Chapter 12

What intermolecular force(s) must be overcome to perform the following? (a) melt ice (b) sublime solid \(\mathrm{I}_{2}\) (c) convert liquid \(\mathrm{NH}_{3}\) to \(\mathrm{NH}_{3}\) vapor

What type of forces must be overcome when solid \(\mathrm{I}_{2}\) dissolves in methanol, \(\mathrm{CH}_{3} \mathrm{OH}\) ? What type of forces must be disrupted between \(\mathrm{CH}_{3} \mathrm{OH}\) molecules when I \(_{2}\) dissolves? What type of forces exist between \(I_{2}\) and \(\mathrm{CH}_{3} \mathrm{OH}\) molecules in solution?

What type of intermolecular forces must be overcome in converting each of the following from a liquid to a gas? (a) liquid \(\mathrm{O}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{I}\) (methyl iodide) (b) mercury (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (ethanol)

What type of intermolecular forces must be overcome in converting each of the following from a liquid to a gas? (a) \(\mathrm{CO}_{2}\) (c) \(\mathrm{CHCl}_{3}\) (b) \(\mathrm{NH}_{3}\) (d) \(\mathrm{CCl}_{4}\)

Rank the following atoms or molecules in order of increasing strength of intermolecular forces in the pure substance. Which of these substances exists as a gas at \(25^{\circ} \mathrm{C}\) and 1 atm? (a) \(\mathrm{Ne}\) (c) CO (b) \(\mathrm{CH}_{4}\) (d) \(\mathrm{CCl}_{4}\)

Rank the following in order of increasing strength of intermolecular forces in the pure substances. Which substance exists as a gas at \(25^{\circ} \mathrm{C}\) and 1 atm? (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (butane) (b) \(\mathrm{CH}_{3} \mathrm{OH}\) (methanol) (c) He

Which of the following compounds would be expected to form intermolecular hydrogen bonds in the liquid state? (a) \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) (dimethyl ether) (b) \(\mathrm{CH}_{4}\) (c) HF (d) \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) (acetic acid) (e) \(\mathrm{Br}_{2}\) (f) \(\mathrm{CH}_{3} \mathrm{OH}\) (methanol)

In each pair of ionic compounds, which is more likely to have the more negative enthalpy of hydration? Briefly explain your reasoning in each case. (a) LiCl or CsCl (b) \(\mathrm{NaNO}_{3}\) or \(\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}\) (c) RbCl or NiCl_

When salts of \(\mathrm{Mg}^{2+}, \mathrm{Na}^{+},\) and \(\mathrm{Cs}^{+}\) are placed in water, the ions are hydrated. Which of these three cations is most strongly hydrated? Which one is least strongly hydrated?

Ethanol, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH},\) has a vapor pressure of \(59 \mathrm{mm}\) Hg at \(25^{\circ} \mathrm{C}\). What quantity of energy as heat is required to evaporate \(125 \mathrm{mL}\) of the alcohol at \(25^{\circ} \mathrm{C} ?\) The enthalpy of vaporization of the alcohol at \(25^{\circ} \mathrm{C}\) is 42.32 \(\mathrm{kJ} / \mathrm{mol} .\) The density of the liquid is \(0.7849 \mathrm{g} / \mathrm{mL}.\)

The enthalpy of vaporization of liquid mercury is \(59.11 \mathrm{kJ} / \mathrm{mol} .\) What quantity of energy as heat is required to vaporize \(0.500 \mathrm{mL}\) of mercury at \(357^{\circ} \mathrm{C},\) its normal boiling point? The density of mercury is \(13.6 \mathrm{g} / \mathrm{mL}.\)

Which member of each of the following pairs of compounds has the higher boiling point? (a) \(\mathrm{O}_{2}\) or \(\mathrm{N}_{2}\) (c) HF or HI (b) \(\mathrm{SO}_{2}\) or \(\mathrm{CO}_{2}\) (d) \(\operatorname{SiH}_{4}\) or \(\operatorname{Ge} \mathrm{H}_{4}\)

Place the following four compounds in order of increasing boiling point: (a) \(\mathrm{SCl}_{2}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{6}\) (b) \(\mathrm{NH}_{3}\) (d) Ne

You are comparing three different substances, \(A, B,\) and C, all liquids. The vapor pressure at \(25^{\circ} \mathrm{C}\) for substance A is less than the vapor pressure for \(B\) at this temperature. Substance C has the highest boiling point of the three substances. List the three substances \(\mathrm{A}, \mathrm{B},\) or \(\mathrm{C}\) in order of the strength of intermolecular forces, from least to greatest.

