MadSci Network: Chemistry
Query:

Re: Why does NaCl have a higher bp. than LiCl

Date: Mon Nov 9 14:32:54 1998
Posted By: Alvan Hengge, Faculty, Chemistry and Biochemistry, Utah State University
Area of science: Chemistry
ID: 909188567.Ch
Message:


When ionic solids like NaCl or LiCl are boiled, the substance is not 
converted into separated positive and negative ions.  The electrostatic 
interaction between the oppositely charged ions remains very strong even in 
the gas phase.  What occurs as these solids are heated first to melting and 
then to boiling temperatures is that the crystal lattice breaks down, first 
into a fluid state in which the units of the salt are sufficiently small 
and mobile to allow the substance to flow.  Of course one does not have 
separate “molecules” in an ionic compound like one has in water, for 
example.  When salt melts it goes from an ordered crystal lattice 
consisting of billions and billions of positive and negatively charged 
units, to smaller and smaller stoichiometric units of the crystal lattice 
which can break free and undergo “molecular” motions: NaCl, Na2Cl2, Na3Cl3, 
etc.  As temperature increases, the units become smaller and smaller and 
finally some have sufficient kinetic energy to leave the surface of the 
liquid and enter the vapor phase; when the pressure of these equals 
atmospheric pressure we say the substance is at its boiling point.  But 
even at these temperatures free separated sodium or chloride ions will be 
rare.

The lower boiling point of LiCl may be largely a result of its smaller 
mass; all other things being equal, mass differences will affect boiling 
points.  In a molecular compound such as water, substitution of deuterium 
for the two hydrogen atoms gives a compound having a molecular weight of 20 
compared with 18 for normal H2O.  The boiling point of D2O is 101.42 C 
compared with 100 C for H2O.  LiCl has a formula weight which is 16 AMU or 
27% less than NaCl which undoubtedly contributes at least partially due to 
its lower boiling point.  Another factor may be crystal packing; sodium and 
chloride ions are much more similar in size and can therefore pack more 
efficiently than Li and Cl which differ greatly in size.  More efficient 
packing allows stronger interactions with neighboring ions.     



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