Forces, intermolecular dispersion

The different kinds of intermolecular forces (dispersion, dipole-dipole, hydrogen bonding, etc. see Section VI-1) may not equally contribute to A-A, B-B, and A-B... 

Molecular interactions are the result of intermolecular forces which are all electrical in nature. It is possible that other forces may be present, such as gravitational and magnetic forces, but these are many orders of magnitude weaker than the electrical forces and play little or no part in solute retention. It must be emphasized that there are three, and only three, different basic types of intermolecular forces, dispersion forces, polar forces and ionic forces. All molecular interactions must be composites of these three basic molecular forces although, individually, they can vary widely in strength. In some instances, different terms have been introduced to describe one particular force which is based not on the type of force but on the strength of the force. Fundamentally, however, there are only three basic types of molecular force. 

Ihe boiling points of different molecular substances are directly related to the strength of the intermolecular forces involved. The stronger the intermolecular forces, the higher the boiling point of the substance. In the remainder of this section, we examine the nature of the three different types of intermolecular forces dispersion forces, dipole forces, and hydrogen bonds. 

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. 

Intermolecular forces = dispersion (van der Waals) forces, which exist between molecules... 

INTERMOLECULAR FORCES We then examine four intermolecular forces dispersion forces, dipole-dipole forces, hydrogen bonds, and ion-dipole forces. 

Types of Intermolecular Forces Dispersion, Dipole-Dipole, and Hydrogen Bonding 425... 

Molecular solids are solids whose composite units are molecules. Ice (solid H2O) and dry ice (solid CO2) are examples of molecular solids. Molecular solids are held together by the kinds of intermolecular forces—dispersion forces, dipole-dipole forces, and hydrogen bonding— that we just discussed in Section 12.6. For example, ice is held together by hydrogen bonds, and dry ice is held together by dispersion forces. Molecular solids as a whole tend to have low to moderately low melting points ice melts at 0 °C and dry ice sublimes at -78.5°C. 

Molecular solids have molecules at the lattice points and often adopt cubic closest packing. Combinations of intermolecular forces (dispersion, dipole-dipole, and H bonding) result in physical properties that vary greatly. 

H—N, H—O, and H—F No. Hydrogen bonding not possible Intermolecular forces dispersion forces... 

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