Effects of Chemical Forces

It should be noted that these compounds are a trivial illustration of the principle stated in the first paragraph. Although all of the molecules contain very strong covalent bonds, none is broken on melting or vaporization, and hence they play no part in determining the melting and boiling points. 

The melting point of a compound is another property to which symmetry is an important contributor. Symmetrical molecules tend to have higher melting points than their less symmetrical isomers. For example, the melting point of neopentane is —17 °C, that of n-pentane -130 °C. If the molecule has very high symmetry, the melting point may be raised until the substance sublimes rather than melts. If you think about some substance that you know sublimes (in addition to SF6 mentioned above), you will note that it has high symmetry. Common examples are l2, CO, and camphor. The extreme example is perhaps dodecahedrane. CjqHjo  

Molecules in polar liquids such as water, liquid ammonia, sulfuric acid, and chloroform are held together by dipole-dipole and hydrogen bonding interactions. For molecules of comparable size, these are stronger than London forces resulting in the familiar trends in boiling points of nonmetal hydrides. For the heavier molecules, such as HjS, H2Se, PH, and HI. dipole effects are not particularly important (the elec- 

Dissociation energies ol rite alkali bolides (or the solid ond gas phases (lei mol-1)  

Increasing the ionic charges will certainly increase the lattice energy of a crystal. For compounds which are predominantly ionic, increased ionic charges will result in increased melting and boiling points. Examples are NaF, mp = 997 °C, and MgO, mp = 2800 °C. 

Crystals held together solely by London dispersion forces melt at comparatively low temperatures and the resulting lii uids vaporize easily. Examples of this type are the noble gases which boil at temperatures ranging Aom -269 C (He) to -62 (Rn). 

Disseclolion nergiei of Iha alkali halidor for Iha solid ond gas phoiti (fcl mor )  

On the other hand, if the covalent bonds are almosi as stable arxl as mimerous in the gas-phase molecules as in the solid, vaporization takes place readily. Examples are the depolymerizalion reactions that take place at a few hundred degrees. For example, red phosphorus sublimes and recondenses as white phosphorus.  

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Effect of Chemical Forces on Physical Properties Table 4.3 Continued... 

Chapters 1 2 Chapter 3 Separation in Effect of chemical force(s) and Bulk flow two phases 11... 

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