Covalent bonds intermolecular forces compared

In linear polymers, cohesion results from weak (compared with covalent bonds) intermolecular attractive forces (Van der Waals) of various types London, Debye, Keesom, and hydrogen bonding. In a first approach, they can be considered undistinguishable, and one can define cohesive energy as the whole energy of intermolecular interactions. For small molecules, cohesive energy is easy to determine from calorimetric measurements since... 

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. 

As simple molecular substances, they are usually gases, liquids or solids with low melting and boiling points. The melting points are low because of the weak intermolecular forces of attraction which exist between simple molecules. These are weaker compared to the strong covalent bonds. Giant... 

You can see the difference between intermolecular forces and intramolecular forces in Figure 3.20. Because pure covalent compounds have low melting and boiling points, you know that the intermolecular forces must be very weak compared with the intramolecular forces. It does not take very much energy to break the bonds that hold the molecules to each other. 

The dipole-dipole forces act between the molecules because the negative end of one molecule will attract the positive end of another molecule. The dipole-dipole forces can comprise up to 1 % of the forces that act between to atoms in a covalent bond. Thus the intermolecular dipole-dipole forces are very week compared to the intramolecular covalent forces. 

These powerful forces, which are interionic, can be overcome only at a very high temperature. In a non-ionic compound, the atoms are held together by covalent bonds and form crystals in which the structural units are molecules. For melting to occur, the forces holding these molecules together must be overcome. These intermolecular forces are weak compared to the electrostatic forces, so... 

The third contribution to equation (7.4) arises from volume changes that occur on demixing polymer segments from the molecules of the dispersion medium on interpenetration. This volume change is a consequence of the dissimilarity in free volumes between the relatively dense polymer and the more expanded dispersion medium (assumed, of course, not to be polymeric in character). The difference in density can be envisaged to arise as follows as a liquid is heated, the van der Waals attractive intermolecular forces become relatively less important and the free volume increases in contrast, a comparable increase in the free volume of a polymer chain is prevented by the concatenation of the polymer segments due to the much stronger covalent bonds. 

A covalent bond is formed as a result of the sharing of a pair of electrons between atoms. Covalent bonds result when the difference in electronegativities between the bonding atoms is very small. Though the intramolecular bonds of covalent compounds are significant, the intermolecular forces are relatively weak. Because of this, covalent compounds have relatively lower boiling and melting points when compared to ionic compoimds. 

This reflects the lower strength of the intermolecular forces of attraction between the water molecules, compared to the covalent bonds within the water molecules. The attractive forces between water molecules are mainly due to hydrogen bonding. 

Note 5.6 (Van der Waals force). Even for nonpolar molecules, the polar character is generated due to instantaneous deviations in the electron orbit. Because of this electric fleld, the neighboring molecules become polarized, and the energy level of the total system becomes lower if the force is attractive rather than repulsive. Frozen carbon dioxide and crystals of iodine h are examples of crystals formed by van der Waals forces, which are known as molecular crystals. Since the van der Waals forces have no orientation, the molecular crystals occur in a closely-packed structure. The van der Waals force V(r) is inversely proportional to the sixth power of the intermolecular distance r V(r) = —C/r . The van der Waals force is extremely small compared to chemical forces such as ionic bonds, covalent bonds and metallic bonds (i.e., less than 1/100). ... 

Hardness and Structure Hardness depends on how easily the structural units of a solid can be moved relative to one another and therefore on the strength of attractive forces between the units. Thus, molecular crystals, with weak intermolecular forces, are rather soft compared with ionic crystals, in which the attractive forces are much stronger. A three-dimensional covalent network solid is usually quite hard because of the rigidity given to the structure by strong covalent bonds throughout it. Diamond and silicon carbide (SiC), three-dimensional covalent networks, are among the hardest substances known. 

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