Boiling points of ionic compounds

The boiling points of ionic compounds are higher still, so high that most ionic organic compounds decompose before they boil. 

Look up the melting and boiling points of ionic compounds in data books. 

Vapor pressure data are not available for the ionic and nonionic surfactants. Some alcohol ethoxylates have been analyzed by high temperature gas chromatography, but the fact that elution temperatures of the higher ethoxylated AEs are above 520 K on a SE 30 boiling point column (Stancher and Favretto, 1978) indicates that the vapor pressure of these compounds is comparatively low. This is consistent with the high boiling points of these compounds. In addition, since surfactants are rather water soluble, their Henry s law constants can be expected to be very low. Actually, no measured Henry s law constants are available. As a result, evaporation of surfactants can be expected to be negligible. 

In extreme cases of Finns effects, as in Bel and transition metal bromides and iodides, the stabilization resulting from covalency is very large. Distortion of the lattice occurs and direct companson with ionic halides is difficult. For metal halides the boiling points of these compounds are comparativeiy low as expected Bel, = S90 C. Znl - 624 C, FeCl - 315 C. The extreme of this trend is for the covalent forces to become so strong as to define discrete molecules even in the solid (e.g., Al Br. mp - 97 C, bp = 263 C). At this point we have come full circle and are back at the SF and CCI4 situation. 

In predominantly covalent compounds the bonds between atoms within a molecule (iwmzmolecular bonds) are relatively strong, but the forces of attraction between molecules (iwtennolecular forces) are relatively weak. As a result, covalent compounds have lower melting and boiling points than ionic compounds. The relation of bonding types to physical properties of liquids and solids will be developed more fully in Chapter 13. 

Covalently bonded compounds have strong internal bonds but weak attractive forces between molecules. Because of these weak attractive forces, the melting and boiling points of these compounds are much lower than compounds with ionic bonds. Therefore, such compounds are much more likely to be liquids or gases at room temperature than ionically bonded compounds. 

How does the Lewis model for covalent bonding account for the relatively low melting and boiling points of molecular compounds (compared to ionic compounds) ... 

The melting and boiling points of the aluminium halides, in contrast to the boron compounds, are irregular. It might reasonably be expected that aluminium, being a more metallic element than boron, would form an ionic fluoride and indeed the fact that it remains solid until 1564 K. when it sublimes, would tend to confirm this, although it should not be concluded that the fluoride is, therefore, wholly ionic. The crystal structure is such that each aluminium has a coordination number of six, being surrounded by six fluoride ions. 

Click Coached Problems for a self-study module on effect of ionic compounds on boiling point... 

Look at the comparison between NaCI and HCI, shown in Table 7.2. Sodium chloride, an ionic compound, is a white solid with a melting point of 801°C and a boiling point of 1413°C. Hydrogen chloride, a covalent compound, is a colorless gas with a melting point of — 115°C and a boiling point of —84.9°C. What accounts for such large differences between the properties of ionic compounds and covalent compounds ... 

Ionic liquids (IL) are a new class of salt-like materials that are entirely composed of ions and that are liquid at unusually low temperatures. For the most commonly used definition of the term ionic liquid the boiling point of water was chosen as a reference point, most likely for emotional reasons The term ionic liquids refers to compounds consisting entirely of ions and existing in the liquid state below 100 °C. In many cases the melting point is even below room temperature. 

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