Chapter 11

For each of the following pairs of substances, predict which will have the higher melting point and indicate why: (a) \(\mathrm{Ar}, \mathrm{Xe} ;\) (b) \(\mathrm{SiO}_{2}, \mathrm{CO}_{2} ;\) (c) \(\mathrm{KBr}, \mathrm{Br}_{2}:\) (d) \(\mathrm{C}_{6} \mathrm{Cl}_{6}, \mathrm{C}_{6} \mathrm{H}_{6}\)

For each of the following pairs of substances, predict which will have the higher melting point, and indicate why: (a) HF, \(\mathrm{HCl} ;\) (b) C (graphite), \(\mathrm{CH}_{4}\); (c) \(\mathrm{KCl}, \mathrm{Cl}_{2}\); (d) \(\mathrm{LiF}, \mathrm{MgF}_{2}\).

As the intermolecular attractive forces between molecules increase in magnitude, do you expect each of the following to increase or decrease in magnitude? (a) vapor pressure, (b) heat of vaporization, (c) boiling point, (d) freezing point, (e) viscosity, (f) surface tension, (g) critical temperature.

Suppose you have two colorless molecular liquids, one boiling at \(-84{ }^{\circ} \mathrm{C}\), the other at \(34{ }^{\circ} \mathrm{C}\), and both at atmospheric pressure. Which of the following statements is correct? For those that are not correct, modify the statement so that it is correct. (a) The higher-boiling liquid has greater total intermolecular forces than the other. (b) The lower boiling liquid must consist of nonpolar molecules. (c) The lower-boiling liquid has a lower molecular weight than the higher-boiling liquid. (d) The two liquids have identical vapor pressures at their normal boiling points. (e) At \(34^{\circ} \mathrm{C}\) both liquids have vapor pressures of \(760 \mathrm{~mm} \mathrm{Hg}\).

In dichloromethane, \(\mathrm{CH}_{2} \mathrm{Cl}_{2}(\mu=1.60 \mathrm{D})\), the dispersion force contribution to the intermolecular attractive forces is about five times larger than the dipole-dipole contribution. Would you expect the relative importance of the two kinds of intermolecular attractive forces to differ (a) in dibromomethane \((\mu=1.43 \mathrm{D})\), (b) in difluoromethane \((\mu=1.93 \mathrm{D}) ?\) Explain.

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

Ethylene glycol \(\left[\mathrm{CH}_{2}(\mathrm{OH}) \mathrm{CH}_{2}(\mathrm{OH})\right]\) is the major component of antifreeze. It is a slightly viscous liquid, not very volatile at room temperature, with a boiling point of \(198^{\circ} \mathrm{C}\). Pentane \(\left(\mathrm{C}_{5} \mathrm{H}_{12}\right)\), which has about the same molecular weight, is a nonviscous liquid that is highly volatile at room temperature and whose boiling point is \(36.1{ }^{\circ} \mathrm{C}\). Explain the differences in the physical properties of the two substances.

Using the following list of normal boiling points for a series of hydrocarbons, estimate the normal boiling point for octane, \(\mathrm{C}_{8} \mathrm{H}_{18}\) : propane \(\left(\mathrm{C}_{3} \mathrm{H}_{8},-42.1{ }^{\circ} \mathrm{C}\right)\), bu- tane \(\left(\mathrm{C}_{4} \mathrm{H}_{10},-0.5^{\circ} \mathrm{C}\right)\), pentane \(\left(\mathrm{C}_{5} \mathrm{H}_{12}, 36.1^{\circ} \mathrm{C}\right)\), hexane \(\left(\mathrm{C}_{6} \mathrm{H}_{14}, 68.7^{\circ} \mathrm{C}\right)\), heptane \(\left(\mathrm{C}_{7} \mathrm{H}_{16}, 98.4{ }^{\circ} \mathrm{C}\right) .\) Explain the trend in the boiling points.

