Chapter 12

For each of the following substances describe the importance of dispersion (London) forces, dipoledipole interactions, and hydrogen bonding: (a) \(HCl;\) (b) \(\mathrm{Br}_{2} ;\) (c) ICl; (d) \(\mathrm{HF} ;\)\ (e) \(\mathrm{CH}_{4}\)

When another atom or group of atoms is substituted for one of the hydrogen atoms in benzene, \(C_{6} H_{6},\) the boiling point changes. Explain the order of the following boiling points: \(\mathrm{C}_{6} \mathrm{H}_{6}, 80^{\circ} \mathrm{C} ; \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}, 132^{\circ} \mathrm{C}\) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}, 156^{\circ} \mathrm{C} ; \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}, 182^{\circ} \mathrm{C}\)

One of the following substances is a liquid at room temperature and the others are gaseous: \(\mathrm{CH}_{3} \mathrm{OH}\) \(\mathrm{C}_{3} \mathrm{H}_{8} ; \mathrm{N}_{2} ; \mathrm{N}_{2} \mathrm{O} .\) Which do you think is the liquid? Explain.

In which of the following compounds do you think that intramolecular hydrogen bonding is an important factor: \((\mathrm{a}) \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} ;(\mathrm{b}) \mathrm{HOOCCH}_{2} \mathrm{CH}_{2}\) \(\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH} ;(\mathrm{c}) \mathrm{CH}_{3} \mathrm{COOH} ;\) (d) ortho-phthalic acid? Explain.

How many water molecules can hydrogen bond to methanol?

What is the maximum number of hydrogen bonds that can form between two acetic acid molecules?

In DNA the nucleic acid bases form hydrogen bonds between them, which are responsible for the formation of the double-stranded helix. Arrange the bases guanine and cytosine to give the maximum number of hydrogen bonds.

Silicone oils, such as \(\mathrm{H}_{3} \mathrm{C}\left[\mathrm{SiO}\left(\mathrm{CH}_{3}\right)_{2}\right]_{\mathrm{n}} \mathrm{Si}\left(\mathrm{CH}_{3}\right),\) are used in water repellents for treating tents, hiking boots, and similar items. Explain how silicone oils function.

Surface tension, viscosity, and vapor pressure are all related to intermolecular forces. Why do surface tension and viscosity decrease with temperature, whereas vapor pressure increases with temperature?

Is there any scientific basis for the colloquial expression "slower than molasses in January"? Explain.

A television commercial claims that a product makes water "wetter" Can there be any basis to this claim? Explain.

Would you predict the surface tension of \(t\) -butyl alcohol, \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH},\) to be greater than or less than that of \(\mathrm{n}\) -butyl alcohol, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) Explain.

Butanol and pentane have approximately the same mass, however, the viscosity (at \(20^{\circ} \mathrm{C}\) ) of butanol is \(\eta=2.948 \mathrm{cP},\) and the viscosity of pentane is \(\eta=0.240 \mathrm{cP.}\) Explain this difference.

As a liquid evaporated from an open container, its temperature was observed to remain roughly constant. When the same liquid evaporated from a thermally insulated container (a vacuum bottle or Dewar flask), its temperature was observed to drop. How would you account for this difference?

Explain why vaporization occurs only at the surface of a liquid until the boiling point temperature is reached. That is, why does vapor not form throughout the liquid at all temperatures?

The enthalpy of vaporization of benzene, \(\mathrm{C}_{6} \mathrm{H}_{6}(\mathrm{l}),\) is \(33.9 \mathrm{kJmol}^{-1}\) at \(298 \mathrm{K}\). How many liters of \(\mathrm{C}_{6} \mathrm{H}_{6}(\mathrm{g})\) measured at \(298 \mathrm{K}\) and \(95.1 \mathrm{mmHg}\), are formed when \(1.54 \mathrm{kJ}\) of heat is absorbed by \(\mathrm{C}_{6} \mathrm{H}_{6}(1)\) at a constant f 298 K?

A vapor volume of 1.17 L forms when a sample of liquid acetonitrile, \(\mathrm{CH}_{3} \mathrm{CN},\) absorbs \(1.00 \mathrm{kJ}\) of heat at its normal boiling point \(\left(81.6^{\circ} \mathrm{C} \text { and } 1 \mathrm{atm}\right) .\) What is \(\Delta H_{\text {vap }}\) in kilojoules per mole of \(\mathrm{CH}_{3} \mathrm{CN} ?\)

