Chem Quiz Chapter 11

What us capillary action? How does it depend on the relative strengths of adhesive and cohesive forces?

Capillary action is the ability of a liquid to flow against gravity up a narrow tube. Capillary action results from a combination of two forces: the attraction between molecules in a liquid, called cohesive forces, and the attraction between these molecules and the surface of the tube, called adhesive forces. The adhesive forces cause the liquid to spread out over the surface of the tube, while the cohesive forces cause the liquid to stay together. If the adhesive forces are greater than the cohesive forces, the attraction to the surface draws the liquid up the tube while the cohesive forces pull along those molecules that are not in direct contact with the tube walls. The liquid rises up the tube until the force of gravity balances the capillary action- the thinner the tube, the higher the rise. If the adhesive forces are smaller than the cohesive forces, the liquid does not rise up the tube at all.

What is dipole dipole force? How can you predict the presence of dipole-dipole forces in a compound?

Dipole-dipole force exists in all molecules that are polar. Polar molecules have permanent dipoles that interact with the permanent dipoles of neighboring molecules. The positive end of one permanent dipole is attracted to the negative end of another; this attraction is the dipole-dipole force

What is the dispersion force? What does the magnitude of the dispersion force depend on? How can you predict the magnitude of the dispersion force for closely related elements or compounds?

Dispersion forces (aka London forces) are the result of fluctuations in the electron distribution within molecules or atoms. Because all atoms and molecules have electrons, they all exhibit dispersion forces. The electron in an atom or molecular may at any one instance, be unevenly distributed. The magnitude of the dispersion force depends on how easily the electrons in the atom or molecule can move or polarize in response to an instantaneous dipole, which in turn depends on the size of the electron cloud. A larger electron cloud results in a greater dispersion force because the electrons are held less tightly by the nucleus and can therefor polarize more easily. If other variables are constant, the dispersion force increase with increasing molar mass because molecules or atoms of higher molar mass generally have more electrons dispersed over a greater volume. The shape of molecules can also effect the magnitude of the dispersion forces. The larger the area of interaction between two molecules, the larger the dispersion forces.

How is the miscibility of two liquids related to their polarity?

Miscibility is the ability to mix without separating into two phases. The rule of thumb is like dissolves like. IN general, polar liquids are miscible with other polar liquids, but are not miscible with nonpolar liquids. Nonplar liquids are miscible with other nonpolar liquids.

Explain what happens when in the process of vaporization and condensation. Why does the rate of vaporization increase with increasing temperature and surface area?

Molecules are in constant motion, the higher the temperature the greater the average energy of the collection of molecules. However, at any one time, some molecules will have more thermal energy than the average and some will have less. The molecules with the highest thermal energy have enough energy to break free from the surface- where molecules are held less tightly than in the interior due to fewer neighbor-neighbor interactions- and into the gas phase. This process is called vaporization, the phase transition from liquid to gas. The greater the temperature, the greater the rate of vaporization. Some of the water molecules in the gas phase, at the low end of the energy distribution curve for the gaseous molecules, can plunge back into the liquid and be captured by intermolecular forces. The process- the opposite vaporization-is called condensation, the phase transition from gas to liquid.

Explain the process of dynamic equilibrium. How id dynamic equilibrium related to vapor pressure?

Molecules are in constant motion. Molecules leave liquid for the gas phase and gas phase molecules condense to become a liquid. Dynamic equilibrium has been reached when the rate of condensation and the rate of vaporization become equal. Although condensation and vaporization continue at equal rates, the concentration of water vapor above the liquid is constant. The pressure of a gas in dynamic equilibrium with its liquid is called its vapor pressure.

What is surface tension? How does surface tension result from intermolecular forces? How is it related to the strength of intermolecular forces?

Surface tension is the tendency of liquids to minimize their surface area. Molecules at the surface have relatively fewer neighbors with which to interact because there are no molecules above the surface. Consequently, molecules at the surface are inherently less stable- they have higher PE- than those in the interior. To increase the surface area of the liquid, some molecules from the interior must be moved to the surface, a process requiring energy. The surface tension of a liquid is the energy required to increase the surface area by a unit amount. Surface tension decreases with decreasing intermolecular forces.

