Chapter 11 Review Questions

17 What is 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 surfaces 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. If the adhesive forces are smaller than the cohesive forces, the liquid does not rise up the tube at all.

25 Define the terms boiling point and normal boiling point.

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

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

The dipole-dipole force exists in all polar molecules. 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.

30 What is the heat of fusion and why is it important?

The heat of fusion (∆Hfus) is the amount of heat required to melt 1 mole of a solid. The heat of fusion of a solid, which is related to the strength of the intermolecular forces, can be used to calculate the amount of heat energy required to melt a given mass of the solid (or the amount of heat given off by the freezing of a given mass of liquid).

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

The heat of vaporization 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 and can be used to compare the volatility of two substances.

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

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

27 Explain what happens to a substance when it is heated in a closed container to its critical temperature.

As the temperature rises, more liquid vaporizes and the pressure within the container increases. As more and more gas is forced into the same amount of space, the density of the gas becomes higher and higher. At the same time, the increasing temperature causes the density of the liquid to become lower and lower. At the critical temperature, the meniscus between the liquid and gas disappears and the gas and liquid phases co-mingle to form a superficial fluid.

10 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. Given that all atoms and molecules have electrons, they all exhibit dispersion forces. The magnitude of the dispersion force depends on the size/volume 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, therefore, polarize more easily in response to an instantaneous dipole (a temporary change in charge distribution). If all other variables are constant, the dispersion force increases with increasing molar mass because molecules or atoms of higher molar mass generally have more electrons dispersed over a greater volume. The shape of the molecules can also affect the magnitude of the dispersion forces. The larger the area of interaction between two molecules, the larger the dispersion forces.

29 What is fusion? Is fusion exothermic or endothermic? Why?

Fusion, or melting, is the phase transition from solid to liquid. The term fusion is used for melting because if you heat several crystals of a solid, they will fuse into a continuous liquid upon melting. Fusion is endothermic because solids have less kinetic energy than liquids, so energy must be added to a solid to get it to melt.

7 Describe the relationship between the state of a substance, its temperature, and the strength of its intermolecular forces.

Given that the most molecular motion occurs in the gas phase and the least molecular motion occurs in the solid phase (atoms are pushed closer together), a substance will be converted from a solid to a liquid to a gas as the temperature increases. The strength of the intermolecular interactions is least in the gas phase because there are large distances between particles and they're moving very fast. Intermolecular forces are stronger in liquids and solids, where molecules are "touching" one another. The strength of the interactions in the condensed phases determines at what temperature the substance will melt and boil.

5 What is the fundamental difference between an amorphous solid and a crystalline solid?

In crystalline solids, atoms or molecules that comprise them are arranged in a well-ordered 3d array. In amorphous solids, the atoms or molecules that compose them have no long-range order.

1 Explain why water drops are spherical in the absence of gravity.

In the absence of gravity, the shape of the blob becomes a sphere because the sphere is the geometrical shape with the lowest surface-area to volume ratio. By forming a sphere, the water molecules maximize their interaction with one another because the sphere results in the min number of molecules being at the surface of the liquid, where fewer interactions occur (compared to the interior of the liquid).

2 Why are intermolecular forces important?

Intermolecular forces are important because they hold many liquids and solids together (examples: water and ice). They determine many of the physical properties of a substance. All living organisms depend on intermolecular forces for many physiological processes. Intermolecular forces are responsible for the very existence of the condensed phases.

8 From what kinds of interactions do intermolecular forces originate?

Intermolecular forces originate from the interactions between charges, partial charges, and temporary charges on molecules (or atoms and ions), much like bonding forces originate from interactions between charged particles in atoms.

9 Why are intermolecular forces generally much weaker than bonding forces?

Intermolecular forces, even the strongest ones, are generally much weaker than bonding forces. The reason for the relative weakness of intermolecular forces compared to bonding forces is related to Coulomb's law (E = 1/4piε₀ * q₁q₂/r). Bonding forces are the result of large charges (the charges on protons and electrons, q₁ and q₂) interacting at very close distances (r). Intermolecular forces are the result of smaller charges interacting as greater distances.

3 What are the main properties of liquids (in contrast to gases and solids)?

Liquids - have much higher densities in comparison to gases and generally have lower densities in comparison to solids (except for water/ice) - have an indefinite shape and assume the shape of their container. - have a finite volume. - are not easily compressed.

12 How is the polarity of a liquid generally related to its miscibility with water?

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

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

Molecules are in constant motion. Molecules leave the liquid for gas phase, and the 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 vapor above the liquid is constant. The pressure of a gas in dynamic equilibrium with its liquid is called its vapor pressure.

18 Explain what happens in the processes 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. Some of the water molecules in the gas phase, at the low end of the energy distribution curve for the gaseous molecule, can plunge back into the liquid and be captured by intermolecular forces. This process-the opposite of vaporization-is called condensation, the phase transition from gas to liquid. The rate of vaporization increases with increasing temperature and surface area because molecules are held less tightly at the surface due to fewer neighbor interactions.

6 What factors cause transitions between the solid and liquid state? The liquid and gas state?

One phase of matter can be transformed to another by changing the temperature, pressure, or both. A liquid can be converted to a gas by heating, and a gas can be condensed into a liquid by cooling. In general, increases in pressure favor the denser phase; so increasing the pressure of a gas sample can result in a transition to the liquid phase. A solid can be converted to a liquid by heating, and a liquid can be converted to a solid by cooling. In general, increases in pressure favor the denser phase (because atoms are pushed closer together); so increasing the pressure of most liquids can result in a transition to the solid phase.

