Chapter 9

It is possible to balance a paper clip on the surface of water in a beaker. If you add a bit of soap to the water, however, the paper clip sinks. Explain how the paper clip can float and why it sinks when soap is added.

Consider a sealed container half-filled with water. Which statement best describes what occurs in the container? a. Water evaporates until the air is saturated with water vapor; at this point, no more water evaporates. b. Water evaporates until the air is overly saturated (supersaturated) with water, and most of this water recondenses; this cycle continues until a certain amount of water vapor is present, and then the cycle ceases. c. Water does not evaporate because the container is sealed. d. Water evaporates, and then water evaporates and recondenses simultaneously and continuously. e. Water evaporates until it is eventually all in vapor form. Explain each choice. Justify your choice, and for choices you did not pick, explain what is wrong with them.

Explain the following: You add \(100 \mathrm{mL}\) water to a \(500-\mathrm{mL}\) round-bottom flask and heat the water until it is boiling. You remove the heat and stopper the flask, and the boiling stops. You then run cool water over the neck of the flask, and the boiling begins again. It seems as though you are boiling water by cooling it.

Is it possible for the dispersion forces in a particular substance to be stronger than the hydrogen bonding forces in another substance? Explain your answer.

Does the nature of intermolecular forces change when a substance goes from a solid to a liquid, or from a liquid to a gas? What causes a substance to undergo a phase change?

Why do liquids have a vapor pressure? Do all liquids have vapor pressures? Explain. Do solids exhibit vapor pressure? Explain. How does vapor pressure change with changing temperature? Explain.

Water in an open beaker evaporates over time. As the water is evaporating, is the vapor pressure increasing, decreasing, or staying the same? Why?

What is the vapor pressure of water at \(100^{\circ} \mathrm{C} ?\) How do you know?

Which are stronger, intermolecular or intramolecular forces for a given molecule? What observation(s) have you made that support this? Explain.

Why does water evaporate?

Rationalize why chalk (calcium carbonate) has a higher melting point than motor oil (large compound made from carbon and hydrogen), which has a higher melting point than water and engages in relatively strong hydrogen bonding interactions.

Hydrogen bonding is a special case of very strong dipoledipole interactions possible among only certain atoms. What atoms in addition to hydrogen are necessary for hydrogen bonding? How does the small size of the hydrogen atom contribute to the unusual strength of the dipole-dipole forces involved in hydrogen bonding?

Atoms are assumed to touch in closest packed structures, yet every closest packed unit cell contains a significant amount of empty space. Why?

Define critical temperature and critical pressure. In terms of the kinetic molecular theory, why is it impossible for a substance to exist as a liquid above its critical temperature?

Use the kinetic molecular theory to explain why a liquid gets cooler as it evaporates from an insulated container.

Will a crystalline solid or an amorphous solid give a simpler X-ray diffraction pattern? Why?

What is an alloy? Explain the differences in structure between substitutional and interstitial alloys. Give an example of each type.

Describe what is meant by a dynamic equilibrium in terms of the vapor pressure of a liquid.

How does each of the following affect the rate of evaporation of a liquid in an open dish? a. intermolecular forces b. temperature c. surface area

A common response to hearing that the temperature in New Mexico is \(105^{\circ} \mathrm{F}\) is, "It's not that bad; it's a dry heat," whereas at the same time the summers in Atlanta, Georgia, are characterized as "dreadful," even though the air temperature is typically lower. What role does humidity play in how our bodies regulate temperature?

Why is a burn from steam typically much more severe than a burn from boiling water?

When wet laundry is hung on a clothesline on a cold winter day, it will freeze but eventually dry. Explain.

Cake mixes and other packaged foods that require cooking often contain special directions for use at high elevations. Typically these directions indicate that the food should be cooked longer above 5000 ft. Explain why it takes longer to cook something at higher elevations.

You have three covalent compounds with three very different boiling points. All of the compounds have similar molar mass and relative shape. Explain how these three compounds could have very different boiling points.

