Chapter 11

Use any one of the phase changes to explain what is meant by dynamic equilibrium.

Define the following terms: (a) molar heat of vaporization, (b) molar heat of fusion, (c) molar heat of sublimation. What are their units?

How is the molar heat of sublimation related to the molar heats of vaporization and fusion? On what law are these relationships based?

What can we learn about the intermolecular forces in a liquid from the molar heat of vaporization?

The greater the molar heat of vaporization of a liquid, the greater its vapor pressure. True or false?

As a liquid is heated at constant pressure, its temperature rises. This trend continues until the boiling point of the liquid is reached. No further rise in temperature of the liquid can be induced by heating. Explain.

What is critical temperature? What is the significance of critical temperature in liquefaction of gases?

What is the relationship between intermolecular forces in a liquid and the liquid's boiling point and critical temperature? Why is the critical temperature of water greater than that of most other substances?

How do the boiling points and melting points of water and carbon tetrachloride vary with pressure? Explain any difference in behavior of these two substances.

Why is solid carbon dioxide called dry ice?

Wet clothes dry more quickly on a hot, dry day than on a hot, humid day. Explain.

Which of the following phase transitions gives off more heat: (a) 1 mole of steam to 1 mole of water at \(100^{\circ} \mathrm{C},\) or (b) 1 mole of water to 1 mole of ice at \(0^{\circ} \mathrm{C} ?\)

A beaker of water is heated to boiling by a Bunsen burner. Would adding another burner raise the boiling point of water? Explain.

How is the rate of evaporation of a liquid affected by (a) temperature, (b) the surface area of a liquid exposed to air, (c) intermolecular forces?

The molar heats of fusion and sublimation of molecular iodine are \(15.27 \mathrm{~kJ} / \mathrm{mol}\) and \(62.30 \mathrm{~kJ} / \mathrm{mol}\), respectively. Estimate the molar heat of vaporization of liquid iodine.

The following compounds, listed with their boiling points, are liquid at \(-10^{\circ} \mathrm{C}:\) butane, \(-0.5^{\circ} \mathrm{C} ;\) ethanol, \(78.3^{\circ} \mathrm{C} ;\) toluene, \(110.6^{\circ} \mathrm{C}\). At \(-10^{\circ} \mathrm{C},\) which of these liquids would you expect to have the highest vapor pressure? Which the lowest? Explain.

Freeze-dried coffee is prepared by freezing brewed coffee and then removing the ice component with a vacuum pump. Describe the phase changes taking place during these processes.

A student hangs wet clothes outdoors on a winter day when the temperature is \(-15^{\circ} \mathrm{C}\). After a few hours, the clothes are found to be fairly dry. Describe the phase changes in this drying process.

Steam at \(100^{\circ} \mathrm{C}\) causes more serious burns than water at \(100^{\circ} \mathrm{C}\). Why?

Vapor pressure measurements at several different temperatures are shown below for mercury. Determine graphically the molar heat of vaporization for mercury. $$ \begin{array}{llllll} t\left({ }^{\circ} \mathrm{C}\right) & 200 & 250 & 300 & 320 & 340 \\ P(\mathrm{mmHg}) & 17.3 & 74.4 & 246.8 & 376.3 & 557.9 \end{array} $$

The vapor pressure of benzene, \(\mathrm{C}_{6} \mathrm{H}_{6},\) is \(40.1 \mathrm{mmHg}\) at \(7.6^{\circ} \mathrm{C}\). What is its vapor pressure at \(60.6^{\circ} \mathrm{C}\) ? The molar heat of vaporization of benzene is \(31.0 \mathrm{~kJ} / \mathrm{mol} .\)

The vapor pressure of liquid \(X\) is lower than that of liquid \(Y\) at \(20^{\circ} \mathrm{C},\) but higher at \(60^{\circ} \mathrm{C}\). What can you deduce about the relative magnitude of the molar heats of vaporization of \(\mathrm{X}\) and \(\mathrm{Y} ?\)

Explain why splashing a small amount of liquid nitrogen (b.p. \(77 \mathrm{~K}\) ) is not as harmful as splashing boiling water on your skin.

What is a phase diagram? What useful information can be obtained from the study of a phase diagram?

Explain how water's phase diagram differs from those of most substances. What property of water causes the difference?

Explain how water's phase diagram differs from those of most substances. What property of water causes the difference?

A length of wire is placed on top of a block of ice. The ends of the wire extend over the edges of the ice, and a heavy weight is attached to each end. It is found that the ice under the wire gradually melts, so that the wire slowly moves through the ice block. At the same time, the water above the wire refreezes. Explain the phase changes that accompany this phenomenon.