4 What are the main properties of solids (in contrast to liquids and gases)?

Solids... - have much higher densities in comparison to gases (and liquids usually) - have a definite shape - do not assume the shape of their container - have a definite volume - are not easily compressed - may be crystalline (ordered) or amorphous (disordered)

28 What is sublimation? Give a common example of sublimation.

Sublimation is the phase transition from solid to gas without going through a liquid phase. A common example of sublimation is the carbon dioxide where "dry ice" converts from a solid to a gas without going through a "wet" (or liquid) phase.

15 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 for 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; so, they're less stable and have higher potential energy than interior molecules. To increase the surface area of the liquid, some molecules from the interior must be moved to the surface, which requires 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.

26 What is the Clausius-Clapeyron equation and why is it important?

The Clausius-Clapeyron equation is the relationship between vapor pressure and temperature. It can be expressed as lnP(vap) = -∆H(vap) / R * (1/T) + lnB In this expression, P(vap) is the vapor pressure, B is a constant that depends on the gas, ∆H(vap) is the heat of evaporation, R is the gas constant (8.314 J/mol K), and T is the temperature in Kelvin. The Clausius-Clapeyron equation gives a linear relationship - not between the vapor pressure and the temperature (which have an exponential relationship), but between the natural log of the vapor pressure and the inverse of temperature. The Clausius-Clapeyron equation leads to a convenient way to measure the heat of vaporization in the laboratory or to calculate the vapor pressure of a liquid at a temperature (if the heat of vaporization and a vapor pressure at one temperature are known).

13 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 molecules containing hydrogen atoms bonded directly to fluorine, oxygen, or nitrogen exhibit an intermolecular force called hydrogen bonding. The large electronegativity difference between hydrogen and these electronegative elements means that the H atoms will have fairly large partial positive charges, while F, O, or N atoms will have fairly large partial negative charges. Because these atoms are small, they can approach one another very closely. The result is a strong attraction between the hydrogen in each of the molecules and the F, O, N on its neighbors: an attraction called a hydrogen bond.

14 What is the 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 and chloride ions interact with water molecules via ion-dipole forces. The positive sodium ions interact with the negative poles of water molecules, which the negative chloride ions interact with the positive poles. Ion-dipole forces are the STRONGEST types of intermolecular forces and are responsible for the ability of ionic substances to form solutions with water.

19 Why is vaporization endothermic? Why is condensation exothermic?

The molecules that leave the liquid (those at the surface) are highest in energy. If no additional heat enters the liquid, the average energy of the entire collection of molecules goes down. So vaporization is an endothermic process; it takes energy to overcome the intermolecular forces that hold liquids together; energy must be absorbed to pull the molecules apart. Condensation is the opposite process, so it must be exothermic. Also, gas particles have more energy than those in the liquid. The least energetic gas particles condense, adding energy to the liquid.

32 Examine the heating curve for water in Section 11.7 (Figure 11.36). Explain the significance of the slopes of each of the three rising segments. Why are the slopes different?

The slopes indicate how much heat is necessary to increase the temperature of the sample. The slope is proportional to 1/Cs (the specific heat capacity of the phase). The amount of molecular motion and the amount of intermolecular interactions are different in each phase, so the specific heat capacity and the slop are different for each phase and each substance.

24 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 the liquid increases, the vapor pressure increases more quickly. As the temperature is decreased, the vapor pressure decreases following the same relationship.

31 Examine the heating curve for water in Section 11.7 (Figure 11.36). Explain why the curve has two segments in which heat is added to the water but the temperature does not rise.

There are two horizontal lines (i.e. heat is added, but the temp stays constant) in the heating curve because there are two endothermic phase changes. The heat that is added is used to change the phase from solid to liquid or from liquid to gas and therefore there's no rise in temperature.

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

Viscosity is the resistance to flow. Viscosity is measured in a unit called poise (P), defined as 1 g/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 longer molecules that can interact over a greater area and possibly become entangled.) Viscosity increases with increasing molar mass (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.

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

Volatility is the tendency of a substance to evaporate. The weaker the intermolecular forces are, the more likely molecules will evaporate at a given temperature, making the liquid more volatile.

34 How do the properties of water differ from those of most other substances?

Water has a low molar mass (18.02 g/mol), yet it is a liquid at room temperature. Water's high boiling point for its molecular mass can be understood by examining the structure of the water molecule. The bent geometry of the water molecule and the highly polar nature of the O-H bonds result in a molecule with a significant dipole moment. Water's two O-H bonds (hydrogen directly bonded to oxygen) allow a water molecule to form strong hydrogen bonds with four other water molecules, resulting in a relatively high boiling point. Water's high polarity also allows it to dissolve many other polar and ionic compounds, and even a number of nonpolar gases such as oxygen and carbon dioxide (by inducing a dipole moment in their molecules). Water has an exceptionally high specific heat capacity. One significant difference between the phases diagram of water and that of other substances is that the fusion curve for water has a negative slope. The fusion curve within the phase diagrams for most substances has a positive slope because increasing pressure favors the denser phase, which for most substances is the solid phase. This negative slope means that ice is less dense than liquid water and so ice floats. The solid phase sinks in the liquid of most other substances.