Chapter 11

(a) Do you expect the viscosity of glycerol, \(\mathrm{C}_{3} \mathrm{H}_{5}(\mathrm{OH})_{3}\) , to be larger or smaller than that of 1 -propanol, \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}\) ? (b) Explain. [ Section 11.3\(]\)

If 42.0 \(\mathrm{kJ}\) of heat is added to a \(32.0-\mathrm{g}\) sample of liquid methane under 1 \(\mathrm{atm}\) of pressure at a temperature of \(-170^{\circ} \mathrm{C}\) , what are the final state and temperature of the methane once the system equilibrates? Assume no heat is lost to the surroundings. The normal boiling point of methane is \(-161.5^{\circ} \mathrm{C}\) The specific heats of liquid and gaseous methane are 3.48 and \(2.22 \mathrm{J} / \mathrm{g}-\mathrm{K}\) , respectively. [ Section 11.4\(]\)

List the three states of matter in order of (a) increasing molecular disorder and (b) increasing intermolecular attraction. (c) Which state of matter is most easily compressed?

(a) How does the average kinetic energy of molecules compare with the average energy of attraction between molecules in solids, liquids, and gases? (b) Why does increasing the temperature cause a solid substance to change in succession from a solid to a liquid to a gas? (c) What happens to a gas if you put it under extremely high pressure?

As a metal such as lead melts, what happens to (a) the average kinetic energy of the atoms and (b) the average distance between the atoms?

At room temperature, Si is a solid, \(\mathrm{CCl}_{4}\) is a liquid, and Ar is gas. List these substances in order of (a) increasing intermolecular energy of attraction and (b) increasing boiling point.

(a) Which type of intermolecular attractive force operates between all molecules? (b) Which type of intermolecular attractive force operates only between polar molecules? (c) Which type of intermolecular attractive force operates only between the hydrogen atom of a polar bond and a nearby small electronegative atom?

(a) Which is generally stronger, intermolecular interactions or intramolecular interactions? (b) Which of these kinds of interactions are broken when a liquid is converted to a gas?

Describe the intermolecular forces that must be overcome to convert these substances from a liquid to a gas: (a) SO \(_{2}\) (b) \(\mathrm{CH}_{3} \mathrm{COOH},(\mathbf{c}) \mathrm{H}_{2} \mathrm{S}\) .

Which type of intermolecular force accounts for each of these differences? (a) \(\mathrm{CH}_{3} \mathrm{OH}\) boils at \(65^{\circ} \mathrm{C} ; \mathrm{CH}_{3} \mathrm{SH}\) boils at \(6^{\circ} \mathrm{C} .(\mathbf{b}) \mathrm{Xe}\) is a liquid at atmospheric pressure and \(120 \mathrm{K},\)whereas Ar is a gas under the same conditions. (c) Kr, atomic weight 84 amu, boils at \(120.9 \mathrm{K},\) whereas \(\mathrm{Cl}_{2},\) molecular weight about 71 amu, boils at 238 \(\mathrm{K}\) . (d) Acetone boils at \(56^{\circ} \mathrm{C},\) whereas 2 -methylpropane boils at \(-12^{\circ} \mathrm{C}\) .

(a) List the following molecules in order of increasing polar-izability: GeCl_ \(_{4}, \mathrm{CH}_{4}, \mathrm{SiCl}_{4}, \mathrm{SiH}_{4},\) and \(\mathrm{GeBr}_{4}\) . (b) Predict the order of boiling points of the substances in part (a).

True or false: (a) For molecules with similar molecular weights, the dispersion forces become stronger as the molecules become more polarizable. (b) For the noble gases the dispersion forces decrease while the boiling points increase as you go down the column in the periodic table. (c) In terms of the total attractive forces for a given substance, dipole- dipole interactions, when present, are always greater than dispersion forces.( \(\mathbf{d}\) ) All other factors being the same, dispersion forces between linear molecules are greater than those between molecules whose shapes are nearly spherical. (e) The larger the atom, the more polarizable it is.

Which member in each pair has the greater dispersion forces? (a) \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{H}_{2} \mathrm{S},(\mathbf{b}) \mathrm{CO}_{2}\) or \(\mathrm{CO},(\mathbf{c}) \operatorname{siH}_{4}\) or \(\mathrm{GeH}_{4}\) .

Which member in each pair has the stronger intermolecular dispersion forces? (a) Br_ or \(\mathrm{O}_{2},\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{SH}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{SH},(\mathbf{c}) \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Clor}\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}\)

Butane and 2 -methylpropane, whose space-filling models are shown here, are both nonpolar and have the same molecular formula, \(\mathrm{C}_{4} \mathrm{H}_{10},\) yet butane has the higher boiling point \(\left(-0.5^{\circ} \mathrm{C}\) compared to \(-11.7^{\circ} \mathrm{C}\right) .\) Explain.

