Chapter 11

List the different phase transitions that are possible and give examples of each.

Describe how you could purify iodine by sublimation.

Describe vapor pressure in molecular terms. What do we mean by saying it involves a dynamic equilibrium?

Explain why \(15 \mathrm{~g}\) of steam at \(100^{\circ} \mathrm{C}\) melts more ice than \(15 \mathrm{~g}\) of liquid water at \(100^{\circ} \mathrm{C}\).

Why is the heat of fusion of a substance smaller than its heat of vaporization?

Explain why evaporation leads to cooling of the liquid.

Describe the behavior of a liquid and its vapor in a closed vessel as the temperature increases.

Gases that cannot be liquefied at room temperature merely by compression are called "permanent" gases. How could you liquefy such a gas?

The pressure in a cylinder of nitrogen continuously decreases as gas is released from it. On the other hand, a cylinder of propane maintains a constant pressure as propane is released. Explain this difference in behavior.

Why does the vapor pressure of a liquid depend on the intermolecular forces?

Explain the surface tension of a liquid in molecular terms. How does the surface tension make a liquid act as though it had a "skin"?

Explain the origin of the London force that exists between two molecules.

Why do molecular substances have relatively low melting points?

Describe the distinguishing characteristics of a crystalline solid and an amorphous solid.

Describe the face-centered cubic unit cell.

Describe the structure of thallium(I) iodide, which has the same structure as cesium chloride.

What is the coordination number of \(\mathrm{Cs}^{+}\) in \(\mathrm{CsCl}\) ? of \(\mathrm{Na}^{+}\) in \(\mathrm{NaCl}\) ? of \(\mathrm{Zn}^{2+}\) in \(\mathrm{ZnS}\) ?

Explain the production of an \(x\) -ray diffraction pattern by a crystal in terms of the interference of waves.

The triple point of a solid is at \(5.2 \mathrm{~atm}\) and \(-57^{\circ} \mathrm{C}\). Under typical laboratory conditions of \(P=0.98\) atm and \(T=23^{\circ} \mathrm{C}\), this solid will a. remain solid indefinitely b. boil c. melt d. sublime e. condense

Under the right conditions, hydrogen gas, \(\mathrm{H}_{2}\), can be liquefied. Which is the most important intermolecular force that is responsible for allowing hydrogen molecules to be liquefied? a. hydrogen bonding b. dipole-dipole interactions c. London (dispersion) forces d. covalent bonds e. ion-dipole forces

An element crystallizes with a simple cubic lattice with atoms at all the lattice points. If the radius of the atom is \(200 . \mathrm{pm}\), what is the volume of the unit cell? a. \(8.00 \times 10^{6} \mathrm{pm}^{3}\) b. \(6.40 \times 10^{7} \mathrm{pm}^{3}\) c. \(4.00 \times 10^{4} \mathrm{pm}^{3}\) d. \(1.60 \times 10^{5} \mathrm{pm}^{3}\) e. \(6.00 \times 10^{7} \mathrm{pm}^{3}\)

a. Is it possible to add heat to a pure substance and not observe a temperature change? If so, provide examples. b. Describe, on a molecular level, what happens to the heat being added to a substance just before and during melting. Do any of these molecular changes cause a change in temperature? Part 2: Consider two pure substances with equal molar masses: substance A, having very strong intermolecular attractions, and substance \(\mathrm{B}\), having relatively weak intermolecular attractions. Draw two separate heating curves for \(0.25\) -mol samples of substance \(A\) and substance \(B\) in going from the solid to the vapor state. You decide on the freezing point and boiling point for each substance, keeping in mind the information provided in this problem. Here is some additional information for constructing the curves. In both cases, the rate at which you add heat is the same. Prior to heating, both substances are at \(-50^{\circ} \mathrm{C}\), which is below their freezing points. The heat capacities of \(\mathrm{A}\) and \(\mathrm{B}\) are very similar in all states. a. As you were heating substances \(\mathrm{A}\) and \(\mathrm{B}\), did they melt after equal quantities of heat were added to each substance? Explain how your heating curves support your answer. b. What were the boiling points you assigned to the substances? Are the boiling points the same? If not, explain how you decided to display them on your curves. C. According to your heating curves, which substance reached the boiling point first? Justify your answer. d. Is the quantity of heat added to melt substance A at its melting point the same as the quantity of heat required to convert all of substance \(\mathrm{A}\) to a gas at its boiling point? Should these quantities be equal? Explain.

