Chapter 6

Concept Review Why does a branched alkane have a lower boiling point than a straight-chain alkane of the same molar mass?

Why do the strengths of London dispersion forces increase with increasing molecular size?

Select the compound in each of the following pairs whose molecules experience stronger London dispersion forces. (a) \(\mathrm{C}_{2} \mathrm{Cl}_{6}\) or \(\mathrm{C}_{2} \mathrm{F}_{6} ;\) (b) \(\mathrm{CH}_{4}\) or \(\mathrm{C}_{3} \mathrm{H}_{8} ;\) (c) \(\mathrm{CS}_{2}\) or \(\mathrm{CO}_{2}\)

The three most abundant gases in air are \(\mathrm{N}_{2}, \mathrm{O}_{2},\) and \(\mathrm{Ar}\). Which of them has the highest boiling point, and which has the lowest boiling point?

Fuels from Crude Oil Petroleum (crude oil) is a complex mixture of mostly hydrocarbons that can be separated into useful fuels by distillation. Common petroleum-based fuels in order of increasing boiling point are gasoline, jet fuel, kerosene, diesel oil, and fuel oil. Which of the fuels contains hydrocarbons with the highest average molar mass?

The permanent dipole moment of \(\mathrm{CH}_{2} \mathrm{F}_{2}(1.93 \mathrm{D})\) is larger than that of \(\mathrm{CH}_{2} \mathrm{Cl}_{2}(1.60 \mathrm{D})\), yet the boiling point of \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\left(40^{\circ} \mathrm{C}\right)\) is much higher than that of \(\mathrm{CH}_{2} \mathrm{F}_{2}\) \(\left(-52^{\circ} \mathrm{C}\right) .\) Why?

How is it that the permanent dipole moment of \(\mathrm{HCl}\) \((1.08 \mathrm{D})\) is larger than the permanent dipole moment of HBr \((0.82 \mathrm{D}),\) yet HBr boils at a higher temperature?

In each of the following pairs of molecules, which one experiences the stronger dispersion forces? (a) \(\mathrm{CCl}_{4}\) or \(\mathrm{CF}_{4}\) (b) \(\mathrm{CH}_{4}\) or \(\mathrm{C}_{3} \mathrm{H}_{8}\)

Concept Review How are water molecules oriented around anions in aqueous solutions?

How are water molecules oriented around cations in aqueous solutions?

Why are dipole-dipole interactions generally weaker than ion-dipole interactions?

Two liquids- one polar, one non polar have the same molar mass. Which one is likely to have the higher boiling point?

Why are hydrogen bonds considered a special class of dipole-dipole interactions?

Can all polar hydrogen-containing molecules form hydrogen bonds?

Suggest two reasons why the boiling point of methyl Auoride, \(\mathrm{CH}_{3} \mathrm{F}\), is higher than the boiling point of methane, \(\mathrm{CH}_{4}.\)

Why is the boiling point of \(\mathrm{Br}_{2}\) lower than that of iodine mono chloride, ICl, even though they have nearly the same molar mass?

Why do molecules of methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right)\) form hydrogen bonds, but molecules of methane (CH, ) do not?

The boiling point of \(\mathrm{H}_{2} \mathrm{S}\) is lower than that of \(\mathrm{H}_{2} \mathrm{O}\) even though \(\mathrm{H}_{2} \mathrm{S}\) has twice the molar mass of \(\mathrm{H}_{2} \mathrm{O} .\) Why?

In which of the following compounds do the molecules experience dipole-dipole interactions? (a) \(\mathrm{CF}_{4} ;\) (b) \(\mathrm{CF}_{2} \mathrm{Cl}_{2}\) (c) \(\mathrm{CCl}_{4} ;\) (d) \(\mathrm{CFCl}_{3}\)

In which of the following compounds do molecules experience dipole-dipole interactions? (a) \(\mathrm{CO}_{2} ;\) (b) \(\mathrm{NO}_{2}\) (c) \(\mathrm{SO}_{2} ;\) (d) \(\mathrm{H}_{2} \mathrm{S}\)

Which of the following molecules can hydrogen-bond among themselves in pure samples of bulk material? (a) methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right) ;\) (b) ethane \(\left(\mathrm{CH}_{3} \mathrm{CH}_{3}\right) ;\) (c) dimethyl cther \(\left(\mathrm{CH}_{3} \mathrm{OCH}_{3}\right) ;\) (d) acctic acid \(\left(\mathrm{CH}_{3} \mathrm{COOH}\right)\)

Which of the following molecules can hydrogen-bond with molecules of water? (a) methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right) ;\) (b) ethane \(\left(\mathrm{CH}_{3} \mathrm{CH}_{3}\right) ;(\mathrm{c})\) dimethyl ether \(\left(\mathrm{CH}_{3} \mathrm{OCH}_{3}\right) ;(\mathrm{d})\) acetic acid \(\left(\mathrm{CH}_{3} \mathrm{COOH}\right)\)

(Before solving the following problems, you may find it useful to review Section 4.1 on the strengths of ionic bonds.) In an aqueous solution containing chloride, bromide, and iodide salts, which anion would you expect to experience the strongest ion-dipole interactions with surrounding water molecules?

In an aqueous solution containing \(\mathrm{Na}^{+}, \mathrm{Mg}^{2+}, \mathrm{K}^{+},\) and \(\mathrm{Ca}^{2+}\) salts, which cation would you expect to experience the strongest ion-dipole interactions?

In a solution prepared by mixing equal masses of water and ethanol (CH \(_{3} \mathrm{CH}_{2} \mathrm{OH}\) ), which ingredient is the solute and which is the solvent?