Equilibrium vapor pressures of benzene, \(\mathrm{C}_{6} \mathrm{H}_{6},\) at various temperatures are given in the table. $$\begin{array}{cc} \text { Temperature }\left(^{\circ} \mathrm{C}\right) & \text { Vapor Pressure }(\mathrm{mm} \mathrm{Hg}) \\ \hline 7.6 & 40 . \\ 26.1 & 100 \\ 60.6 & 400 \\\ 80.1 & 760 \\ \hline \end{array}$$ (a) What is the normal boiling point of benzene? (b) Plot these data so that you have a graph resembling the one in Figure \(12.17 .\) At what temperature does the liquid have an equilibrium vapor pressure of \(250 \mathrm{mm}\) Hg? At what temperature is the vapor pressure \(650 \mathrm{mm}\) Hg? (c) Calculate the molar enthalpy of vaporization for benzene using the the Clausius-Clapeyron equation.

Vapor pressure data are given here for octane, \(\mathrm{C}_{8} \mathrm{H}_{18}\). $$\begin{array}{cc} \text { Temperature }\left(^{\circ} \mathrm{C}\right) & \text { Vapor Pressure }(\mathrm{mm} \mathrm{Hg}) \\ \hline 25 & 13.6 \\ 50 . & 45.3 \\ 75 & 127.2 \\ 100 . & 310.8 \\ \hline \end{array}$$ Use the Clausius-Clapeyron equation to calculate the molar enthalpy of vaporization of octane and its normal boiling point.

Can carbon monoxide \(\left(T_{c}=132.9 \mathrm{K} ; P_{c}=34.5 \mathrm{atm}\right)\) be liquefied at or above room temperature? Explain briefly.

Methane \(\left(\mathrm{CH}_{4}\right)\) cannot be liquefied at room temperature, no matter how high the pressure. Propane \(\left(\mathrm{C}_{3} \mathrm{H}_{8}\right),\) another simple hydrocarbon, has a critical pressure of 42 atm and a critical temperature of \(96.7^{\circ} \mathrm{C} .\) Can this compound be liquefied at room temperature?

What is surface tension? Give an example illustrating the phenomenon of surface tension. Explain why surface tension is the consequence of intermolecular forces.

What factors affect the viscosity of a substance? Which of the following substances, water \(\left(\mathrm{H}_{2} \mathrm{O}\right),\) ethanol \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\right),\) ethylene glycol \(\left(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\right),\) and glycerol \(\left(\mathrm{HOCH}_{2} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{OH}\right),\) is expected to have the highest viscosity? Should viscosity of a substance be affected by temperature? Explain your answers.

If a piece of filter paper (an absorbent paper used in laboratories) is suspended above a beaker of water and just touching the surface, water will slowly climb up the paper. What is the name given to this phenomenon, and how is this behavior explained?

When water is placed in a buret it forms a concave meniscus at the surface. In contrast, mercury (in a manometer for example) forms a convex meniscus (Figure \(12.22) .\) Explain why this phenomenon occurs, and why the two liquids give different results. Predict the meniscus shape if the buret is filled with ethylene glycol \(\left(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\right).\)

Rank the following substances in order of increasing strength of intermolecular forces: (a) Ar, (b) \(\mathrm{CH}_{3} \mathrm{OH}\), and (c) \(\mathrm{CO}_{2}\)

What types of intermolecular forces are important in the liquid phase of (a) \(\mathrm{C}_{2} \mathrm{H}_{6}\) and (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHOH} .\)

Which of the following salts, \(\mathrm{Li}_{2} \mathrm{SO}_{4}\) or \(\mathrm{Cs}_{2} \mathrm{SO}_{4},\) is expected to have the more exothermic enthalpy of hydration?