Consider a substance \(\mathrm{X}\) with a \(\Delta H_{\mathrm{vap}}=20.3 \mathrm{~kJ} / \mathrm{mol}\) and \(\Delta H_{\text {fius }}=9.0 \mathrm{~kJ} / \mathrm{mol}\). The melting point, freezing point, and heat capacities of both the solid and liquid \(\mathrm{X}\) are identical to those of water. a. If you place one beaker containing \(50 \mathrm{~g}\) of \(\mathrm{X}\) at \(-10^{\circ} \mathrm{C}\) and another beaker with \(50 \mathrm{~g}\) of \(\mathrm{H}_{2} \mathrm{O}\) at \(-10^{\circ} \mathrm{C}\) on a hot plate and start heating them, which material will reach the boiling point first? b. Which of the materials from part a, \(\mathrm{X}\) or \(\mathrm{H}_{2} \mathrm{O}\), would completely boil away first? c. On a piece of graph paper, draw the heating curve for \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{X}\). How do the heating curves reflect your answers from parts a and b?

Using the information presented in this chapter, explain why farmers spray water above and on their fruit trees on still nights when they know the temperature is going to drop below \(0^{\circ} \mathrm{C}\). (Hint: Totally frozen fruit is what the farmers are trying to avoid.)

You are presented with three bottles, each containing a different liquid: bottle A, bottle \(\mathrm{B}\), and bottle \(\mathrm{C}\). Bottle A's label states that it is an ionic compound with a boiling point of \(35^{\circ} \mathrm{C}\). Bottle B's label states that it is a molecular compound with a boiling point of \(29.2^{\circ} \mathrm{C}\). Bottle \(\mathrm{C}^{+}\) s label states that it is a molecular compound with a boiling point of \(67.1^{\circ} \mathrm{C}\). a. Which of the compounds is most likely to be incorrectly identified? b. If Bottle A were a molecular compound, which of the compounds has the strongest intermolecular attractions? c. If Bottle A were a molecular compound, which of the compounds would have the highest vapor pressure?

If you place water at room temperature in a wellinsulated cup and allow some of the water to evaporate, the temperature of the water in the cup will drop lower than room temperature. Come up with an explanation for this observation.

The heats of vaporization for water and carbon disulfide are \(40.7 \mathrm{~kJ} / \mathrm{mol}\) and \(26.8 \mathrm{~kJ} / \mathrm{mol}\), respectively. A vapor (steam) bum occurs when the concentrated vapor of a substance condenses on your skin. Which of these substances, water or carbon disulfide, will result in the most severe burn if identical quantities of each vapor at a temperature just above their boiling point came in contact with your skin?

When hypothetical element \(\mathrm{X}\) forms a solid, it can crystallize in three ways: with unit cells being either simple cubic, face-centered cubic, or body-centered cubic. a. Which crystalline form of solid \(\mathrm{X}\) has the highest density? b. Which crystalline form of solid \(X\) has the most empty space?

Consider two flasks that contain different pure liquids at \(20^{\circ} \mathrm{C} .\) The liquid in one flask, substance \(A\), has a molar mass of \(100 \mathrm{~g} / \mathrm{mol}\) and has hydrogen bonding. The liquid in the other flask, substance \(\mathrm{B}\), has a molar mass of \(105 \mathrm{~g} / \mathrm{mol}\) and has dipole-dipole interactions. a. If the molecular structures of the compounds are very similar, which flask probably contains substance \(\mathrm{A}\) ? b. If you were to increase the temperature of each of the flasks by \(15^{\circ} \mathrm{C}\), how would the pictures change (assume that you stay below the boiling points of the liquids)?

Identify the phase transition occurring in each of the following. a. The water level in an aquarium tank falls continuously (the tank has no leak). b. A mixture of scrambled eggs placed in a cold vacuum chamber slowly turns to a powdery solid. C. Chlorine gas is passed into a very cold test tube where it turns to a yellow liquid. d. When carbon dioxide gas under pressure exits from a small orifice, it turns to a white "snow." e. Molten lava from a volcano cools and turns to solid rock.