Compare and contrast the structures of the following solids. a. diamond versus graphite b. silica versus silicates versus glass

Compare and contrast the structures of the following solids. a. \(\mathrm{CO}_{2}(s)\) versus \(\mathrm{H}_{2} \mathrm{O}(s)\) b. \(\mathrm{NaCl}(s)\) versus \(\mathrm{CsCl}(s) ;\) see Exercise 69 for the structures.

How could you tell experimentally if \(\operatorname{Ti} \mathrm{O}_{2}\) is an ionic solid or a network solid?

A common prank on college campuses is to switch the salt and sugar on dining hall tables, which is usually easy because the substances look so much alike. Yet, despite the similarity in their appearance, these two substances differ greatly in their properties, since one is a molecular solid and the other is an ionic solid. How do the properties differ and why?

A plot of \(\ln \left(P_{\text {vap }}\right)\) versus \(1 / T(\mathrm{K})\) is linear with a negative slope. Why is this the case?

Iodine, like most substances, exhibits only three phases: solid, liquid, and vapor. The triple point of iodine is at 90 torr and \(115^{\circ} \mathrm{C} .\) Which of the following statements concerning liquid \(\mathrm{I}_{2}\) must be true? Explain your answer. a. \(\mathrm{I}_{2}(l)\) is more dense than \(\mathrm{I}_{2}(g).\) b. \(\mathrm{I}_{2}(l)\) cannot exist above \(115^{\circ} \mathrm{C}.\) c. \(\mathrm{I}_{2}(l)\) cannot exist at 1 atmosphere pressure. d. \(\mathrm{I}_{2}(l)\) cannot have a vapor pressure greater than 90 torr. e. \(\mathrm{I}_{2}(l)\) cannot exist at a pressure of 10 torr.

Identify the most important types of interparticle forces present in the solids of each of the following substances. a. \(\mathrm{Ar}\) b. HCI c. HF d. \(\mathrm{CaCl}_{2}\) \(\mathbf{e} . \mathrm{CH}_{4}\) f. co g. \(\mathrm{NaNO}_{3}\)

Identify the most important types of interparticle forces present in the solids of each of the following substances. a. \(\mathrm{BaSO}_{4}\) b. \(\mathrm{H}_{2} \mathrm{S}\) c. Xe d. \(C_{2} H_{6}\) $$\begin{array}{l} \text { e. } \mathrm{CsI} \\ \text { f. } \mathrm{P}_{4} \\ \text { g. } \mathrm{NH}_{3} \end{array}$$

Predict which substance in each of the following pairs would have the greater intermolecular forces. a. \(\mathrm{CO}_{2}\) or \(\mathrm{OCS}\) b. \(\operatorname{SeO}_{2}\) or \(\mathrm{SO}_{2}\) \(\mathbf{c .} \cdot \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) or \(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) d. \(\mathrm{CH}_{3} \mathrm{CH}_{3}\) or \(\mathrm{H}_{2} \mathrm{CO}\) e. \(\mathrm{CH}_{3} \mathrm{OH}\) or \(\mathrm{H}_{2} \mathrm{CO}\)

In each of the following groups of substances, pick the one that has the given property. Justify your answer. a. highest boiling point: HBr, \(\mathrm{Kr},\) or \(\mathrm{Cl}_{2}\) b. highest freezing point: \(\mathrm{H}_{2} \mathrm{O}, \mathrm{NaCl},\) or \(\mathrm{HF}\) c. lowest vapor pressure at \(25^{\circ} \mathrm{C}: \mathrm{Cl}_{2}, \mathrm{Br}_{2},\) or \(\mathrm{I}_{2}\) d. lowest freezing point: \(\mathrm{N}_{2}, \mathrm{CO},\) or \(\mathrm{CO}_{2}\) e. lowest boiling point: \(\mathrm{CH}_{4}, \mathrm{CH}_{3} \mathrm{CH}_{3},\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) f. highest boiling point: HF, HCl, or HBr g.(EQUATION CAN'T COPY)