What is the difference, if there is any, between the terms miscible and soluble?

Which of the following substances have little solubility in water? (a) benzenc, \(\mathrm{C}_{6} \mathrm{H}_{6} ;\) (b) \(\mathrm{KBr} ;\) (c) \(\mathrm{Ar}\)

In each of the following pairs of compounds, which compound is likely to be more soluble in \(\mathrm{CCl}_{4} ?\) a. \(C C 1_{4}\) or \(C H C l_{3}\) b. \(\mathrm{CH}_{3} \mathrm{OH}\) or \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{4} \mathrm{CH}_{2} \mathrm{OH}\) c. \(\mathrm{NaF}\) or \(\mathrm{MgO}\) d. \(\mathrm{CaF}_{2}\) or \(\mathrm{BaF}_{2}\)

In each of the following pairs of compounds, which compound is likely to be more soluble in \(\mathrm{CCl}_{4} ?\) a. \(\mathrm{Br}_{2}\) or \(\mathrm{NaBr}\) b. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) c. \(\mathrm{CS}_{2}\) or \(\mathrm{KOH}\) d. \(\overline{\mathrm{I}}_{2}\) or \(\mathrm{CaF}_{2}\)

Which of the following pairs of substances are likely to be miscible? a. \(\quad \mathrm{Br}_{2}\) and benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\) b. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}\) (diethyl ether) and \(\mathrm{CH}_{3} \mathrm{COOH}\) (acetic acid) c. \(\mathrm{C}_{6} \mathrm{H}_{12}\) (cyclohexane) and hexane \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\right)\) d. \(\mathrm{CS}_{2}\) (carbon disulfide) and \(\mathrm{CCl}_{4}\) (carbon tetrachloride)

Which of the following pairs of substances is likely to be miscible? a. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (ethanol) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}\) (diethyl ether) b. \(\mathrm{CH}_{3} \mathrm{OH}\) (methanol) and methyl amine \(\left(\mathrm{CH}_{3} \mathrm{NH}_{2}\right)\) c. \(\mathrm{CH}_{3} \mathrm{CN}\) (acetonitrile) and acetone \(\left(\mathrm{CH}_{3} \mathrm{COCH}_{3}\right)\) d. \(\mathrm{CF}_{3} \mathrm{CHF}_{2}\) (a Freon replacement) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (pentane)

Which of the following compounds is likely to be the most soluble in water? (a) \(\mathrm{NaCl} ;\) (b) \(\mathrm{KI} ;\) (c) \(\mathrm{Ca}(\mathrm{OH})_{2} ;\) (d) \(\mathrm{CaO}\)

Which sulfur compound would you predict to be more soluble in non polar solvents: \(\mathrm{SO}_{2}\) or \(\mathrm{CS}_{2} ?\)

Which of the following compounds is the most soluble in water? a. \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{2} \mathrm{O}\left(\mathrm{CH}_{2}\right)_{2} \mathrm{CH}_{3}\) b. \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{3} \mathrm{O}\left(\mathrm{CH}_{2}\right)_{2} \mathrm{CH}_{3}\) c. \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) d. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}\)

Phase Diagrams: Inter molecular Forces at Work Explain the difference between sublimation and evaporation.

Can ice be melted merely by applying pressure? How about dry ice? Explain your answers.

At what temperatures and pressures does a substance behave as a supercritical fluid?

Preserving Food Freeze-drying is used to preserve food at low temperature with minimal loss of flavor. Freeze-drying works by freezing the food and then lowering the pressure with a vacuum pump to sublime the ice. Must the pressure be lower than the pressure at the triple point of \(\mathrm{H}_{2} \mathrm{O} ?\)

Solid helium cannot be converted directly into the vapor phase. Does the phase diagram of He have a triple point?

What phase changes, if any, does liquid water at \(100^{\circ} \mathrm{C}\) undergo if the initial pressure of 5.0 atm is reduced to 0.5 atm at constant temperature?

What phase changes, if any, occur if a sample of \(\mathrm{CO}_{2}\) initially at \(-80^{\circ} \mathrm{C}\) and 5.0 atm is warmed to \(-25^{\circ} \mathrm{C}\) and 5.0 atm?

Below what temperature can solid \(\mathrm{CO}_{2}\) (dry ice) be converted into \(\mathrm{CO}_{2}\) gas simply by lowering the pressure?

What is the maximum pressure at which solid \(\mathrm{CO}_{2}\) (dry ice) can be converted into \(\mathrm{CO}_{2}\) gas without melting?

Predict the phase of water that exists under the following conditions: a. 2 atm of pressure and \(110^{\circ} \mathrm{C}\) b. 0.5 atm of pressure and \(80^{\circ} \mathrm{C}\) c. \(7 \times 10^{-3}\) atm of pressure and \(3^{\circ} \mathrm{C}\)

Which phase or phases of water exist under the following conditions? a. 0.32 atm and \(70^{\circ} \mathrm{C}\) b. 300 atm and \(400^{\circ} \mathrm{C}\) c. 1 atm and \(0^{\circ} \mathrm{C}\)

List the steps you would take to convert a sample of water at \(25^{\circ} \mathrm{C}\) and 1 atm pressure to water at its triple point.

List the steps you would take to convert a \(10.0 \mathrm{g}\) sample of water at \(25^{\circ} \mathrm{C}\) and 2 atm pressure to ice at 1 atm pressure. At what temperature would the water freeze?

Concept Review Explain why a needle floats on the surface of water but sinks in a container of methanol.

Explain why different liquids do not reach the same height in capillary tubes of the same diameter.

Explain why the plumbing in a house may burst if the temperature in the house drops below \(0^{\circ} \mathrm{C}.\)