Polar molecular substances

It should be noted that self-ionisation is not an essential prerequisite for a satisfactory polar solvent. Liquids such as acetonitrile CH3CN or dimethylsulphoxide SO(CH3)2 appear not to ionise but they make very useful solvents for electrolytes as well as for polar molecular substances. As with H20, NH3, H2S04 etc., they owe their solvent powers to their polarity, leading to dipole-dipole interaction in the case of polar molecules as solutes and ion-dipole attraction in the case of electrolytes. There may in addition be considerable covalent bonding, via coordinate bond formation, in the case of cations. In solvents which do undergo appreciable self-ionisation, coordination often needs to be considered explicitly in discussing acid/base and other reactions and equilibria. 

Polar molecular substances are also soluble in polar water molecules. When a molecular substance such as ethyl alcohol (C2H5OH) dissolves in water (H20), polar ethyl alcohol molecules bond with polar water molecules. In general, likes dissolve likes. Polar solutes will dissolve in polar solvents. In addition, nonpolar solutes dissolve in nonpolar solvents. Nonpolar octane (C8H18) dissolves in nonpolar carbon tetrachloride (CC14). It follows that solutes and solvents of opposite polarity do not form solutions. Nonpolar oil does not dissolve in polar water. (Polar means bearing a charge.)... 

Which of the following represents a polar molecular substance ... 

When a polar molecular substance, such as hydrogen chloride, is heated to convert it from liquid to gas, dipole-dipole attractions are broken. The molecules themselves remain unchanged. For example, when liquid FiCl is boiled, the dipole-dipole attractions between FiCl molecules are broken, but the covalent bonds between the hydrogen atoms and the chlorine atoms within the FiCl molecules are unaffected. 

When a polar molecular substance, such as hydrogen chloride, is heated to... 

It is more difficult to predict the solubility of polar molecular substances than to predict the solubility of ionic compounds and nonpolar molecular substances. Many polar molecular substances are soluble in both water and hexane. For example, ethanol is miscible with both water and hexane. The following generalization is helpfiil ... 

Nonconductors of electricity when pure. Molecules are uncharged, so they cannot carry an electric current. In most cases (e.g., iodine, I2, and ethyl alcohol, C2H5OH), water solutions of molecular substances are also nonconductors. A few polar molecules, including HC1, react with water to form ions ... 

As a result of these dipole-dipole forces of attraction, polar molecules will tend to attract one another more at room temperature than similarly sized non-polar molecules would. The energy required to separate polar molecules from one another is therefore greater than that needed to separate non-polar molecules of similar molar mass. This is indicated hy the extreme difference in melting and boiling points of these two types of molecular substances. (Recall that melting and boiling points are physical properties of substances.)... 

Not surprisingly, the strength of a given dipole-dipole interaction depends on the sizes of the dipole moments involved. The more polar the substance, the greater the strength of its dipole-dipole interactions. Butane, for instance, is a nonpolar molecule with a molecular mass of 58 amu and a boiling point of —0.5°C, while acetone has the same molecular mass yet boils 57°C higher because it is polar. 

Molecular substances tend to dissolve in non polar solvents attraction between the molecules and the solvent can easily balance the small positive AH° for the breakup of the lattice, and entropy will favour dissolution. Molecular substances that consist of polar molecules, or of molecules having polar bonds, may also dissolve in polar solvents. 

The Hg-C bond is very weak, partly on account of the large size of the Hg atom and the low polarity of the bond (the mercury dialkyls are molecular substances, with linear C—Hg-C skeletons). Despite the low atomisation enthalpy of mercury compared with other metallic elemental substances, reactions of this kind are usually exothermic. The mercury dialkyls are relatively insensitive to air and moisture and are therefore convenient to store and use. They are prepared from Grignard reagents ... 

Polarity is the extent to which a substance, at molecular level, is characterized by a non-symmetrical distribution of electron density. Polarity is often expressed as dipole moment, which is a function of the magnitude of the partial charges on the molecule, and the distance between the charges. Substances that have larger dipole moments have greater polarity than substances with lower dipole moments. Water and acetone, for example, have dipole moments of 1.85 and 2.80, respectively. Benzene and carbon tetrachloride are nonpolar and have dipole moments of zero. 

