Attractive Forces in Compounds

We U encounter the hydrogen bond, a special kind of intermolecular attractive force in compounds containing O H, N H, orF H bonds. 

VSEPR Theory) Electronegativity and Polarity Attractive Forces in Compounds Changes of State... 

Rh(0H)3H20 (161, 236) and hexachlororhodate(III) have also been reported (19). The carboxylates are initially isolated as the solvent adduct, but heating under vacuum is adequate to prepare the anhydrous product. A variety of donors can occupy the terminal positions of the copper acetate type structure found in these rhodium dimers. No simple explanation of the bond order in these compounds seems adequate to describe the metal-metal attractive force in view of the number of electrons available and the prominent role of axial ligands in the total bonding scheme (200). 

The stabilising action of the adsorption layers from high molecular substances (protective colloids) is related to the decrease in the forces of molecular attraction. Hence, films from aqueous solution of polyvinyl alcohol obtained between drops of cyclohexane have thickness of 80 nm and respectively, a very low attraction force, in contrast to black films [513]. Along with that the adsorption layers from such compounds possess visco-elasticity properties with modulus of elasticity 104 N m"2, impeding the film thinning and drop coalescence [503]. 

Differences in properties are a result of differences in attractive forces. In a covalent compound, the covalent bond between atoms in molecules is quite strong, but the attraction between individual molecules is relatively weak. The weak forces of attraction between individual molecules are known as inter-molecular forces, or van der Waals forces. Intermolecular forces, which are discussed at length in Chapter 13, vary in strength but are weaker than the bonds that join atoms in a molecule or ions in an ionic compound. 

Arts. The carboxylate salts have considerably higher boiling and melting points since they are ionic compounds while the carboxylic acids are polar compounds. Intermolecular attractive forces in ionic compounds are greater than those in the most polar compounds, even those with hydrogen bonding forces. 

Ans. Butanoic acid > 1-pentanol > pentanal > methyl propanoate > hexane. The order is based on differences in intermolecular forces since the compounds have very nearly the same molecular mass. Butanoic acid and 1-pentanol have hydrogen bonding but that in the acid is more extensive because of the special interaction between the carbonyl and HO groups. Aldehydes boil higher than esters because the polarity of the carbonyl group is greater than that of an ester group. The attractive forces in alkanes, the dispersion forces, are the weakest. 

Sublimation is usually a property of relatively nonpolar substances that also have highly symmetrical structures. Symmetrical compounds have relatively high melting points and high vapor pressures. The ease with which a substance can escape from the solid state is determined by the strength of intermolecular forces. Symmetrical molecular structures have a relatively uniform distribution of electron density and a small dipole moment. A smaller dipole moment means a higher vapor pressure because of lower electrostatic attractive forces in the crystal. 

Methane is a gas at room temperature and atmospheric pressure. It can be converted to a liquid if cooled to - 164°C, and to a solid if further cooled to - 182°C. The fact that methane (or any other compound, for that matter) can exist as a Uquid or a solid depends upon intermolecular attraction. Although the forces of attraction are generally electrostatic, they vaiy widely in relative strength. The strongest attractive forces are between ions, as for example between Na and Cl" in NaCl [787 kj (188 kcal)/mol]. Dipole-dipole interactions and hydrogen bonding [8-42 kJ (2-10 kcal)/mol] are weaker. We shall have more to say about these intermolecular attractive forces in Chapter 10. 

Solubility is defined as the maximum amount of solute that will dissolve in a given quantity of solvent at a specific temperature. Not all ionic compounds dissolve in water. Whether or not an ionic compound is water soluble depends on the relative magnitudes of the water molecules attraction to the ions, and the ions attraction for each other. We will learn more about the magnitudes of attractive forces in ionic compounds in Chapter 8, but for now it is useful to learn some... 

For a molecule to escape these same attractive forces in the liquid state and enter the gas phase, more heat has to be applied. When the vapor pressure of a liquid equals atmospheric pressure, boiling occurs. Boiling points of compounds are also relatively high if the intermolecular forces are relatively large. These effects lead to the smooth increase in boiling points seen in Figure 2-5. 

A new Chemistry Link to Health, Attractive Forces in Biological Compounds, is added. 

Physisorption occurs when, as a result of energy differences and/or electrical attractive forces (weak van der Waals forces), the adsorbate molecules become physically fastened to the adsorbent molecules. This type of adsorption is multilayered that is, each molecular layer forms on top of the previous layer with the number of layers being proportional to the contaminant concentration. More molecular layers form with higher concentrations of contaminant in solution. When a chemical compound is produced by the reaction between the adsorbed molecule and the adsorbent, chemisorption occurs. Unlike physisorption, this process is one molecule thick and irreversible... 

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