

possibility of the reaction Na(s) + Cl 2 (g) -> NaCl 2 (s) occurring

under any conditions is extremely remote.

The main factor responsible for the large positive value of AHj^

for NaCl 2 is the high second ionisation energy of sodium. Since for

any element, the second ionisation energy is much larger than the

first, we might ask the question : Why do elements from Group II

form ionic dichlorides? The enthalpy changes for the formation of

MgCl, MgCl 2 and MgCl 3 are given below (standard enthalpies in

kJ):

MgCl MgCl 2 MgCl 3

Ahj Mg(s)^Mg(g) +146 +146 +146

Ah 2 Mg(g) -* Mg" + (g) 4- ne ~ + 736 +2184 + 9924

Ah 3 iitC! 2 (g) -> nCl(g) +121 4-242 4-363

Ah 4 nCl(g) + ne' -> nCHg) - 364 - 728 - 1092

Ah 5 Mg"+(g) + fid ~(g) -> MgCln - 753 - 2502 - 5440

AHf = Ahj + Ah 2 + Ah 3 + Ah 4 + Ah 5 - 1 14 -658 4-3901

The values of AH^ indicate that it is extremely unlikely that

MgCl 3 (s) can be prepared under any conditions, but both MgCl(s)

and MgCl 2 (s) appear to be energetically stable with respect to

magnesium and chlorine.

MgCl(s), however, is not energetically stable with respect to dis-

proportionation. The following energy cycle enables the enthalpy

of disproportionation to be calculated, i.e.

2AHf(MgClXs) + AH (disproportionation) - AHf(MgCl 2 ) = 0

Mg(s)

2Mg(s) + Cl 2 (g)

Hence
^(disproportionation) == ~ ^2 / KJ
We see, therefore, that magnesium normally forms a dichloride
and not a mono- or tri-chloride. Similar calculations can be made
for many systems, but greater uncertainties arise, especially when