Name the kinds of attractive forces that must be overcome in order to (a) boil liquid ammonia, (b) melt solid phosphorus \(\left(\mathrm{P}_{4}\right),\) (c) dissolve CsI in liquid \(\mathrm{HF}\), (d) melt potassium metal.

Which of the following properties indicates very strong intermolecular forces in a liquid: (a) very low surface tension, (b) very low critical temperature, (c) very low boiling point, or (d) very low vapor pressure?

At \(-35^{\circ} \mathrm{C}\), liquid \(\mathrm{HI}\) has a higher vapor pressure than liquid HF. Explain.

Classify the unit cell of molecular iodine.

A \(\mathrm{CO}_{2}\) fire extinguisher is located on the outside of a building in Massachusetts. During the winter months, one can hear a sloshing sound when the extinguisher is gently shaken. In the summertime there is often no sound when it is shaken. Explain. Assume that the extinguisher has no leaks and that it has not been used.

A flask of water is connected to a powerful vacuum pump. When the pump is turned on, the water begins to boil. After a few minutes, the same water begins to freeze. Eventually, the ice disappears. Explain what happens at each step.

The liquid-vapor boundary line in the phase diagram of any substance always stops abruptly at a certain point. Why?

Which has a greater density, crystalline \(\mathrm{SiO}_{2}\) or amorphous \(\mathrm{SiO}_{2}\) ? Why?

The vapor pressure of a liquid in a closed container depends on which of the following: (a) the volume above the liquid, (b) the amount of liquid present, (c) temperature, (d) intermolecular forces between the molecules in the liquid?

A student is given four solid samples labeled \(W, X, Y\), and \(Z\). All except \(Z\) have a metallic luster. She is told that the solids could be gold, lead sulfide, quartz \(\left(\mathrm{SiO}_{2}\right),\) and iodine. The results of her investigations are as follows: (a) \(\mathrm{W}\) is a good electrical conductor; \(\mathrm{X}\), \(\mathrm{Y},\) and \(\mathrm{Z}\) are poor electrical conductors. (b) When the solids are hit with a hammer, W flattens out, X shatters into many pieces, \(Y\) is smashed into a powder, and \(Z\) is cracked. (c) When the solids are heated with a Bunsen burner, Y melts with some sublimation, but \(\mathrm{X}, \mathrm{W},\) and \(\mathrm{Z}\) do not melt. (d) In treatment with \(6 \mathrm{M}\) \(\mathrm{HNO}_{3}, \mathrm{X}\) dissolves; there is no effect on \(\mathrm{W}, \mathrm{Y},\) or \(\mathrm{Z}\) On the basis of these test results, identify the solids.

Which of the following statements are false? (a) Dipole-dipole interactions between molecules are greatest if the molecules possess only temporary dipole moments. (b) All compounds containing hydrogen atoms can participate in hydrogen-bond formation. (c) Dispersion forces exist between all atoms, molecules, and ions. (d) The extent of ioninduced dipole interaction depends only on the charge on the ion.

Select the substance in each pair that should have the higher boiling point. In each case identify the principal intermolecular forces involved and account briefly for your choice. (a) \(\mathrm{K}_{2} \mathrm{~S}\) or \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N},\) (b) \(\mathrm{Br}_{2}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\)

A small drop of oil in water assumes a spherical shape. Explain. (Hint: Oil is made up of nonpolar molecules, which tend to avoid contact with water.)

Under the same conditions of temperature and density, which of the following gases would you expect to behave less ideally: \(\mathrm{CH}_{4}, \mathrm{SO}_{2} ?\) Explain.

The fluorides of the second-period elements and their melting points are: \(\mathrm{LiF}, 845^{\circ} \mathrm{C} ; \mathrm{BeF}_{2}\), \(800^{\circ} \mathrm{C} ; \mathrm{BF}_{3},-126.7^{\circ} \mathrm{C} ; \mathrm{CF}_{4},-184^{\circ} \mathrm{C} ; \mathrm{NF}_{3}\) \(-206.6^{\circ} \mathrm{C} ; \quad \mathrm{OF}_{2},-223.8^{\circ} \mathrm{C} ; \quad \mathrm{F}_{2},-219.6^{\circ} \mathrm{C}\) Classify the type(s) of intermolecular forces present in each compound.