Propyl alcohol \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\right)\) and isopropyl alcohol \(\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHOH}\right],\) whose space- filling models are shown, have boiling points of 97.2 and \(82.5^{\circ} \mathrm{C}\) , respectively. Explain why the boiling point of propyl alcohol is higher, even though both have the molecular formula, \(\mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O}\) .

(a) What atoms must a molecule contain to participate in hydrogen bonding with other molecules of the same kind? (b) Which of the following molecules can form hydrogen bonds with other molecules of the same kind: \(\mathrm{CH}_{3} \mathrm{F}, \mathrm{CH}_{3} \mathrm{NH}_{2}, \mathrm{CH}_{3} \mathrm{OH}, \mathrm{CH}_{3} \mathrm{B}\) ?

Rationalize the difference in boiling points in each pair: (a) HF \(\left(20^{\circ} \mathrm{C}\right)\) and \(\mathrm{HCl}\left(-85^{\circ} \mathrm{C}\right),\) (b) CHCl \(_{3}\left(61^{\circ} \mathrm{C}\right)\) and \(\mathrm{CHBr}_{3}\left(150^{\circ} \mathrm{C}\right),(\mathbf{c}) \mathrm{Br}_{2}\left(59^{\circ} \mathrm{C}\right)\) and \(\mathrm{ICl}\left(97^{\circ} \mathrm{C}\right)\)

Based on the type or types of intermolecular forces, predict the substance in each pair that has the higher boiling point:(a) propane \(\left(\mathrm{C}_{3} \mathrm{H}_{8}\right)\) or \(n\) -butane \(\left(\mathrm{C}_{4} \mathrm{H}_{10}\right),(\mathbf{b})\) diethyl ether \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}\right)\) or 1 -butanol \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\right)\) (c) sulfur dioxide \(\left(\mathrm{SO}_{2}\right)\) or sulfur trioxide \(\left(\mathrm{SO}_{3}\right),(\mathbf{d})\) phosgene \(\left(\mathrm{Cl}_{2} \mathrm{CO}\right)\) or formaldehyde \(\left(\mathrm{H}_{2} \mathrm{CO}\right)\)

Look up and compare the normal boiling points and normal melting points of \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{H}_{2} \mathrm{S}\) . Based on these physical properties, which substance has stronger intermolecular forces? What kinds of intermolecular forces exist for each molecule?

Carbon tetrachloride, \(\mathrm{CCl}_{4},\) and chloroform, \(\mathrm{CHCl}_{3},\) are common organic liquids. Carbon tetrachloride's normal boiling point is \(77^{\circ} \mathrm{C} ;\) chloroform's normal boiling point is \(61^{\circ} \mathrm{C} .\) Which statement is the best explanation of these data? (a) Chloroform can hydrogen-bond, but carbon tetrachloride cannot. (b) Carbon tetrachloride has a larger dipole moment than chloroform. (c) Carbon tetrachloride is more polarizable than chloroform.

A number of salts containing the tetrahedral polyatomic anion, \(\mathrm{BF}_{4}^{-}\) , are ionic liquids, whereas salts containing the somewhat larger tetrahedral ion \(\mathrm{SO}_{4}^{2-}\) do not form ionic liquids. Explain this observation.

(a) What is the relationship between surface tension and temperature? (b) What is the relationship between viscosity and temperature? (c) Why do substances with high surface tension also tend to have high viscosities?

Based on their composition and structure, list \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3},\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) in order of \((\mathbf{a})\) increasing intermolecular forces, (\mathbf{b} ) increasing viscosity , ( c ) increasing surface tension.

Hydrazine \(\left(\mathrm{H}_{2} \mathrm{NNH}_{2}\right),\) hydrogen peroxide \((\mathrm{HOOH}),\) and water \(\left(\mathrm{H}_{2} \mathrm{O}\right)\) all have exceptionally high surface tensions compared with other substances of comparable molecular weights. (a) Draw the Lewis structures for these three compounds. (b) What structural property do these substances have in common, and how might that account for the high surface tensions?

The boiling points, surface tensions, and viscosities of water and several alcohols are as shown below:(a) From ethanol to propanol to \(n\) -butanol the boiling points, surface tensions, and viscosities all increase. What is the reason for this increase? (b) How do you explain the fact that propanol and ethylene glycol have similar molecular weights (60 versus 62 amu), yet the viscosity of ethylene glycol is more than 10 times larger than propanol? (c) How do you explain the fact that water has the highest surface tension but the lowest viscosity?

Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (a) When ice is heated, it turns to water. (b) Wet clothes dry on a warm summer day. (c) Frost appears on a window on a cold winter day. (d) Droplets of water appear on a cold glass of lemonade.

Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (a) Bromine vapor turns to bromine liquid as it is cooled. (b) Crystals of iodine disappear from an evaporating dish as they stand in a fume hood. (c) Rubbing alcohol in an open container slowly disappears. (d) Molten lava from a volcano turns into solid rock.

(a) What phase change is represented by the "heat of fusion" of a substance? (b) Is the heat of fusion endothermic or exothermic? (c) If you compare a substance's heat of fusion to its heat of vaporization, which one is generally larger?

Ethyl chloride \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\right)\) boils at \(12^{\circ} \mathrm{C}\) . When liquid \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\) under pressure is sprayed on a room-temperature \(\left(25^{\circ} \mathrm{C}\right)\) surface in air, the surface is cooled considerably. (a) What does this observation tell us about the specific heat of \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}(g)\) as compared with that of \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}(l) ?\) Assume that the heat lost by the surface is gained by ethyl chloride. What enthalpies must you consider if you were to calculate the final temperature of the surface?

For many years drinking water has been cooled in hot climates by evaporating it from the surfaces of canvas bags or porous clay pots. How many grams of water can be cooled from 35 to \(20^{\circ} \mathrm{C}\) by the evaporation of 60 \(\mathrm{g}\) of water?(The heat of vaporization of water in this temperature range is 2.4 \(\mathrm{kJ} / \mathrm{g} .\) The specific heat of water is \(4.18 \mathrm{J} / \mathrm{g}-\mathrm{K}\) .

Compounds like \(\mathrm{CCl}_{2} \mathrm{F}_{2}\) are known as chlorofluorocarbons, or CFCs. These compounds were once widely used as refrigerants but are now being replaced by compounds that are believed to be less harmful to the environment. The heat of vaporization of \(\mathrm{CCl}_{2} \mathrm{F}_{2}\) is 289 \(\mathrm{J} / \mathrm{g}\) . What mass of this substance must evaporate to freeze 200 \(\mathrm{g}\) of water initially at \(15^{\circ} \mathrm{C} ?\) (The heat of fusion of water is \(334 \mathrm{J} / \mathrm{g} ;\) the specific heat of water is \(4.18 \mathrm{J} / \mathrm{g}-\mathrm{K}\) .

Ethanol \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\right)\) melts at \(-114^{\circ} \mathrm{C}\) and boils at \(78^{\circ} \mathrm{C}\) . The enthalpy of fusion of ethanol is \(5.02 \mathrm{kJ} / \mathrm{mol},\) and its enthalpy of vaporization is 38.56 \(\mathrm{kJ} / \mathrm{mol}\) . The specific heats of solid and liquid ethanol are 0.97 and \(2.3 \mathrm{J} / \mathrm{g}-\mathrm{K},\) respectively. (a) How much heat is required to convert 42.0 \(\mathrm{g}\) of ethanol at \(35^{\circ} \mathrm{C}\) to the vapor phase at \(78^{\circ} \mathrm{C} ?(\mathbf{b})\) How much heat is required to convert the same amount of ethanol at \(-155^{\circ} \mathrm{C}\) to the vapor phase at \(78^{\circ} \mathrm{C} ?\)

The fluorocarbon compound \(\mathrm{C}_{2} \mathrm{Cl}_{3} \mathrm{F}_{3}\) has a normal boiling point of \(47.6^{\circ} \mathrm{C}\) . The specific heats of \(\mathrm{C}_{2} \mathrm{Cl}_{3} \mathrm{F}_{3}(l)\) and \(\mathrm{C}_{2} \mathrm{Cl}_{3} \mathrm{F}_{3}(g)\) are 0.91 and \(0.67 \mathrm{J} / \mathrm{g}-\mathrm{K}\) , respectively. The heat of vaporization for the compound is 27.49 \(\mathrm{kJ} / \mathrm{mol}\) . Calculate the heat required to convert 35.0 \(\mathrm{g}\) of \(\mathrm{C}_{2} \mathrm{Cl}_{3} \mathrm{F}_{3}\) from a liquid at \(10.00^{\circ} \mathrm{C}\) to a gas at \(105.00^{\circ} \mathrm{C}\) .

Indicate whether each statement is true or false: (a) The critical pressure of a substance is the pressure at which it turns into a solid at room temperature. (b) The critical temperature of a substance is the highest temperature at which the liquid phase can form. (c) Generally speaking, the higher the critical temperature of a substance, the lower its critical pressure. (\boldsymbol{d} ) In general the more intermolecular forces there are in a substance, the higher its critical temperature and pressure.

Which of the following affects the vapor pressure of a liquid? (a) Volume of the liquid, (b) surface area, (c) intermolecular attractive forces, (d) temperature, (e) density of the liquid.

