Intermolecular forces boiling

The dotted lines represent hydrogen bonds. The high boiling point and viscosity of the pure acid indicate strong intermolecular forces of this kind. 

Because so many factors contribute to the net intermolecular attractive force it is not always possible to predict which of two compounds will have the higher boiling point We can however use the boiling point behavior of selected molecules to inform us of the relative importance of various intermolecular forces and the structural features that influence them... 

Thermodynamic Effects 4.16.2.4.1 Intermolecular forces (i) Melting points and boiling points... 

The degree of polarity has considerable influence on the physical properties of covalent compounds and it can also affect chemical reactivity. The melting point (mp) and boiling point (bp) are higher in ionic substances due to the strong nature of the interionic forces, whereas the covalent compounds have lower values due to the weak nature of intermolecular forces. 

The generally low melting and boiling points of molecular substances reflect the fact that the forces between molecules (intermolecular forces) are weak. To melt or boil a molecular... 

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. 

When iodine chloride is heated to 27°C, the weak intermolecular forces are unable to keep the molecules rigidly aligned, and the solid melts. Dipole forces are still important in the liquid state, because the polar molecules remain close to one another. Only in the gas, where the molecules are far apart, do the effects of dipole forces become negligible. Hence boiling points as well as melting points of polar compounds such as Id are somewhat higher than those of nonpolar substances of comparable molar mass. This effect is shown in Table 9.3. 

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. 

In Chapter 4 we considered gases, in which intermolecular forces play only a minor role. Here, we deal with liquids and solids, in which the forces that hold molecules together are of crucial importance for determining the physical properties of bulk samples. Individual water molecules, for instance, are not wet, but bulk water is wet because water molecules are attracted to other substances and spread over their surfaces. Individual water molecules neither freeze nor boil, but bulk water does, because in the process of freezing molecules stick together and form a rigid array and in boiling they separate from one another and form a gas. 

O 3 Predict the relative order of the boiling points of two substances from the strengths of their intermolecular forces (Examples 5.1 and 5.2). 

The lower the vapor pressure, the higher the boiling point. Therefore, a high normal boiling point is a sign of strong intermolecular forces. 

Boiling occurs when the vapor pressure of a liquid is equal to the atmospheric pressure. Strong intermolecular forces usually lead to high normal boiling points. 

Solutions in which intermolecular forces are stronger in the solution than in the pure components have negative deviations from Raoulfs law some form maximum-boiling azeotropes. Solutions in which intermolecular forces are weaker in the solution than in the pure components have positive deviations from Raoulfs law some form minimum-boiling azeotropes. 

Alkanes with long, unbranched chains tend to have higher melting points, boiling points, and enthalpies of vaporization than those of their branched isomers. The difference arises because, compared with unbranched molecules, the atoms of neighboring branched molecules cannot get as close together (Fig. 18.5). As a result, molecules with branched chains have weaker intermolecular forces than their unbranched isomers. 

Cl l CH2CI 12CH2OCHv (a) Draw a Lewis structure for each molecule, name it, and classify it by functional group, (b) Which molecules are isomers of each other Are any chiral If so, which ones (c) For each molecule, list the types of intermolecular forces that are present, (d) Use your answers to parts (a) and (b) to predict the relative boiling points, from lowest to highest. 

The boiling point of a substance depends on the magnitude of its intermolecular forces, which in turn depends on the polarizability of its electron cloud. Monatomic gases contain atoms rather than molecules, so we must assess interatomic forces for these substances. 

Methanol has a considerably higher boiling point than its alkane relatives, methane and ethane, consistent with significant intermolecular forces. Ammonia dissolves readily in acetone, also consistent with significant intermolecular forces. 

In Chapter 6, the polarizability of molecules was considered as one factor related to both London and dipole-induced dipole intermolecular forces. The data shown in Table 9.6 confirm many of the observations that can be made about physical properties. For example, in the case of F2, Cl2, and Br2, the London forces that arise from the increase in polarizability result in a general increase in boiling point. 

This structure contains a total of five bonds, which is an average of 2.5 bonds per NO unit. Therefore, there is no net increase in the number of bonds in the dimer compared to two separate molecules. The result is that there is not much energy advantage if dimers form. The melting point of NO is -164 °C and the boiling point is -152 °C. The low boiling point and small liquid range, about 12 °C, is indicative of only very weak intermolecular forces. The Lewis structure of the molecule can be shown as... 

The normal melting points and boiling points generally increase as the intermolecular forces between the molecules in the compounds increase. 

To confirm the above reasoning, use boiling points given below to indicate strengths of intermolecular forces. Remember, the lower the boiling point is, the weaker the intermolecular forces present in the liquid, and the higher the vapor pressure at a specific temperature. 

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