How many liters of \(\mathrm{CH}_{4}(\mathrm{g}),\) measured at \(23.4^{\circ} \mathrm{C}\) and \(768 \mathrm{mmHg},\) must be burned to provide the heat needed to vaporize 3.78 L of water at \(100^{\circ} \mathrm{C}\) ? \(\Delta \mathrm{H}_{\text {combustion }}=\) \(-8.90 \times 10^{2} \mathrm{kJmol}^{-1} \mathrm{CH}_{4} \quad\) For \(\quad \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \quad\) at \(\quad 100^{\circ} \mathrm{C}\) \(d=0.958 \mathrm{g} \mathrm{cm}^{-3},\) and \(\Delta H_{\mathrm{vap}}=40.7 \mathrm{kJmol}^{-1}\)

Equilibrium is established between \(\mathrm{Br}_{2}(\mathrm{l})\) and \(\mathrm{Br}_{2}(\mathrm{g})\) at \(25.0^{\circ} \mathrm{C} .\) A \(250.0 \mathrm{mL}\) sample of the vapor weighs 0.486 g. What is the vapor pressure of bromine at \(25.0^{\circ} \mathrm{C},\) in millimeters of mercury?

A double boiler is used when a careful control of temperature is required in cooking. Water is boiled in an outside container to produce steam, and the steam condenses on the outside walls of an inner container in which cooking occurs. (A related laboratory device is called a steam bath.) (a) How is heat energy conveyed to the food to be cooked in a double boiler? (b) What is the maximum temperature that can be reached in the inside container?

One popular demonstration in chemistry labs is performed by boiling a small quantity of water in a metal can (such as a used soda can), picking up the can with tongs and quickly submerging it upside down in cold water. The can collapses with a loud and satisfying pop. Give an explanation of this crushing of the can. (Note: If you try this demonstration, do not heat the can over an open flame.)

A 25.0 L volume of \(\mathrm{He}(\mathrm{g})\) at \(30.0^{\circ} \mathrm{C}\) is passed through \(6.220 \mathrm{g}\) of liquid aniline \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\right)\) at \(30.0^{\circ} \mathrm{C} .\) The liquid remaining after the experiment weighs \(6.108 \mathrm{g}\) Assume that the He(g) becomes saturated with aniline vapor and that the total gas volume and temperature remain constant. What is the vapor pressure of aniline at \(30.0^{\circ} \mathrm{C} ?\)

A 7.53 I. sample of \(\mathrm{N}_{2}(\mathrm{g})\) at \(742 \mathrm{mmHg}\) and \(45.0^{\circ} \mathrm{C}\) is bubbled through \(\mathrm{CCl}_{4}(1)\) at \(45.0^{\circ} \mathrm{C} .\) Assuming the gas becomes saturated with \(\mathrm{CCl}_{4}(\mathrm{g}),\) what is the volume of the resulting gaseous mixture, if the total pressure remains at \(742 \mathrm{mm} \mathrm{Hg}\) and the temperature remains at \(45^{\circ} \mathrm{C} ?\) The vapor pressure of \(\mathrm{CCl}_{4}\) at \(45^{\circ} \mathrm{C}\) is \(261 \mathrm{mmHg}\)

Some vapor pressure data for Freon- \(12, \mathrm{CCl}_{2} \mathrm{F}_{2}\), once a common refrigerant, are \(-12.2^{\circ} \mathrm{C}, 2.0 \mathrm{atm} ; 16.1^{\circ} \mathrm{C}\) 5.0 atm; \(42.4^{\circ} \mathrm{C}, 10.0\) atm; \(74.0^{\circ} \mathrm{C}, 20.0\) atm. Also, \(\mathrm{bp}=-29.8^{\circ} \mathrm{C}, \quad T_{\mathrm{c}}=111.5^{\circ} \mathrm{C}, \quad P_{\mathrm{c}}=39.6 \mathrm{atm} .\) Use these data to plot the vapor pressure curve of Freon-12 What approximate pressure would be required in the compressor of a refrigeration system to convert Freon- 12 vapor to liquid at \(25.0^{\circ} \mathrm{C}\) ?

Cyclohexanol has a vapor pressure of \(10.0 \mathrm{mmHg}\) at \(56.0^{\circ} \mathrm{C}\) and \(100.0 \mathrm{mmHg}\) at \(103.7^{\circ} \mathrm{C} .\) Calculate its enthalpy of vaporization, \(\Delta H_{\mathrm{vap}}\)

The normal boiling point of acetone, an important laboratory and industrial solvent, is \(56.2^{\circ} \mathrm{C}\) and its \(\Delta H_{\text {vap }}\) is \(25.5 \mathrm{kJmol}^{-1} .\) At what temperature does acetone have a vapor pressure of \(375 \mathrm{mmHg} ?\)

The vapor pressure of trichloromethane (chloroform) is 40.0 Torr at \(-7.1^{\circ} \mathrm{C}\). Its enthalpy of vaporization is \(29.2 \mathrm{kJ} \mathrm{mol}^{-1} .\) Calculate its normal boiling point.

