Molecular Substances Intermolecular Forces

Molecules are the characteristic structural units of gases, most liquids, and many solids. As a class, molecular substances tend to have the fallowing characteristics. 

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  

Insoluble in water but soluble in nonpolar solvents such as CCl4 or benzene. Iodine is typical of most molecular substances it is only slightly soluble in water (0.0013 mol/L at 25°C), much more soluble in benzene (0.48 mol/L). A few molecular substances, including ethyl alcohol, are very soluble in water. As you will see later in this section, such substances have intermolecular forces similar to those in water. 

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 

Effect of pressure on the melting point of a solid, (a) When ihe solid is die more dense phase, an increase in pressure converts liquid to solid the melting point increases, (b) If the liquid is the more dense phase, an increase in pressure converts solid to liquid and the melting point decreases. 

Low melting and boiling. Many molecular substances are gases at 25°C and 1 atm (e.g., N2,02, and CO2), which means that they have boiling points below 25°C. Others (such as H2O and CCh) are Hquids with melting (freezing) points below room temperature. Of the 

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 substance, the molecules must be set free from one another. This requires only that enough energy be supplied to overcome the weak attractive forces between molecules. The strong covalent bonds within molecules remain intact when a molecular substance melts or boils. 

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Molecular solids Intermolecular forces cause molecular substances to condense to form solids and liquids. 

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. 

The most common type of intermolecular force, found in all molecular substances, is referred to as a dispersion force. It is basically electrical in nature, involving an attraction between temporary or induced dipoles in adjacent molecules. To understand the origin of dispersion forces, consider Figure 9.8. 

The system is dynamic because molecular transfers continue, and it has reached equilibrium because no further net change occurs. The pressure of the vapor at dynamic equilibrium is called the vapor pressure (v p) of the substance. The vapor pressure of any substance increases rapidly with temperature because the kinetic energies of the molecules increase as the temperature rises. Table lists the vapor pressures for water at various temperatures. We describe intermolecular forces and vapor pressure in more detail in Chapter 11. 

When molecular energies are nearly sufficient to overcome intermolecular forces, molecules of a substance move relatively freely between the liquid phase and the vapor phase. We describe these phase changes in Section 11-1. 

Molecular solids are aggregates of molecules bound together by intermolecular forces. Substances that are gases under normal conditions form molecular solids when they condense at low temperature. Many larger molecules have sufficient dispersion forces to exist as solids at room temperature. One example is naphthalene (Cio Hg), a white solid that melts at 80 °C. Naphthalene has a planar structure like that of benzene (see Section 10-), with a cloud of ten delocalized n electrons that lie above and below the molecular plane. Naphthalene molecules are held in the solid state by strong dispersion forces among these highly polarizable n electrons. The molecules in... 

In molecular covalent compounds, intermolecular forces are very weak in comparison with intramolecular forces. For this reason, most covalent substances with a low molecular mass are gaseous at room temperature. Others, with higher molecular masses may be liquids or solids, though with relatively low melting and boiling points. 

Boiling points of substances increase with increasing intermolecular forces. All the given compounds are non-polar. We know that the non-polar molecules possess van der Waals forces and these forces are proportional to the molecular masses of the compounds. Therefore CH4, having the smallest molecular mass, has the lowest boiling point. So the boiling point order is ... 

The thermal motion of molecules of a given substance in a solvent medium causes dispersion and migration. If dispersion takes place by intermolecular forces acting within a gas, fluid, or solid, molecular diffusion takes place. In a turbulent medium, the migration of matter within it is defined as turbulent diffusion or eddy diffusion. Diffusional flux J is the product of linear concentration gradient dCldX multiphed by a proportionality factor generally defined as diffusion coefficient (D) (see section 4.11) ... 

At the same time that Dalton proposed his ideas on partial pressure, he developed the concept of vapor pressure. A vapor is the gaseous form of a substance that normally exists as a solid or liquid. A gas is a substance that exists in the gaseous states under normal conditions of temperature and pressure. The vapor pressure of a liquid is the partial pressure of the liquid s vapor at equilibrium. Liquids with strong inter-molecular forces exert lower vapor pressures than those with weak intermolecular forces. In liquids with strong intermolecular forces, it is more difficult for the molecules to leave the liquid state and enter the gaseous state. 

Why is it that some substances readily mix to form solutions while others do not Whether one substance dissolves in another substance is largely dependent on the inter-molecular forces present in the substances. For a solution to form, the solute particles must become dispersed throughout the solvent. This process requires the solute and solvent to initially separate and then mix. Another way of thinking of this is that the solute particles must separate from each other and disperse throughout the solvent. The solvent may separate to make room for the solute particles or the solute particles may occupy the space between the solvent particles. Determining whether one substance dissolves in another requires examining three different intermolecular forces present in the substances—between the... 

A familiarity with intermolecular forces is crucial to building insight into matter. We saw in Chapter 4 that a major goal in chemistry is to trace the connection between individual atoms and molecules and the bulk substances they form. There we dealt with gases, in which intermolecular forces play only a minor role. Here we deal with liquids and solids, for which the forces that hold molecules together are of crucial importance. Individual water molecules, for instance, are not wet, but bulk water is wet. Individual water molecules neither freeze nor boil, but bulk water does. We have to refine our atomic and molecular model of matter to see how properties like these, which we observe when we examine samples consisting of huge numbers of molecules, can be interpreted in terms of the properties of individual molecules, such as their size, shape, and polarity. 

A supercritical fluid exists when a substance is heated above its critical temperature and pressure and is unable to be condensed to a liquid by pressure alone. A typical supercritical fluid is carbon dioxide, which, at temperatures above 31°C and pressures above 73 atm, exists in a supercritical fluid state where individual molecules of the compound are held by less restrictive intermolecular forces and molecular movement resembles that of a gas (1 atm =101 325 Pa). 

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