Solid intermolecular attractive forces

Molecules have kinetic energy (they move around), and they also have intermolecular attractive forces (they stick to each other). The relationship between these two determines whether a collection of molecules will be a gas, liquid, or solid. 

In a solid, the energy of intermolecular attractive forces is much stronger than the kinetic energy of the molecules, so kinetic energy and kinetic molecular theory are not very important. As temperature increases in a solid, the vibrations of individual molecules grow more intense and the molecules spread slightly further apart, decreasing the density of the solid. 

B. A gas at STP has a normal boiling point under 0 °C. The substance with the lowest boiling point will have the weakest intermolecular attractive forces and will be the most likely gas at STP. F2 has the lowest molecular weight, is not a salt, metal, or covalent network solid, and is non-polar, indicating the weakest intermolecular attractive forces of the four choices. F2 actually is a gas at STP, and the other three are solids. 

Let us now consider the flow of gas in a tube. The molecules of a substance in the liquid or gaseous state are in rapid motion, and in the gaseous state the number of particles in unit volume is much smaller than in either the solid or liquid states. Intermolecular attractive forces are consequently much reduced, and a mass of gas in an enclosed space will diffuse in all directions to fill the entire system. 

However, as RH further increases, tablet strength decreases for most tested tablets, and it was suggested that condensed water on the solid surface at high RH weakens intermolecular attraction forces between particles in the tablets and the further softened particles and solid bonds cause the tensile strength to decrease. " In addition, at high RH, water may form multilayers on the solid surface, which can act as a lubricant and reduce the frictional forces between particles, thus decreasing tensile strength as well. Ultimately, the effects of adsorbed moisture on particle surfaces are very complex, affected by many factors, especially the properties of the tablet excipients. 

We have seen (Section 12-15) how the presence of strong attractive forces between gas molecules can cause gas behavior to become nonideal when the molecules get close together. In liquids and solids the molecules are much closer together than in gases. As a result, properties of liquids, such as boiling point, vapor pressure, viscosity, and heat of vaporization, depend markedly on the strengths of the intermolecular attractive forces. These forces are also directly related to the properties of solids, such as melting point and heat of fusion. Let us preface our study of these condensed phases with a discussion of the types of attractive forces that can exist between molecules and ions. 

Whether a substance exists as a gas, liquid, or solid depends on the nature of its intermolecular attractive forces and on its temperature and pressure. A phase diagram is a graphical way to summarize the environmental conditions under which the different states of a substance are stable. The diagram is divided into three areas representing the three possible states of the substance (gas, liquid, or solid). Temperature and pressure determine the phase of a substance and are shown on the x-axis and y-axis of the phase diagram, respectively. 

Sucrose, C12H22O11, is a molecule that contains eight 0— H bonds. When water is added to solid sucrose, each of the 0 — H bonds on the sucrose molecule is a possible site for hydrogen bonding with water. The intermolecular attractive forces between sucrose molecules are overcome and replaced by water-sucrose intermolecular attractive forces. This is why sugar is highly soluble. 

Whether a substance exists as a gas, liquid, or solid depends on the nature of its intermolecular attractive forces and on its temperature and pressure. This information is often visualized as a phase diagram for the substance. 

The time course of the heating will show the temperature of the ice in the tube remain at 0 °C until all the ice melts. During this time, the heat added to the ice from the room is not causing an increase in temperature. Instead it is acting to break the intermolecular attractive forces of the solid so that only liquid remains. Then the added heat from the room will increase water temperature until it returns to room temperature. 

Molecular solids. The imits that comprise a molecular solid, molecules, are held together by intermolecular attractive forces (London forces, dipole-dipole interactions, and hydrogen bonding). Molecular solids are usually soft and have low melting points. They are frequently volatile and are poor electrical conductors. A common example is ice (solid water). 

Polar compounds have strong intermolecular attractive forces. Higher temperatures are needed to overcome these forces and convert the solid to a liquid hence, we predict higher melting points for polar compounds when compared to nonpolar compounds. 

A measure of the intermolecular attraction forces in a material is provided by the cohesive energy. Approximately, this equals the heat of vaporization (for liquids) or sublimation (for solids) per mol. The cohesive energy density in the liquid state is thus AEyfV, in which AE-o is the molar energy of vaporization and V is the molar volume of the liquid. The square root of this cohesive energy density is known as the solubility parameter (d), that is,... 

A molecular solid consists of atoms or molecules held togetiier by intermolecular attractive forces. In molecular solids, the attractive forces include hydrogen bonds and dipole-dipole forces (e.g.. Ice [H O (5)]). 

Over the years, examination of different solvent-solute combinations has led to an important generalization Substances with similar intermolecular attractive forces tend to be soluble in one another. This generalization is often simply stated as like dissolves liker Nonpolar substances are more likely to be soluble in nonpolar solvents ionic and polar solutes are more likely to be soluble in polar solvents. Network solids such as diamond and quartz are not soluble in either polar or nonpolar solvents because of the strong bonding forces within the solid. 

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