Can \(\mathrm{SO}_{2}\) be maintained as a liquid under a pressure of 100 atm at \(0^{\circ} \mathrm{C} ?\) Can liquid methane be obtained under the same conditions?

The normal melting point of copper is \(1357 \mathrm{K}\), and \(\Delta \mathrm{H}_{\text {fus }}\) of \(\mathrm{Cu}\) is \(13.05 \mathrm{kJ} \mathrm{mol}^{-1}\). (a) How much heat, in kilojoules, is evolved when a \(3.78 \mathrm{kg}\) sample of molten Cu freezes? (b) How much heat, in kilojoules, must be absorbed at 1357 K to melt a bar of copper that is \(75 \mathrm{cm} \times\) \(15 \mathrm{cm} \times 12 \mathrm{cm} ?\) (Assume \(d=8.92 \mathrm{g} / \mathrm{cm}^{3}\) for \(\mathrm{Cu}\).)

Sketch a plausible phase diagram for hydrazine \(\left(\mathrm{N}_{2} \mathrm{H}_{4}\right)\) from the following data: triple point \(\left(2.0^{\circ} \mathrm{C}\right)\) and \(3.4 \mathrm{mm} \mathrm{Hg}\) ), the normal melting point \(\left(2^{\circ} \mathrm{C}\right),\) the normal boiling point \(\left(113.5^{\circ} \mathrm{C}\right),\) and the critical point \(\left(380^{\circ} \mathrm{C} \text { and } 145 \mathrm{atm}\right) .\) The density of the liquid is less than that of the solid. Label significant data points on this diagram. Are there any features of the diagram that remain uncertain? Explain.

A \(2.50 \mathrm{g}\) sample of \(\mathrm{H}_{2} \mathrm{O}(\mathrm{l})\) is sealed in a \(5.00 \mathrm{L}\) flask at \(120.0^{\circ} \mathrm{C}\) (a) Show that the sample exists completely as vapor. (b) Estimate the temperature to which the flask must be cooled before liquid water condenses.

Trace the phase changes that occur as a sample of \(\mathrm{H}_{2} \mathrm{O}(\mathrm{g}),\) originally at \(1.00 \mathrm{mmHg}\) and \(-0.10^{\circ} \mathrm{C},\) is compressed at constant temperature until the pressure reaches 100 atm.

You decide to cool a can of soda pop quickly in the freezer compartment of a refrigerator. When you take out the can, the soda pop is still liquid; but when you open the can, the soda pop immediately freezes. Explain why this happens.

Why is the triple point of water (ice-liquid-vapor) a better fixed point for establishing a thermometric scale than either the melting point of ice or the boiling point of water?

Diamond is often used as a cutting medium in glass cutters. What property of diamond makes this possible? Could graphite function as well?

Are the fullerenes network covalent solids? What makes them different from diamond and graphite? It has been shown that carbon can form chains in which every other carbon atom is bonded to the next carbon atom by a triple bond. Is this allotrope of carbon a network covalent solid? Explain.

Explain why there are \(t w o\) arrangements for the closest packing of spheres rather than a single one.

Argon, copper, sodium chloride, and carbon dioxide all crystallize in the fcc structure. How can this be when their physical properties are so different?

Germanium has a cubic unit cell with a side edge of \(565 \mathrm{pm} .\) The density of germanium is \(5.36 \mathrm{g} / \mathrm{cm}^{3}\) What is the crystal system adopted by germanium?

Silicon tetrafluoride molecules are arranged in a body-centered cubic unit cell. How many silicon atoms are in the unit cell?

Potassium chloride has the same crystal structure as NaCl. Careful measurement of the internuclear distance between \(K^{+}\) and \(C l^{-}\) ions gave a value of 314.54 pm. The density of \(\mathrm{KCl}\) is \(1.9893 \mathrm{g} / \mathrm{cm}^{3}\). Use these data to evaluate the Avogadro constant, \(N_{\mathrm{A}}\)

Without doing calculations, indicate how you would expect the lattice energies of \(\mathrm{LiCl}(\mathrm{s}), \mathrm{KCl}(\mathrm{s}), \mathrm{RbCl}(\mathrm{s}),\) and \(\mathrm{CsCl}(\mathrm{s})\) to compare with the value of \(-787 \mathrm{kJmol}^{-1}\) determined for \(\mathrm{NaCl}(\mathrm{s})\) on page \(543 .\) [ Hint: Assume that the enthalpies of sublimation of the alkali metals are comparable in value. What atomic properties from Chapter 9 should you compare?