Define the terms boiling point and normal boiling point.

The boiling point of a liquid is the temp. at which its vapor pressure equals the external pressure. The normal boiling point of a liquid is the temperature at which it s vapor pressure equals 1 atm.

What is the heat of vaporization for a liquid and why is it useful?

The heat of vaporization (change in H vap) is the amount of heat required to vaporize 1 mole of a liquid to a gas. The heat of vaporization of a liquid can be used to calculate the amount of heat energy required to vaporize a given mass of the liquid (or amount of heat given off by condensation of a given mass of liquid) and can be used to compare the volatility of the two substances.

What is hydrogen bonding? How can you predict the presence of hydrogen bonding in a compound?

The hydrogen bond is a sort of super dipole-dipole force. Polar molecuels contating hydroen atoms bonded directly to fluorine, oxygen, or nitrgen exhibit an intermolecular dorce called hydrogen bonding. The large electroneagtivity difference between hydorgen and these electronegative elements means the H atoms will have fairly large partial positive charges, while the F, O, or N atoms will have fairly large partial negative charges. In addition, because these atoms are quite small, they can approach on another very closely. The result is a strong attraction between the hydrogen in each of these molecules an the F,O, or N on its neighbors, attraction called a hydrogen bond.

What is ion-dipole force? Why is it important?

The ion-dipole force occurs when an ionic compound is mixed with a polar compound and is especially important in aqueous solutions of ionic compounds. For example, when sodium chloride is mixed with water, the sodium chloride ions interact with water molecules via ion-dipole forces. The positive sodium ions interact with the negative poles of water molecules, while the negative chloride ions interact with the positive poles. Ion-dipole forces are the strongest types of intermolecular forces discussed here and are responsible for the ability of ionic substances to form solutions with water.

Why is vaporization endothermic? Why is condensation exothermic?

The molecules that leave the liquid are at the high end of the energy curve- the most energetic. If no additional heat enters the liquid, the average of the entire collection of molecules goes down. So vaporization is an endothermic process; it takes energy to vaporize the molecules in a liquid. Also, vaporization requires overcoming the intermolecular forces that hold liquids together. Because energy mist be absorbed to pull the molecules apart, the process is endothermic. Condensation is the opposite process so it must be an exothermic. also, gas particles have more energy than those in the liquid. It is the least energetic of these that condense, adding energy to the liquid.

How is vapor pressure related to temperature? What happens to the vapor pressure of a substance when the temperature is increased? Decreased?

The vapor pressure of a liquid increases with increasing temperature. However, the relationship is not linear; it is exponential. As the temperature of a liquid increases, the vapor increases more and more quickly. As the temperature is decreased, the vapor pressure decreases following this same relationship.

How is the volatility of a substance related to the intermolecular forces present within the substance?

The weaker the intermolecular forces are, the more likely molecules will evaporate at a given temperature, making the liquid more volatile.

What is viscosity? How does viscosity depend on intermolecular forces? What other factors affect viscosity?

Viscosity is the resistance of a liquid to flow. Viscosity is measured in a unit called poise (P), defined as 1g/cm(s). The centipoise (cP) is a convenient unit because the viscosity of water at room temperature is approximately one centipoise. Viscosity is greater in substances with stronger intermolecular forces because molecules are more strongly attracted to each other. Preventing them from flowing around each other as freely. Viscosity also depends on molecular shape, increasing in loger molecules that can interact over a greater area and possibly become entangles. Viscosity increases with increasing molar masses (and therefore increasing magnitude of dispersion forces) and with increasing length (and therefore increasing potential for molecular entanglement). Viscosity also depends on temperature because thermal energy partially overcomes the intermolecular forces, allowing molecules to flow past each other more easily.

What happens to a system in dynamic equilibrium when it is disturbed in some way?

When a system in dynamic equilibrium is distributed, the system responds so as to minimize the disturbance and return to a state of equilibrium.