MadSci Network: Chemistry |
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.
Try the links in the MadSci Library for more information on Chemistry.