Covalent compounds: boiling points

Ionic compounds tend to be hard, crystalline substances with relatively high melting and boiling points. Covalently bonded substances tend to be gases, liquids, or relatively soft solids, with much lower melting and boiling points. 

Both boron and aluminium chlorides can be prepared by the direct combination of the elements. Boron trichloride can also be prepared by passing chlorine gas over a strongly heated mixture of boron trioxide and carbon. Like boron trifluoride, this is a covalent compound and a gas at ordinary temperature and pressure (boiling point 285 K). It reacts vigorously with water, the mechanism probably involving initial co-ordination of a water molecule (p, 152). and hydrochloric acid is obtained ... 

Table 14.2 shows that all three elements have remarkably low melting points and boiling points—an indication of the weak metallic bonding, especially notable in mercury. The low heat of atomisation of the latter element compensates to some extent its higher ionisation energies, so that, in practice, all the elements of this group can form cations in aqueous solution or in hydrated salts anhydrous mercuryfll) compounds are generally covalent. 

The degree of polarity has considerable influence on the physical properties of covalent compounds and it can also affect chemical reactivity. The melting point (mp) and boiling point (bp) are higher in ionic substances due to the strong nature of the interionic forces, whereas the covalent compounds have lower values due to the weak nature of intermolecular forces. 

Ionic compounds typically have higher boiling points and lower vapor pressures than covalent compounds. Predict which compound in the following pairs has the lower vapor pressure at room temperature (a) CEO or Na,0 (b) InCl, or SbCl, (c) LiH or HC1 (d) MgCl, or PCI,. 

Boron and hydrogen form many compounds and they exhibit unusual structural forms. Several of the boranes are listed in Table 13.2. Covalent hydrides are generally compounds that have low boiling points. Consequently, they are often referred to as volatile hydrides. 

Ionic compounds, as compared to covalent compounds, tend to have greater densities, higher melting and boiling points, and can be soluble in the very polar solvent, water, if the ionic bond is not too strong. 

In molecular covalent compounds, intermolecular forces are very weak in comparison with intramolecular forces. For this reason, most covalent substances with a low molecular mass are gaseous at room temperature. Others, with higher molecular masses may be liquids or solids, though with relatively low melting and boiling points. 

Organic compounds contain covalent bonds. In general, compared to inorganic compounds organic compounds have low melting and boiling points, tend to be flammable, have relatively low densities, do not dissolve readily in water, and are primarily nonelectrolytes. 

Problem 1.2 How do the boiling points, melting points, and solubilities of covalent organic compounds differ from those of salts Account for the differences. 

Polarity is a physical property of a compound, which relates other physical properties, e.g. melting and boiling points, solubility and intermolecular interactions between molecules. Generally, there is a direct correlation between the polarity of a molecule and the number and types of polar or nonpolar covalent bond that are present. In a few cases, a molecule having polar bonds, but in a symmetrical arrangement, may give rise to a nonpolar molecule, e.g. carbon dioxide (CO2). 

The hydrogen and halogen carbonyls and the nitroso carbonyls Co(NO)(CO)3, in which all bonds are covalent, are non-polar compounds with relatively low boiling points. The dimeric carbonyl, too, is non-polar, but its boiling point is much higher than that of the monomeric carbonyls because of the larger size of the molecule. 

Cadmium Halides. Cadmium halides show a steadily increasing covalency of the metal—halide bond proceeding from fluoride through to iodide. Bond lengths increase through the series F, 0.197 nm Cl, 0.221 nm Br, 0.237 nm I, 0.255 nm. The fluoride is much less soluble in water than the others (see Table 1) and the Cl, Br, and I compounds dissolve to a significant extent in alcohols, ethers, acetone, and liquid ammonia. Boiling points and... 

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 ... 

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