Determine the lattice energy of \(\mathrm{KF}(\mathrm{s})\) from the following data: \(\Delta \mathrm{H}_{\mathrm{f}}[\mathrm{KF}(\mathrm{s})]=-567.3 \mathrm{kJ} \mathrm{mol}^{-1} ;\) enthalpy of sub- limation of \(\mathrm{K}(\mathrm{s}), 89.24 \mathrm{kJ} \mathrm{mol}^{-1} ;\) enthalpy of dissociation of \(\mathrm{F}_{2}(\mathrm{g}), 159 \mathrm{kJ} \mathrm{mol}^{-1} \mathrm{F}_{2} ; I_{1}\) for \(\mathrm{K}(\mathrm{g}), 418.9 \mathrm{kJmol}^{-1}\) EA for \(\mathrm{F}(\mathrm{g}),-328 \mathrm{kJ} \mathrm{mol}^{-1}\)

In ionic compounds with certain metals, hydrogen exists as the hydride ion, \(\mathrm{H}^{-}\). Determine the electron affinity of hydrogen; that is, \(\Delta H\) for the process \(\mathrm{H}(\mathrm{g})+e^{-} \rightarrow \mathrm{H}^{-}(\mathrm{g}) .\) To do so, use data from Section \(12-7 ;\) the bond energy of \(\mathrm{H}_{2}(\mathrm{g})\) from table 10.3 \(-812 \mathrm{kJmol}^{-1} \mathrm{NaH}\) for the lattice energy of \(\mathrm{NaH}(\mathrm{s})\) and \(-57 \mathrm{kJmol}^{-1}\) NaH for the enthalpy of formation of \(\mathrm{NaH}(\mathrm{s})\)

When a wax candle is burned, the fuel consists of gaseous hydrocarbons appearing at the end of the candle wick. Describe the phase changes and processes by which the solid wax is ultimately consumed.

A supplier of cylinder gases warns customers to determine how much gas remains in a cylinder by weighing the cylinder and comparing this mass to the original mass of the full cylinder. In particular, the customer is told not to try to estimate the mass of gas available from the measured gas pressure. Explain the basis of this warning. Are there cases where a measurement of the gas pressure can be used as a measure of the remaining available gas? If so, what are they?

Because solid \(p\) -dichlorobenzene, \(\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{Cl}_{2},\) sublimes rather easily, it has been used as a moth repellent. From the data given, estimate the sublimation pressure of \(\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{Cl}_{2}(\mathrm{s})\) at \(25^{\circ} \mathrm{C} .\) For \(\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{Cl}_{2} ; \mathrm{mp}=\) \(53.1^{\circ} \mathrm{C} ;\) vapor pressure of \(\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{Cl}_{2}(1)\) at \(54.8^{\circ} \mathrm{C}\) is \(10.0 \mathrm{mmHg} ; \Delta H_{\text {fus }}=17.88 \mathrm{kJ} \mathrm{mol}^{-1} ; \Delta H_{\text {vap }}=\) \(72.22 \mathrm{k}] \mathrm{mol}^{-1}\)

One handbook lists the sublimation pressure of solid benzene as a function of Kelvin temperature, \(T\), as \(\log \mathrm{P}(\mathrm{mmHg})=9.846-2309 / \mathrm{T} .\) Another hand- book lists the vapor pressure of liquid benzene as a function of Celsius temperature, \(t,\) as \(\log P(\mathrm{mmHg})=\) \(6.90565-1211.033 /(220.790+t) .\) Use these equations to estimate the normal melting point of benzene, and compare your result with the listed value of \(5.5^{\circ} \mathrm{C}\)

Estimate the boiling point of water in Leadville, Colorado, elevation 3170 m. To do this, use the barometric formula relating pressure and altitude: \(P=P_{0} \times 10^{-\mathrm{Mgh} / 2.303 \mathrm{RT}} \quad(\text { where } P=\text { pressure } \mathrm{in}\) atm; \(P_{0}=1 \mathrm{atm} ; g=\) acceleration due to gravity; molar mass of air, \(M=0.02896 \mathrm{kg} \mathrm{mol}^{-1} ; \quad R=\) \(8.3145 \mathrm{Jmol}^{-1} \mathrm{K}^{-1} ;\) and \(T\) is the Kelvin temperature). Assume the air temperature is \(10.0^{\circ} \mathrm{C}\) and that \(\Delta H_{\text {vap }}=41 \mathrm{kJ} \mathrm{mol}^{-1} \mathrm{H}_{2} \mathrm{O}\)

A cylinder containing 151 lb \(\mathrm{Cl}_{2}\) has an inside diameter of 10 in. and a height of 45 in. The gas pressure is 100 psi \((1 \mathrm{atm}=14.7 \mathrm{psi})\) at \(20^{\circ} \mathrm{C} . \mathrm{Cl}_{2}\) melts at \(-103^{\circ} \mathrm{C},\) boils at \(-35^{\circ} \mathrm{C},\) and has its critical point at \(144^{\circ} \mathrm{C}\) and \(76 \mathrm{atm} .\) In what state \((\mathrm{s})\) of matter does the \(\mathrm{Cl}_{2}\) exist in the cylinder?