True or false: (a) \(\mathrm{CBr}_{4}\) is more volatile than \(\mathrm{CCl}_{4} .(\mathbf{b}) \mathrm{CBr}_{4}\) has a higher boiling point than \(\mathrm{CCl}_{4}\) . (c) CBr. has weaker intermolecular forces than \(\mathrm{CCl}_{4}\) . (d) \(\mathrm{CBr}_{4}\) has a higher vapor pressure at the same temperature than \(\mathrm{CCl}_{4}\) .

You are high up in the mountains and boil water to make some tea. However, when you drink your tea, it is not as hot as it should be. You try again and again, but the water is just not hot enough to make a hot cup of tea. Which is the best explanation for this result? (a) High in the mountains, it is probably very dry, and so the water is rapidly evaporating from your cup and cooling it. (b) High in the mountains, it is probably very windy, and so the water is rapidly evaporating from your cup and cooling it. (c) High in the mountains, the air pressure is significantly less than 1 atm, so the boiling point of water is much lower than at sea level. (d) High in the mountains, the air pressure is significantly less than 1 atm, so the boiling point of water is much higher than at sea level.

Appendix \(\mathrm{B}\) lists the vapor pressure of water at various external pressures. (a) Plot the data in Appendix B, vaporpressure (torr) versus temperature \(\left(^{\circ} \mathrm{C}\right) .\) From your plot, estimate the vapor pressure of water at body temperature, \(37^{\circ} \mathrm{C}\) (b) Explain the significance of the data point at 760.0 torr, \(100^{\circ} \mathrm{C}\) (c) A city at an altitude of 5000 \(\mathrm{ft}\)above sea level has a barometric pressure of 633 torr. To what temperature would you have to heat water to boil it in this city? (d) A city at an altitude of 500 ft below sea level would have a barometric pressure of 774 torr. To what temperature would you have to heat water to boil it in this city?

(a) What is the significance of the critical point in a phase diagram? (b) Why does the line that separates the gas and liquid phases end at the critical point?

(a) What is the significance of the triple point in a phase diagram? (b) Could you measure the triple point of water by measuring the temperature in a vessel in which water vapor, liquid water, and ice are in equilibrium under 1 atm of air? Explain.

At \(25^{\circ} \mathrm{C}\) gallium is a solid with a density of 5.91 \(\mathrm{g} / \mathrm{cm}^{3} .\) Its melting point, \(29.8^{\circ} \mathrm{C},\) is low enough that you can melt it by holding it in your hand. The density of liquid gallium just above the melting point is 6.1 \(\mathrm{g} / \mathrm{cm}^{3} .\) Based on this information, what unusual feature would you expect to find in the phase diagram of gallium?

In terms of the arrangement and freedom of motion of the molecules, how are the nematic liquid crystalline phase and an ordinary liquid phase similar? How are they different?

Indicate whether each statement is true or false: (a) The liquid crystal state is another phase of matter, just like solid, liquid, and gas. (b) Liquid crystalline molecules are generally spherical in shape. (c) Molecules that exhibit a liquid crystalline phase do so at well-defined temperatures and pressures. (d) Molecules that exhibit a liquid crystalline phase show weaker- than-expected intermolecular forces. (e) Molecules containing only carbon and hydrogen are likely to form liquid crystalline phases. (f) Molecules can exhibit more than one liquid crystalline phase.

For a given substance, the liquid crystalline phase tends to be more viscous than the liquid phase. Why?

Describe how a cholesteric liquid crystal phase differs from a nematic phase.

The smectic liquid crystalline phase can be said to be more highly ordered than the nematic phase. In what sense is this true?

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 each statement that is not correct, modify the statement so that it is correct. (a) The higher-boiling liquid has greater total intermolecular forces than the lower- boiling liquid. (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 \(-84^{\circ}\) both liquids have vapor pressures of 760 \(\mathrm{mm} \mathrm{Hg}\) .

Ethylene glycol \(\left(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\right)\) and pentane \(\left(\mathrm{C}_{5} \mathrm{H}_{12}\right)\) are both liquids at room temperature and room pressure, and have about the same molecular weight. (a) One of these liquids is much more viscous than the other. Which one do you predict is more viscous? (b) One of these liquids has a much lower normal boiling point \(\left(36.1^{\circ} \mathrm{C}\right)\) compared to the other one \(\left(198^{\circ} \mathrm{C}\right) .\) Which liquid has the lower normal boiling point? (c) One of these liquids is the major component in antifreeze in automobile engines. Which liquid would you expect to be used as antifreeze? (d) One of these liquids is used as a "blowing agent" in the manufacture of polystyrene foam because it is so volatile. Which liquid would you expect to be used as a blowing agent?