Select the substance in each of the following pairs that should have the higher boiling point: (a) \(\mathrm{Br}_{2}\) or \(\mathrm{ICl}\) (b) neon or krypton (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (ethanol) or \(\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{O}\) (ethylene oxide, structure below) (EQUATION CAN'T COPY)

Rank the following compounds in order of increasing molar enthalpy of vaporization: \(\mathrm{CH}_{3} \mathrm{OH}, \mathrm{C}_{2} \mathrm{H}_{6}, \mathrm{HCl}.\)

Rank the following molecules in order of increasing intermolecular forces: \(\mathrm{CH}_{3} \mathrm{Cl}, \mathrm{HCO}_{2} \mathrm{H}\) (formic acid), and \(\mathrm{CO}_{2}.\)

Mercury and many of its compounds are dangerous poisons if breathed, swallowed, or even absorbed through the skin. The liquid metal has a vapor pressure of \(0.00169 \mathrm{mm}\) Hg at \(24^{\circ} \mathrm{C} .\) If the air in a small room is saturated with mercury vapor, how many atoms of mercury vapor occur per cubic meter?

You are going to prepare a silicone polymer, and one of the starting materials is dichlorodimethylsilane, \(\mathrm{SiCl}_{2}\left(\mathrm{CH}_{3}\right)_{2} .\) You need its normal boiling point and to measure equilibrium vapor pressures at various temperatures. $$\begin{array}{cc} \text { Temperature }\left(^{\circ} \mathrm{C}\right) & \text { Vapor Pressure }(\mathrm{mm} \mathrm{Hg}) \\ \hline-0.4 & 40 . \\\ +17.5 & 100 \\ 51.9 & 400 \\ 70.3 & 760 \\ \hline \end{array}$$ (a) What is the normal boiling point of dichlorodimethylsilane? (b) Plot these data as In \(P\) versus \(1 / T\) so that you have a plot resembling the one in Figure \(12.18 .\) At what temperature does the liquid have an equilibrium vapor pressure of 250 mm Hg? At what temperature is it 650 mm Hg? (c) Calculate the molar enthalpy of vaporization for dichlorodimethylsilane using the Clausius-Clapeyron equation.

A "hand boiler" can be purchased in toy stores or at science supply companies. If you cup your hand around the bottom bulb, the volatile liquid in the boiler boils, and the liquid moves to the upper chamber. Using your knowledge of kinetic molecular theory and intermolecular forces, explain how the hand boiler works. (IMAGE CAN'T COPY)

If you place \(1.0 \mathrm{L}\) of ethanol \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\right)\) in a small laboratory that is \(3.0 \mathrm{m}\) long, \(2.5 \mathrm{m}\) wide, and \(2.5 \mathrm{m}\) high, will all the alcohol evaporate? If some liquid remains, how much will there be? The vapor pressure of ethyl alcohol at \(25^{\circ} \mathrm{C}\) is \(59 \mathrm{mm}\) Hg, and the density of the liquid at this temperature is \(0.785 \mathrm{g} / \mathrm{cm}^{3} .\)

Acetone, \(\mathrm{CH}_{3} \mathrm{COCH}_{3},\) is a common laboratory solvent. It is usually contaminated with water, however. Why does acetone absorb water so readily? Draw molecular structures showing how water and acetone can interact. What intermolecular force(s) is(are) involved in the interaction? (EQUATION CAN'T COPY)

Cooking oil floats on top of water. From this observation, what conclusions can you draw regarding the polarity or hydrogen-bonding ability of molecules found in cooking oil?

Liquid ethylene glycol, HOCH \(_{2} \mathrm{CH}_{2} \mathrm{OH},\) is one of the main ingredients in commercial antifreeze. Do you predict its viscosity to be greater or less than that of ethanol, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) ?

Liquid methanol, \(\mathrm{CH}_{3} \mathrm{OH}\), is placed in a glass tube. Is the meniscus of the liquid concave or convex? Explain briefly.

Account for these facts: (a) Although ethanol \(\left.\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\right) \text { (bp, } 80^{\circ} \mathrm{C}\right)\) has a higher molar mass than water \(\left(\mathrm{bp}, 100^{\circ} \mathrm{C}\right),\) the alcohol has a lower boiling point. (b) Mixing 50 mL of ethanol with 50 mL of water produces a solution with a volume slightly less than \(100 \mathrm{mL}\)

Rationalize the observation that \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) 1-propanol, has a boiling point of \(97.2^{\circ} \mathrm{C},\) whereas a compound with the same empirical formula, methyl ethyl ether \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{3}\right),\) boils at \(7.4^{\circ} \mathrm{C}\)

Cite two pieces of evidence to support the statement that water molecules in the liquid state exert considerable attractive force on one another.