PPX films containing PbO nanocrystals also show increase in their conductivity under action of small quantities of ammonia and ethanol vapors from gaseous environment [89, 102, 103]. This effect takes place in the presence of water vapors only and so, most probably, is due to the formation of ionized or highly polarized molecular complexes NH3 H2O and C2H5OH H2O. The responses of conductivity to ammonia and ethanol are also reversible the film conductivity returns to its initial value after the removal of these substances from the surrounding atmosphere. 

Water H20 is the most abundant molecular substance on Earth. It is highly polar compound. Its physical properties are dominated by hydrogen bonding. It is an excellent solvent for ionic substances and reactions. 

A passage of the blood-brain barrier has, however, not to be expected due to their polarity and molecular size. Due to the particular structure of the brain capillaries only lipoid-soluble low molecular substances are able to enter the brain vessels from the blood. This fact is called the blood-brain or blood-liquor barrier which indeed cannot be overcome by polymers but by free low-molecular active substances like mezcalin or aminoantipyrine. 

When a substance is placed in an electric field, such as exists between the plates of a charged capacitor, it becomes to some extent electrically polarized. The polarization results at least in part from a displacement of electron clouds relative to atomic nuclei polarization resulting from this cause is termed electronic polarization. For molecular substances, atomic polarization may also be present, owing to a distortion of the molecular skeleton. Taken together, these two kinds of polarization are called distortion polarization. Finally, when molecules possessing permanent dipoles are present in a liquid or gas, application of an electric field produces a small preferential orientation of the dipoles in the field direction, leading to orientation polarization. 

Use Figure 4-la and your knowledge of molecular geometry and polarity to determine whether or not the following substances are ionic compounds, non-polar molecular compounds, or polar molecular compounds. 

When there are no polar bonds in a molecule, there is no permanent charge difference between one part of the molecule and another, and the molecule is nonpolar. For example, the CI2 molecule has no polar bonds because the two atoms in each Cl-Cl bond attract electrons equally. It is therefore a nonpolar molecule. None of the bonds in hydrocarbon molecules, such as hexane, C6Ffi4, are significantly polar, so hydrocarbons are nonpolar molecular substances. Lastly,... 

Given the name or chemical formula for a substance, (1) categorize the substance as a metallic element, carbon in the diamond form, another nonmetallic element, an ionic compound, a polar molecular compound with hydrogen bonds, a polar molecular compound without hydrogen bonds, or a nonpolar molecular compound (2) identify the type of particle that forms its fundamental structure and (3) identify the type of attraction holding its particles in the solid and liquid form. 

Nonpolar substances are not likely to dissolve to a significant degree in polar solvents. For example, nonpolar molecular substances, such as hydrocarbons, are likely to be insoluble in water. 

The term athermal mixtures refers to those in which the heat of mixing A i H is very small but S differs considerably from zero, for example, polymer solutions. In regular mixtures differs considerably from zero but S is negligible, for example, mixtures of highly polar low-molecular substances such as nitriles/esters. 

The analysis of the low adhesive properties of iPP leads to the two different approaches of explanation (Brewis Mathieson, 2002 Chodak Novak, 1999, Kinloch, 1987). By the first explanation the low adhesion of iPP consists in a formation of thin layer of low-molecular substances on the interfacial boundary. The primary function of modification is then a removal of the thin low-molecular substance layer from the polymer surface, while the chemical modification itself is of a secondary importance. The second explanation attributes the low adhesive properties of iPP to its non-polar character and low surface energy, stressing the dependence of the adhesive properties of iPP on their super molecular structure. The chemical changes resulted in the increase of the polarity and surface energy are considering for the most important in the modification of iPP. 

Whether a molecular compound is a gas, liquid, or solid is a function of the strength of the attractions between molecules. Only when there are huge molecules, with attractions between many polar regions in the molecule, as in polymers (Figure 14.11), do we find very hard and strong molecular substances. 

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