Intermolecular forces Iodine compounds

When iodine chloride is heated to 27°C, the weak intermolecular forces are unable to keep the molecules rigidly aligned, and the solid melts. Dipole forces are still important in the liquid state, because the polar molecules remain close to one another. Only in the gas, where the molecules are far apart, do the effects of dipole forces become negligible. Hence boiling points as well as melting points of polar compounds such as Id are somewhat higher than those of nonpolar substances of comparable molar mass. This effect is shown in Table 9.3. 

Most simple covalent compounds whose intermolecular forces are London (dispersion) forces, for example iodine (Figure 4-92) and the halogenoalkanes, are poorly soluble in water, but are soluble in less polar or non-polar solvents. Simple covalent compounds whose intermolecular forces are hydrogen bonds are often soluble in water, for example amines, carboxylic acids, amides and sugars, provided they have relatively low molar mass or can form multiple hydrogen bonds. 

Covalently bonded substances with a simple molecular structure, for example water and ammonia, are usually liquids or gases. This is because the forces between the molecules are weak. It does not take much energy to overcome these intermolecular forces, so these substances have low melting points, low boiling points and low enthalpy changes of vaporisation compared with ionic compounds. Some substances that have covalently bonded molecules maybe solids at room temperature, for example iodine and poly(ethene). These are usually molecules where the van der Waals forces are considerable. However, the melting points of these substances are still fairly low compared with ionic compounds or most metals. 

The molecules in crystalline chlorine, bromine and iodine are packed in a different manner, as shown in Fig. 11.1. The rather different distances between atoms of adjacent molecules are remarkable. If we take the van der Waals distance, such as observed in organic and inorganic molecular compounds, as reference, then some of the intermolecular contacts in the b-c plane are shorter, whereas they are longer to the molecules of the next plane. We thus observe a certain degree of association of the halogen molecules within the b-c plane (dotted in Fig. 11.1, top left). This association increases from chlorine to iodine. The weaker attractive forces between the planes show up in the plate-like habit of the crystals and in their easy cleavage parallel to the layers. Similar association tendencies are also observed for the heavier elements of the fifth and sixth main groups. 

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