Identify the phase transition occurring in each of the following. a. Mothballs slowly become smaller and eventually disappear. b. Rubbing alcohol spilled on the palm of the hand feels cool as the volume of liquid decreases. C. A black deposit of tungsten metal collects on the inside of a lightbulb whose filament is tungsten metal. d. Raindrops hit a cold metal surface, which becomes covered with ice. e. Candle wax turns to liquid under the heat of the candle flame.

An electric heater coil provided heat to a \(15.5\) -g sample of iodine, \(\mathrm{I}_{2}\), at the rate of \(3.48 \mathrm{~J} / \mathrm{s}\). It took \(4.54 \mathrm{~min}\) from the time the iodine began to melt until the iodine was completely melted. What is the heat of fusion per mole of iodine?

Isopropyl alcohol, \(\mathrm{CH}_{3} \mathrm{CHOHCH}_{3}\), is used in rubbing alcohol mixtures. Alcohol on the skin cools by evaporation. How much heat is absorbed by the alcohol if \(2.25 \mathrm{~g}\) evaporates? The heat of vaporization of isopropyl alcohol is \(42.1\) \(\mathrm{kJ} / \mathrm{mol} .\)

Liquid butane, \(\mathrm{C}_{4} \mathrm{H}_{10}\), is stored in cylinders to be used as a fuel. Suppose \(39.3 \mathrm{~g}\) of butane gas is removed from a cylinder. How much heat must be provided to vaporize this much gas? The heat of vaporization of butane is \(21.3 \mathrm{~kJ} / \mathrm{mol}\)

Water at \(0^{\circ} \mathrm{C}\) was placed in a dish inside a vessel maintained at low pressure by a vacuum pump. After a quantity of water had evaporated, the remainder froze. If \(9.31 \mathrm{~g}\) of ice at \(0^{\circ} \mathrm{C}\) was obtained, how much liquid water must have evaporated? The heat of fusion of water is \(6.01 \mathrm{~kJ} / \mathrm{mol}\) and its heat of vaporization is \(44.9 \mathrm{~kJ} / \mathrm{mol}\) at \(0^{\circ} \mathrm{C}\).

A quantity of ice at \(0.0^{\circ} \mathrm{C}\) was added to \(33.6 \mathrm{~g}\) of water at \(21.0^{\circ} \mathrm{C}\) to give water at \(0.0^{\circ} \mathrm{C}\). How much ice was added? The heat of fusion of water is \(6.01 \mathrm{~kJ} / \mathrm{mol}\) and the specific heat is \(4.18 \mathrm{~J} /\left(\mathrm{g} \cdot{ }^{\circ} \mathrm{C}\right)\).

A quantity of ice at \(0^{\circ} \mathrm{C}\) is added to \(64.3 \mathrm{~g}\) of water in a glass at \(55^{\circ} \mathrm{C}\). After the ice melted, the temperature of the water in the glass was \(15^{\circ} \mathrm{C}\). How much ice was added? The heat of fusion of water is \(6.01 \mathrm{~kJ} / \mathrm{mol}\) and the specific heat is \(4.18 \mathrm{~J} /\left(\mathrm{g} \cdot{ }^{\circ} \mathrm{C}\right)\).

Steam at \(100^{\circ} \mathrm{C}\) was passed into a flask containing \(275 \mathrm{~g}\) of water at \(21^{\circ} \mathrm{C}\), where the steam condensed. How many grams of steam must have condensed if the temperature of the water in the flask was raised to \(83^{\circ} \mathrm{C}\) ? The heat of vaporization of water at \(100^{\circ} \mathrm{C}\) is \(40.7 \mathrm{~kJ} / \mathrm{mol}\) and the specific heat is \(4.18 \mathrm{~J} /\left(\mathrm{g} \cdot{ }^{\circ} \mathrm{C}\right)\)

Methanol, \(\mathrm{CH}_{3} \mathrm{OH}\), a colorless, volatile liquid, was formerly known as wood alcohol. It boils at \(65.0^{\circ} \mathrm{C}\) and has a heat of vaporization of \(37.4 \mathrm{~kJ} / \mathrm{mol}\). What is its vapor pressure at \(22.0^{\circ} \mathrm{C} ?\)

White phosphorus, \(\mathrm{P}_{4}\), is normally a white, waxy solid melting at \(44^{\circ} \mathrm{C}\) to a colorless liquid. The liquid has a vapor pressure of \(400.0 \mathrm{mmHg}\) at \(251.0^{\circ} \mathrm{C}\) and \(760.0 \mathrm{mmHg}\) at \(280.0^{\circ} \mathrm{C}\). What is the heat of vaporization of this substance?

