13. Liquids, Solids & Intermolecular Forces

Which organic compound has the lowest viscosity? (LO 11.2)

Arrange the following molecules from lowest to highest boiling point. (LO 8.9) (I)

(II)

(III)

(IV)

(a) I 6 II 6 III 6 IV (b) II 6 III 6 I 6 IV (c) IV 6 II 6 III 6 I (d) IV 6 I 6 II 6 III

Which type of intermolecular force accounts for each of these differences? (d) Acetone boils at 56 °C, whereas 2-methylpropane boils at -12 °C.

(a) List the following molecules in order of increasing polar- izability: GeCl4, CH4, SiCl4, SiH4, and GeBr4. (b) Predict the order of boiling points of the substances in part (a).

Water flows quickly through the narrow neck of a bottle, but maple syrup flows sluggishly. Is this different behavior due to a difference in viscosity or in surface tension for the liquids?

Predict which substance in each pair has the highest surface tension. (a) CCl4 or CH2Br2

Predict which substance in each pair has the highest viscosity. (b) Pentane or neopentane

Predict which substance in each pair has the highest viscosity. (a) Hexane 1CH3CH2CH2CH2CH2CH32 or 1-hexanol 1CH3CH2CH2CH2CH2CH2OH2

The chemical structure for oleic acid, the primary component of olive oil, is shown. Explain why olive oil has a higher vis- cosity than water.

The viscosity of water at 20 °C is 1.00 * 10-3 1N • s>m2 2 higher than dimethyl sulfide 31CH322 S4 2.8 * 10-4 1N • s>m2 2. Explain the difference in viscosity based on chemical structure.

Ethylene glycol (HOCH2CH2OH), the major substance in antifreeze, has a normal boiling point of 198 °C. By comparison, ethyl alcohol (CH3CH2OH) boils at 78 °C at atmospheric pressure. Ethylene glycol dimethyl ether (CH3OCH2CH2OCH3) has a normal boiling point of 83 °C, and ethyl methyl ether (CH3CH2OCH3) has a nomral boiling point of 11 °C. (a) Explain why replacement of a hydrogen on the oxygen by a CH3 group generally results in a lower boiling point.

Based on the type or types of intermolecular forces, predict the substance in each pair that has the higher boiling point: (d) phosgene (Cl2CO) or formaldehyde (H2CO)

The following diagram shows a close-up view of part of the vapor-pressure curves for a solvent (red curve) and a solution of the solvent with a second liquid (green curve). Is the second liquid more volatile or less volatile than the solvent?

The generic structural formula for a 1-alkyl-3-methylimid- azolium cation is where R is a -CH2(CH2)nCH3 alkyl group. The melting points of the salts that form between 1-alkyl-3-methylimidazolium cation and the PF6- anion are as follows: R = CH2CH3 (m.p. = 60 °C), R = CH2CH2CH3 (m.p. = 40 °C), r = CH2CH2CH2CH3 (m.p. = 10 °C), and R = CH2CH2CH2CH2CH2CH3 (m.p. = -61 °C). Why does the melting point decrease as the length of alkyl group increases?

(b) What is the relationship between viscos- ity and temperature?

Based on their composition and structure, list CH2Cl2, CH3CH2CH3, and CH3CH2OH in order of (c) increasing surface tension

Liquids can interact with flat surfaces just as they can with capillary tubes; the cohesive forces within the liquid can be stronger or weaker than the adhesive forces between liquid and surface: (i)

(ii)

(b) Which of these diagrams, i or ii, rep- resents what happens when water is on a nonpolar surface?

Two dichloroethylene molecules with the same chemical formula 1C2H2Cl22, but different arrangements of atoms are shown.

(c) Which form of dichloroethylene has the highest boiling point?

The boiling points, surface tensions, and viscosities of water and several alcohols are as shown below: (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 propa- nol?

In each pair of compounds, pick the one with the higher boiling point. Explain your reasoning. c. CH4 or CH3CH3

In each pair of compounds, pick the one with the higher boiling point. Explain your reasoning. b. CS2 or CO2

In each pair of compounds, pick the one with the higher vapor pressure at a given temperature. Explain your reasoning. c. CH3OH or H2CO

In each pair of compounds, pick the one with the higher vapor pressure at a given temperature. Explain your reasoning. b. CH3CH2CH2OH or CH3OH

Which compound would you expect to have greater surface tension: acetone [(CH3)2CO] or water (H2O)? Explain.

The critical temperatures and pressures of a series of haloge- nated methanes are as follows: (c) Predict the critical temperature and pres- sure for CCl4 based on the trends in this table. Look up the experimentally determined critical temperatures and pressures for CCl4, using a source such as the CRC Hand- book of Chemistry and Physics, and suggest a reason for any discrepancies.

The structures of two isomers of heptane are shown. Which of these two compounds would you expect to have the greater viscosity?

Explain why the viscosity of multigrade motor oils is less temperature- dependent than that of single-grade motor oils.

(a) Place the following substances in order of increasing volatility: CH4, CBr4, CH2Cl2, CH3Cl, CHBr3, and CH2Br2. (b) How do the boiling points vary through this series? (c) Explain your answer to part (b) in terms of intermolec- ular forces.

When a thin glass tube is put into water, the water rises 1.4 cm. When the same tube is put into hexane, the hexane rises only 0.4 cm. Explain.

Which evaporates more quickly: 55 mL of water (H2O) in a beaker or 55 mL of acetone [(CH3)2CO] in an identical beaker under identical conditions? Is the vapor pressure of the two substances different? Explain.

As the intermolecular attractive forces between molecules increase in magnitude, do you expect each of the following to increase or decrease in magnitude? (g) critical temperature.

Suppose you have two colorless molecular liquids, one boil- ing at - 84 °C, the other at 34 °C, and both at atmospheric 6 pressure. Which of the following statements is correct? For each statement that is not correct, modify the statement so that it is correct. (e) At - 84 °C both liquids have vapor pressures of 760 mm Hg.

Suppose you have two colorless molecular liquids, one boil- ing at - 84 °C, the other at 34 °C, and both at atmospheric 6 pressure. Which of the following statements is correct? For each statement that is not correct, modify the statement so that it is correct. (d) The two liquids have iden- tical vapor pressures at their normal boiling points.

The table below shows the normal boiling points of benzene and benzene derivatives. (e) Why is the boiling point of phenol the highest of all?

(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.

Explain the observed trend in the melting points of the hydrogen halides. HI -50.8 °C HBr -88.5 °C HCl -114.8 °C HF -83.1 °C

Explain the observed trend in the boiling points of these compounds. H2Te -2 °C H2Se -41.5 °C H2S -60.7 °C H2O 100 °C

In Table 11.3, we saw that the viscosity of a series of hydro- carbons increased with molecular weight, doubling from the six-carbon molecule to the ten-carbon molecule.

(a) The eight-carbon hydrocarbon, octane, has an isomer, isooctane. Would you predict that isooctane would have a larger or smaller viscosity than octane? Why?

For each of the following substances, identify the intermolecular force or forces that predominate. Using your knowledge of the relative strengths of the various forces, rank the substances in order of their normal boiling points: Al2O3, F2, H2O, Br2, ICl, NaCl.