Intermolecular forces types compared

The emphasis in this chapter is on bimolecular reactimis, i.e. reactions of the type A + B. Both the theoretical framework and the experimental techniques differ for reactions between an ion and a neutral molecule and between two neutral molecules, and they are discussed separately, in Sects. 3.3 and 3.4 respectively. In ion + neutral molecule reactions, the intermolecular forces are comparatively Imig range and theory concentrates on the calculation of the so-called capture rate coefficient. 

We have now discussed three types of intermolecular forces dispersion forces, dipole forces, and hydrogen bonds. You should bear in mind that all these forces are relatively weak compared with ordinary covalent bonds. Consider, for example, the situation in HzO. The total intermolecular attractive energy in ice is about 50 kj/mol. In contrast, to dissociate one mole of water vapor into atoms requires the absorption of928 kj of energy, that is, 2(OH bond energy). This explains why it is a lot easier to boil water than to decompose it into the elements. Even at a temperature of 1000°C and 1 atm, only about one H20 molecule in a billion decomposes to hydrogen and oxygen atoms. 

There are several types of intermolecular forces ion-dipole, dipole-dipole, hydrogen bonding, dipole-induced dipole, and dispersion forces. As we discuss these intermolecular forces (also called van der Waals forces), look at Table 12.2, which compares them with the stronger intramolecular (bonding) forces. 

In order to learn how to achieve the selectivity required to resolve a pair of enantiomorphs, the mechanism of retention must be fully understood. This means that the molecular forces that control retention must be defined, their mode of action identified, and their effect on the distribution coefficient (K) examined. The magnitude of (K) depends on the relative affinity of the solute for the two phases. Consequently, the stationary phase must be chosen to interact strongly with the solutes to achieve a separation i.e. the intermolecular forces between solute and stationary phase must be relatively large). In contrast, the interactions between the solute molecules and the mobile phase should be chosen to be relatively weak, to allow the stronger forces to dominate in the stationary phase and produce the required retention and selectivity. This will naturally occur in GC, as the probability of interaction (collision between solute and gas molecules) is very small compared with that in a liquid, and due to the small mass of the mobile phase molecules, the strength of any interactions that do occur will be extremely weak. This will not be true in LC, and the mobile phase must be chosen so that the type of interactions that take place with the solute will be weaker than those that take place between the solute and the stationary phase. This will become clearer when the different types of molecular interactions are understood. 

The concept like dissolves like can be helpful in predicting the solubiUty of a snbstance in a given solvent. What this expression means is that two substances with intermolecular forces of similar type and magnitude are likely to be soluble in each other. For example, both carbon tetrachloride (CCI4) and benzene (CeHe) are nonpolar liqnids. The only intermolecular forces present in these substances are the dispersion forces discussed in Section 4.6. When these two Uquids are mixed, they readily dissolve in each other, because the CCLt-CeHe attractions are comparable in magnitude to the CCLt-CCLt forces and to the CeHg-CeHe forces. 

Since intermolecular Van der Waals-type forces which hold molecules in a crystalline lattice are very weak, when compared with intramolecular covalent forces, lattice dynamics of polymers is made easy by considering firstly the vibrations of infinite isolated polymer chains [16-18]. Three-dimensional order is considered later as a small perturbation of the nonual modes of the one-dimensional chain by the weak intermolecular forces [49]. In other words, we are considering the dynamics of one chain in vacuo . 

What type of physical properties would you need to consider in comparing the strength of intermolecular forces in solids and in liquids ... 

We have now discussed three types of intermolecular forces dispersion forces, dipole forces, and hydrogen bonds. You should bear in mind that all these forces are relatively weak compared with ordinary covalent bonds. Consider, for example, the... 

In general, when comparing the properties of different substances, we must consider the various types of intermolecular forces and the factors that affect the strength of each type of force. Although it wasn t important here, the three-dimensional shape of a molecule is usually a very important consideration and it is usually necessary to sketch the molecular structure to see how molecular shape plays a role. 

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