(a) When you exercise vigorously, you sweat. How does this help your body cool? (b) A flask of water is connected to a vacuum pump. A few moments after the pump is turned on, the water begins to boil. After a few minutes, the water begins to freeze. Explain why these processes occur.

$$ \begin{aligned} &\begin{aligned} &\text { The following table gives the vapor pressure of hexaflu- } \\ &\text { orobenzene }\left(\mathrm{C}_{6} \mathrm{~F}_{6}\right) \text { as a function of temperature: } \end{aligned}\\\ &\begin{aligned} &4 \end{aligned}\\\ &\begin{array}{lc} \hline \text { Temperature (K) } & \text { Vapor Pressure (torr) } \\ \hline 280.0 & 32.42 \\ 300.0 & 9247 \\ 320.0 & 225.1 \\ 330.0 & 334.4 \\ 340.0 & 482.9 \\ \hline \end{array} \end{aligned} $$ (a) By plotting these data in a suitable fashion, determine whether the Clausius-Clapeyron equation is obeyed. If it is obeyed, use your plot to determine \(\Delta H_{\text {vap }}\) for \(C_{6} F_{6}\) (b) Use these data to determine the boiling point of the compound.

Suppose the vapor pressure of a substance is measured at two different temperatures. (a) By using the ClausiusClapeyron equation, Equation \(11.1\), derive the following relationship between the vapor pressures, \(P_{1}\) and \(P_{2}\), and the absolute temperatures at which they were measured, \(T_{1}\) and \(T_{2}\) $$ \ln \frac{P_{1}}{P_{2}}=-\frac{\Delta H_{\mathrm{vap}}}{R}\left(\frac{1}{T_{1}}-\frac{1}{T_{2}}\right) $$ (b) Gasoline is a mixture of hydrocarbons, a major component of which is octane, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2}\) \(\mathrm{CH}_{2} \mathrm{CH}_{3} .\) Octane has a vapor pressure of \(13.95\) torr at \(25{ }^{\circ} \mathrm{C}\) and a vapor pressure of \(144.78\) torr at \(75^{\circ} \mathrm{C}\). Use these data and the equation in part (a) to calculate the heat of vaporization of octane. (c) By using the equation in part (a) and the data given in part (b), calculate the normal boiling point of octane. Compare your answer to the one you obtained from Exercise \(11.86 .\) (d) Calculate the vapor pressure of octane at \(-30^{\circ} \mathrm{C}\).

The following data present the temperatures at which certain vapor pressures are achieved for dichloromethane \(\left(\mathrm{CH}_{2} \mathrm{Cl}_{2}\right)\) and methyl iodide \(\left(\mathrm{CH}_{3} \mathrm{I}\right)\) : $$ \begin{array}{lcccc} \hline \begin{array}{l} \text { Vapor Pressure } \\ \text { (torr): } \end{array} & \mathbf{1 0 . 0} & \mathbf{4 0 . 0} & \mathbf{1 0 0 . 0} & \mathbf{4 0 0 . 0} \\ \hline T \text { for } \mathrm{CH}_{2} \mathrm{Cl}_{2}\left({ }^{\circ} \mathrm{C}\right): & -43.3 & -22.3 & -6.3 & 24.1 \\ T \text { for } \mathrm{CH}_{3} \mathrm{I}\left({ }^{\circ} \mathrm{C}\right): & -45.8 & -24.2 & -7.0 & 25.3 \end{array} $$ (a) Which of the two substances is expected to have the greater dipole-dipole forces? Which is expected to have the greater London dispersion forces? Based on your answers, explain why it is difficult to predict which compound would be more volatile. (b) Which compound would you expect to have the higher boiling point? Check your answer in a reference book such as the \(C R C\) Handbook of Chemistry and Physics. (c) The order of volatility of these two substances changes as the temperature is increased. What quantity must be different for the two substances in order for this phenomenon to occur? (d) Substantiate your answer for part (c) by drawing an appropriate graph.

The elements xenon and gold both have solid-state structures with face- centered cubic unit cells, yet Xe melts at \(-112^{\circ} \mathrm{C}\) and gold melts at \(1064^{\circ} \mathrm{C}\). Account for these greatly different melting points.

(a) Explain why X-rays can be used to measure atomic distances in crystals but visible light cannot. (b) Why can't \(\mathrm{CaCl}_{2}\) have the same crystal structure as \(\mathrm{NaCl} ?\)

Acetone, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CO}\), is widely used as an industrial solvent. (a) Draw the Lewis structure for the acetone molecule, and predict the geometry around each carbon atom. (b) Is the acetone molecule polar or nonpolar? (c) What kinds of intermolecular attractive forces exist between acetone molecules? (d) 1-Propanol, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\), has a molecular weight that is very similar to that of acetone, yet acetone boils at \(56.5^{\circ} \mathrm{C}\) and 1 -propanol boils at \(97.2^{\circ} \mathrm{C}\). Explain the difference.