During thunderstorms in the Midwest, very large hailstones can fall from the sky. (Some are the size of golf balls!) To preserve some of these stones, we put them in the freezer compartment of a frost-free refrigerator. Our friend, who is a chemistry student, tells us to use an older model that is not frost- free. Why?

What quantity of energy is evolved (in joules) when 1.00 mol of liquid ammonia cools from \(-33.3^{\circ} \mathrm{C}\) (its boiling point) to \(-43.3^{\circ} \mathrm{C} ?\) (The specific heat capacity of liquid \(\mathrm{NH}_{3}\) is \(4.70 \mathrm{J} / \mathrm{g} \cdot \mathrm{K}\).) Compare this with the quantity of heat evolved by 1.00 mol of liquid water cooling by exactly \(10^{\circ} \mathrm{C} .\) Which evolves more heat per mole on cooling \(10^{\circ} \mathrm{C},\) liquid water or liquid ammonia? (The underlying reason for the difference in energy evolved is scientifically illuminating and interesting. You can learn more by searching the Internet for specific heat capacity and its dependence on molecular properties.)

A fluorocarbon, \(\mathrm{CF}_{4},\) has a critical temperature of -45.7 \(^{\circ} \mathrm{C}\) and a critical pressure of 37 atm. Are there any conditions under which this compound can be a liquid at room temperature? Explain briefly.

List four properties of liquids that are directly determined by intermolecular forces.

List the following ions in order of hydration energies: \(\mathrm{Na}^{+}, \mathrm{K}^{+}, \mathrm{Mg}^{2+}, \mathrm{Ca}^{2+} .\) Explain how you determined this order.

Compare the boiling points of the various isomeric hydrocarbons shown in the table below. Notice the relationship between boiling point and structure; branched-chain hydrocarbons have lower boiling points than the unbranched isomer. Speculate on possible reasons for this trend. Why might the intermolecular forces be slightly different in these compounds? $$\begin{array}{lc} \text { Compound } & \text { Boiling point }\left(^{\circ} \mathrm{C}\right) \\ \hline \text { Hexane } & 68.9 \\ \text { 3-Methylpentane } & 63.2 \\ \text { 2-Methylpentane } & 60.3 \\ \text { 2,3-Dimethylbutane } & 58.0 \\ \text { 2,2-Dimethylbutane } & 49.7 \\ \hline \end{array}$$

In which of the following organic molecules might we expect hydrogen bonding to occur? (a) methyl acetate, \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{CH}_{3}\) (b) acetaldehyde (ethanal), \(\mathrm{CH}_{3} \mathrm{CHO}\) (c) acetone (2-propanone) (see Question 8) (d) benzoic acid \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{H}\right)\) (e) acetamide (CH \(_{3} \mathrm{CONH}_{2}\) an amide formed from acetic acid and ammonia) (f) \(\mathrm{N}, \mathrm{N}\) -dimethylacetamide \(\left[\mathrm{CH}_{3} \mathrm{CON}\left(\mathrm{CH}_{3}\right)_{2}\right.\),an amide formed from acetic acid and dimethylamine]

Vapor pressures of \(\mathrm{NH}_{3}(\ell)\) at several temperatures are given in the table below. Use this information to calculate the enthalpy of vaporization of ammonia. $$\begin{array}{cc} \text { Temperature }\left(^{\circ} \mathrm{C}\right) & \text { Vapor Pressure }(\mathrm{atm}) \\ \hline-68.4 & 0.132 \\ -45.4 & 0.526 \\ -33.6 & 1.000 \\\ -18.7 & 2.00 \\ 4.7 & 5.00 \\ 25.7 & 10.00 \\ 50.1 & 20.00 \\ \hline \end{array}$$

Water \((10.0 \mathrm{g})\) is placed in a thick walled glass tube whose internal volume is \(50.0 \mathrm{cm}^{3} .\) Then all the air is removed, the tube is sealed, and then the tube and contents are heated to \(100^{\circ} \mathrm{C}\) (a) Describe the appearance of the system at \(100^{\circ} \mathrm{C}\) (b) What is the pressure inside the tube? (c) At this temperature, liquid water has a density of \(0.958 \mathrm{g} / \mathrm{cm}^{3} .\) Calculate the volume of liquid water in the tube. (d) Some of the water is in the vapor state. Determine the mass of water in the gaseous state.