Carbon disulfide, \(\mathrm{CS}_{2}\), is a volatile, flammable liquid. It has a vapor pressure of \(400.0 \mathrm{mmHg}\) at \(28.0^{\circ} \mathrm{C}\) and \(760.0 \mathrm{mmHg}\) at \(46.5^{\circ} \mathrm{C}\). What is the heat of vaporization of this substance?

Use graph paper and sketch the phase diagram of argon, Ar, from the following information: normal melting point, \(-187^{\circ} \mathrm{C}\); normal boiling point, \(-186^{\circ} \mathrm{C} ;\) triple point, \(-189^{\circ} \mathrm{C}\), \(0.68\) atm; critical point, \(-122^{\circ} \mathrm{C}, 48 \mathrm{~atm} .\) Label each phase region on the diagram.

Krypton, Kr, has a triple point at \(-169^{\circ} \mathrm{C}\) and \(133 \mathrm{mmHg}\) and a critical point at \(-63^{\circ} \mathrm{C}\) and \(54 \mathrm{~atm}\). The density of the solid is \(2.8 \mathrm{~g} / \mathrm{cm}^{3}\), and the density of the liquid is \(2.4 \mathrm{~g} / \mathrm{cm}^{3} .\) Sketch a rough phase diagram of krypton. Circle the correct word in each of the following sentences (and explain your answers). a. Solid krypton at \(130 \mathrm{mmHg}\) (melts, sublimes without melting) when the temperature is raised. b. Solid krypton at \(760 \mathrm{mmHg}\) (melts, sublimes without melting) when the temperature is raised.

The heats of vaporization of liquid \(\mathrm{Cl}_{2}\), liquid \(\mathrm{H}_{2}\), and liquid \(\mathrm{N}_{2}\) are \(20.4 \mathrm{~kJ} / \mathrm{mol}, 0.9 \mathrm{~kJ} / \mathrm{mol}\), and \(5.6 \mathrm{~kJ} / \mathrm{mol}\), respectively. Are the relative values as you would expect? Explain.

The heats of vaporization of liquid \(\mathrm{O}_{2}\), liquid \(\mathrm{Ne}\), and liquid methanol, \(\mathrm{CH}_{3} \mathrm{OH}\), are \(6.8 \mathrm{~kJ} / \mathrm{mol}, 1.8 \mathrm{~kJ} / \mathrm{mol}\), and \(34.5 \mathrm{~kJ} / \mathrm{mol}\), respectively. Are the relative values as you would expect? Explain.

Arrange the following substances in order of increasing magnitude of the London forces: \(\mathrm{SiCl}_{4}, \mathrm{CCl}_{4}, \mathrm{GeCl}_{4}\)

Arrange the following substances in order of increasing magnitude of the London forces: Xe, Ne, Kr.

Methane, \(\mathrm{CH}_{4}\), reacts with chlorine, \(\mathrm{Cl}_{2}\), to produce a series of chlorinated hydrocarbons: methyl chloride \(\left(\mathrm{CH}_{3} \mathrm{Cl}\right)\), methylene chloride \(\left(\mathrm{CH}_{2} \mathrm{Cl}_{2}\right)\), chloroform \(\left(\mathrm{CHCl}_{3}\right)\), and carbon tetrachloride \(\left(\mathrm{CCl}_{4}\right) .\) Which compound has the lowest vapor pressure at room temperature? Explain.

The halogens form a series of compounds with each other, which are called interhalogens. Examples are bromine chloride (BrCl), iodine bromide (IBr), bromine fluoride (BrF), and chlorine fluoride (CIF). Which compound is expected to have the lowest boiling point at any given pressure? Explain.

Predict the order of increasing vapor pressure at a given temperature for the following compounds: a. \(\mathrm{FCH}_{2} \mathrm{CH}_{2} \mathrm{~F}\) b. \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) c. \(\mathrm{FCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) Explain why you chose this order.