The table shown here lists the molar heats of vaporization for several organic compounds. Use specific examples from this list to illustrate how the heat of vaporization varies with (a) molar mass, (b) molecular shape, (c) molecular polarity, (d) hydrogen-bonding interactions. Explain these comparisons in terms of the nature of the intermolecular forces at work. (You may find it helpful to draw out the structural formula for each compound.) $$ \begin{array}{ll} \hline \text { Compound } & \begin{array}{l} \text { Heat of } \\ \text { Vaporization (kJ/mol) } \end{array} \\ \hline \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3} & 19.0 \\ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} & 27.6 \\ \mathrm{CH}_{3} \mathrm{CHBrCH}_{3} & 31.8 \\ \mathrm{CH}_{3} \mathrm{COCH}_{3} & 32.0 \\ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br} & 33.6 \\ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} & 47.3 \\ \hline \end{array} $$

Liquid butane, \(\mathrm{C}_{4} \mathrm{H}_{10}\), is stored in cylinders, to be used as a fuel. The normal boiling point of butane is listed as \(-0.5^{\circ} \mathrm{C}\). (a) Suppose the tank is standing in the sun and reaches a temperature of \(35^{\circ} \mathrm{C}\). Would you expect the pressure in the tank to be greater or less than atmospheric pressure? How does the pressure within the tank depend on how much liquid butane is in it? (b) Suppose the valve to the tank is opened and a few liters of butane are allowed to escape rapidly. What do you expect would happen to the temperature of the remaining liquid butane in the tank? Explain. (c) How much heat must be added to vaporize \(250 \mathrm{~g}\) of butane if its heat of vaporization is \(21.3 \mathrm{~kJ} / \mathrm{mol}\) ? What volume does this much butane occupy at 755 torr and \(35^{\circ} \mathrm{C}\) ?

In a certain type of nuclear reactor, liquid sodium metal is employed as a circulating coolant in a closed system, protected from contact with air or water. Much like the coolant that circulates in an automobile engine, the liquid sodium carries heat from the hot reactor core to heat exchangers. (a) What properties of the liquid sodium are of special importance in this application? (b) The viscosity of liquid sodium varies with temperature as follows: $$ \begin{array}{ll} \hline \text { Temperature }\left({ }^{\circ} \mathrm{C}\right) & \text { Viscosity }\left(\mathrm{kg} \mathrm{m}^{-1} \mathrm{~s}^{-1}\right) \\ \hline 100 & 7.05 \times 10^{-4} \\ 200 & 4.50 \times 10^{-4} \\ 300 & 3.45 \times 10^{-4} \\ 600 & 2.10 \times 10^{-4} \\ \hline \end{array} $$ What forces within the liquid sodium are likely to be the major contributors to the viscosity? Why does viscosity decrease with increasing temperature?

The vapor pressure of a volatile liquid can be determined by slowly bubbling a known volume of gas through it at a known temperature and pressure. \(\operatorname{In}\) an experiment, \(5.00 \mathrm{~L}\) of \(\mathrm{N}_{2}\) gas is passed through \(7.2146 \mathrm{~g}\) of liquid benzene, \(\mathrm{C}_{6} \mathrm{H}_{6}\), at \(26.0^{\circ} \mathrm{C}\). The liquid remaining after the experiment weighs \(5.1493 \mathrm{~g}\). Assuming that the gas becomes saturated with benzene vapor and that the total gas volume and temperature remain constant, what is the vapor pressure of the benzene in torr?

Use a reference source such as the CRC Handbook of Chemistry and Physics to compare the melting and boiling points of the following pairs of inorganic substances: (a) \(\mathrm{W}\) and \(\mathrm{WF}_{6}\), (b) \(\mathrm{SO}_{2}\) and \(\mathrm{SF}_{4}\), (c) \(\mathrm{SiO}_{2}\) and \(\mathrm{SiCl}_{4}\). Account for the major differences observed in terms of likely structures and bonding.