In each of the following groups of substances, pick the one that has the given property. Justify each answer. a. highest boiling point: \(\mathrm{CCl}_{4}, \mathrm{CF}_{4}, \mathrm{CBr}_{4}\) b. lowest freezing point: \(\mathrm{LiF}, \mathrm{F}_{2}, \mathrm{HCl}\) c. smallest vapor pressure at \(25^{\circ} \mathrm{C}: \mathrm{CH}_{3} \mathrm{OCH}_{3}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) d. greatest viscosity: \(\mathrm{H}_{2} \mathrm{S}, \mathrm{HF}, \mathrm{H}_{2} \mathrm{O}_{2}\) e. greatest heat of vaporization: \(\mathrm{H}_{2} \mathrm{CO}, \mathrm{CH}_{3} \mathrm{CH}_{3}, \mathrm{CH}_{4}\) f. smallest enthalpy of fusion: \(\mathrm{I}_{2}, \mathrm{CsBr}, \mathrm{CaO}\)

Explain why water forms into beads on a waxed car finish.

Hydrogen peroxide \(\left(\mathrm{H}_{2} \mathrm{O}_{2}\right)\) is a syrupy liquid with a relatively low vapor pressure and a normal boiling point of \(152.2^{\circ} \mathrm{C}\). Rationalize the differences of these physical properties from those of water.

X rays from a copper X-ray tube \((\lambda=154 \mathrm{pm})\) were diffracted at an angle of 14.22 degrees by a crystal of silicon. Assuming first-order diffraction \((n=1\) in the Bragg equation), what is the interplanar spacing in silicon?

The second-order diffraction \((n=2)\) for a gold crystal is at an angle of \(22.20^{\circ}\) for \(\mathrm{X}\) rays of \(154 \mathrm{pm}\). What is the spacing between these crystal planes?

A topaz crystal has an interplanar spacing \((d)\) of \(1.36 Å\) \(\left(1Å =1 \times 10^{-10} \mathrm{m}\right) .\) Calculate the wavelength of the X ray that should be used if \(\theta=15.0^{\circ}\) (assume \(n=1\) ).

X rays of wavelength 2.63 Å were used to analyze a crystal. The angle of first-order diffraction \((n=1\) in the Bragg equation) was 15.55 degrees. What is the spacing between crystal planes, and what would be the angle for second- order diffraction \((n=2) ?\)

Calcium has a cubic closest packed structure as a solid. Assuming that calcium has an atomic radius of \(197 \mathrm{pm},\) calculate the density of solid calcium.

Nickel has a face-centered cubic unit cell. The density of nickel is \(6.84 \mathrm{g} / \mathrm{cm}^{3} .\) Calculate a value for the atomic radius of nickel.

A certain form of lead has a cubic closest packed structure with an edge length of \(492 \mathrm{pm} .\) Calculate the value of the atomic radius and the density of lead.

You are given a small bar of an unknown metal X. You find the density of the metal to be \(10.5 \mathrm{g} / \mathrm{cm}^{3} .\) An X-ray diffraction experiment measures the edge of the face-centered cubic unit cell as \(4.09 Å\left(1 Å=10^{-10} \mathrm{m}\right) .\) Identify X.

Titanium metal has a body-centered cubic unit cell. The density of titanium is \(4.50 \mathrm{g} / \mathrm{cm}^{3} .\) Calculate the edge length of the unit cell and a value for the atomic radius of titanium. (Hint: In a body-centered arrangement of spheres, the spheres touch across the body diagonal.)

Barium has a body-centered cubic structure. If the atomic radius of barium is \(222 \mathrm{pm},\) calculate the density of solid barium.

The radius of gold is \(144 \mathrm{pm},\) and the density is \(19.32 \mathrm{g} / \mathrm{cm}^{3}\) Does elemental gold have a face-centered cubic structure or a body-centered cubic structure?

What fraction of the total volume of a cubic closest packed structure is occupied by atoms? (Hint: \(V_{\text {sphere }}=\frac{4}{3} \pi r^{3} .\) ) What fraction of the total volume of a simple cubic structure is occupied by atoms? Compare the answers.

Iron has a density of \(7.86 \mathrm{g} / \mathrm{cm}^{3}\) and crystallizes in a bodycentered cubic lattice. Show that only \(68 \%\) of a body-centered lattice is actually occupied by atoms, and determine the atomic radius of iron.