Thursday, 1 October 2015

inorganic chemistry - Why does lithium from the nitride whereas the other alkali metal forms the azide?



What property of lithium makes them forms the nitride, lithium nitride LiX3N whereas the other group 1 elements forms the azide, like sodium azide and potassium azide - NaNX3 and KNX3 respectively?


Does lithium azide exist? If so, what is its structure?



Answer



The formation of LiX3N can be explained thermodynamically. The enthalpy of formation of the NX3 ion is extremely endothermic (it involves three successive additions of electrons) and with the larger counterions NaX+ or KX+ the lattice energy (which is proportional to q+qr++r) is not sufficiently large to compensate for the large ΔfH of NX3. The same argument explains why magnesium also forms a nitride.


According to my very brief research on the Internet, lithium azide does exist but is unstable. It can be prepared via reaction of NaNX3 and LiX2SOX4: see Acta Chem. Scand. 1957, 11, 581 for more details.


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