The distance between \(\mathrm{Li}^{+}\) and \(\mathrm{Cl}^{-}\) is \(257 \mathrm{pm}\) in solid \(\mathrm{LiCl}\) and \(203 \mathrm{pm}\) in a \(\mathrm{LiCl}\) unit in the gas phase. Explain the difference in the bond lengths.

Heat of hydration, that is, the heat change that occurs when ions become hydrated in solution, is largely due to ion-dipole interactions. The heats of hydration for the alkali metal ions are \(\mathrm{Li}^{+},-520 \mathrm{~kJ} /\) mol; \(\mathrm{Na}^{+},-405 \mathrm{~kJ} / \mathrm{mol} ; \mathrm{K}^{+},-321 \mathrm{~kJ} / \mathrm{mol} .\) Account for the trend in these values.

If water were a linear molecule, (a) would it still be polar, and (b) would the water molecules still be able to form hydrogen bonds with one another?

Calculate the \(\Delta H^{\circ}\) for the following processes at \(25^{\circ} \mathrm{C}\) (a) \(\mathrm{Br}_{2}(l) \longrightarrow \mathrm{Br}_{2}(g)\) and (b) \(\mathrm{Br}_{2}(g) \longrightarrow\) \(2 \mathrm{Br}(g) .\) Comment on the relative magnitudes of these \(\Delta H^{\circ}\) values in terms of the forces involved in each case. \\{Hint: See Table \(9.4,\) and given that \(\left.\Delta H_{\mathrm{f}}^{\circ}\left[\mathrm{Br}_{2}(g)\right]=30.7 \mathrm{~kJ} / \mathrm{mol} .\right\\}\)

Gaseous or highly volatile liquid anesthetics are often preferred in surgical procedures because once inhaled, these vapors can quickly enter the bloodstream through the alveoli and then enter the brain. Shown here are several common gaseous anesthetics with their boiling points. Based on intermolecular force considerations, explain the advantages of using these anesthetics. (Hint: The brain barrier is made of membranes that have a nonpolar interior region.)

A beaker of water is placed in a closed container. Predict the effect on the vapor pressure of the water when (a) its temperature is lowered, (b) the volume of the container is doubled, (c) more water is added to the beaker.

A sample of limestone \(\left(\mathrm{CaCO}_{3}\right)\) is heated in a closed vessel until it is partially decomposed. Write an equation for the reaction and state how many phases are present.

Silicon used in computer chips must have an impurity level below \(10^{-9}\) (that is, fewer than one impurity atom for every \(10^{9} \mathrm{Si}\) atoms \() .\) Silicon is prepared by the reduction of quartz \(\left(\mathrm{SiO}_{2}\right)\) with coke (a form of carbon made by the destructive distillation of coal) at about \(2000^{\circ} \mathrm{C}\) : $$\mathrm{SiO}_{2}(s)+2 \mathrm{C}(s) \longrightarrow \mathrm{Si}(I)+2 \mathrm{CO}(g)$$ Next, solid silicon is separated from other solid impurities by treatment with hydrogen chloride at \(350^{\circ} \mathrm{C}\) to form gaseous trichlorosilane \(\left(\mathrm{SiCl}_{3} \mathrm{H}\right)\) $$ \mathrm{Si}(s)+3 \mathrm{HCl}(g) \longrightarrow \mathrm{SiCl}_{3} \mathrm{H}(g)+\mathrm{H}_{2}(g) $$ Finally, ultrapure \(\mathrm{Si}\) can be obtained by reversing the above reaction at \(1000^{\circ} \mathrm{C}\) : $$ \mathrm{SiCl}_{3} \mathrm{H}(g)+\mathrm{H}_{2}(g) \longrightarrow \mathrm{Si}(s)+3 \mathrm{HCl}(g) $$ (a) Trichlorosilane has a vapor pressure of 0.258 atm at \(-2^{\circ} \mathrm{C} .\) What is its normal boiling point? Is trichlorosilane's boiling point consistent with the type of intermolecular forces that exist among its molecules? (The molar heat of vaporization of trichlorosilane is \(28.8 \mathrm{~kJ} / \mathrm{mol} .\) ) (b) What types of crystals do Si and \(\mathrm{SiO}_{2}\) form? (c) Silicon has a diamond crystal structure (see Figure 11.28 ). Each cubic unit cell (edge length \(a=543 \mathrm{pm}\) ) contains eight Si atoms. If there are \(1.0 \times 10^{13}\) boron atoms per cubic centimeter in a sample of pure silicon, how many Si atoms are there for every \(\mathrm{B}\) atom in the sample? Does this sample satisfy the \(10^{-9}\) purity requirement for the electronic grade silicon?