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Problem 24
Which member in each pair has the stronger intermolecular dispersion forces? (a) Br2 or O2 (b) CH3CH2CH2CH2SH or CH3CH2CH2CH2CH2SH (c) CH3CH2CH2Cl or (CH3)2CHCl
Problem 27
(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: CH3F, CH3NH2, CH3OH, CH3Br?
- Rationalize the difference in boiling points in each pair: (a) HF (20 °C) and HCl (-85 °C) (b) CHCl3 (61 °C) and CHBr3 (150 °C) (c) Br2 (59 °C) and ICl (97 °C)
Problem 28
Problem 29a
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.
Problem 29b
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. (b) What are the major factors responsible for the difference in boiling points of the two ethers?
Problem 30
Based on the type or types of intermolecular forces, predict the substance in each pair that has the higher boiling point: (a) propane (C3H8) or n-butane (C4H10) (b) diethyl ether (CH3CH2OCH2CH3) or 1-butanol (CH3CH2CH2CH2OH) (c) sulfur dioxide (SO2) or sulfur trioxide (SO3) (d) phosgene (Cl2CO) or formaldehyde (H2CO)
- Look up and compare the normal boiling points and normal melting points of H2O and H2S. Based on these physical properties, which substance has stronger intermolecular forces? What kinds of intermolecular forces exist for each molecule?
Problem 31
Problem 33
A number of salts containing the tetrahedral polyatomic anion, BF4-, are ionic liquids, whereas salts containing the somewhat larger tetrahedral ion SO42- do not form ionic liquids. Explain this observation.
Problem 34
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?
Problem 35b
(b) What is the relationship between viscosity and temperature?
Problem 35c
(c) Why do substances with high surface tension also tend to have high viscosities?
Problem 36a,c
Based on their composition and structure, list CH2Cl2, CH3CH2CH3, and CH3CH2OH in order of (a) increasing intermolecular forces (c) increasing surface tension
Problem 37b
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:
(b) Which of these diagrams, i or ii, rep- resents what happens when water is on a nonpolar surface?
Problem 39b
The boiling points, surface tensions, and viscosities of water and several alcohols are listed in the following table:
b. How do you explain the fact that propanol and ethylene glycol have similar molecular weights (60 vs. 62 amu), yet the viscosity of ethylene glycol is more than 10 times larger than propanol?
- 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.
Problem 41
Problem 42c
Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (c) Rubbing alcohol in an open container slowly disappears.
Problem 42d
Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (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?
Problem 43
Problem 44a
Ethyl chloride (C2H5Cl) boils at 12 °C. When liquid C2H5Cl under pressure is sprayed on a room-temperature (25 °C) surface in air, the surface is cooled considerably. (a) What does this observation tell us about the specific heat of C2H5Cl(g) as compared with that of C2H5Cl(l)?
Problem 44b
Ethyl chloride (C2H5Cl) boils at 12 °C. When liquid C2H5Cl under pressure is sprayed on a room-temperature (25 °C) surface in air, the surface is cooled considerably. (b) 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?
Problem 45
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 °C by the evaporation of 60 g of water? (The heat of vaporization of water in this temperature range is 2.4 kJ/g. The specific heat of water is 4.18 J/g-K).
- Compounds like CCl2F2 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 CCl2F2 is 289 J/g. What mass of this substance must evaporate to freeze 200 g of water initially at 15 °C? (The heat of fusion of water is 334 J/g; the specific heat of water is 4.18 J/g-K.)
Problem 46
- Ethanol (C2H5OH) melts at -114 °C and boils at 78 °C. The enthalpy of fusion of ethanol is 5.02 kJ/mol, and its enthalpy of vaporization is 38.56 kJ/mol. The specific heats of solid and liquid ethanol are 0.97 and 2.3 J/g-K, respectively. (a) How much heat is required to convert 42.0 g of ethanol at 35 °C to the vapor phase at 78 °C? (b) How much heat is required to convert the same amount of ethanol at -155 °C to the vapor phase at 78 °C?
Problem 47
- The fluorocarbon compound C2Cl3F3 has a normal boiling point of 47.6 °C. The specific heats of C2Cl3F3(l) and C2Cl3F3(g) are 0.91 and 0.67 J/g-K, respectively. The heat of vaporization for the compound is 27.49 kJ/mol. Calculate the heat required to convert 35.0 g of C2Cl3F3 from a liquid at 10.00 °C to a gas at 105.00 °C.
Problem 48
- 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. (d) In general, the more intermolecular forces there are in a substance, the higher its critical temperature and pressure.
Problem 49
Problem 50a
The critical temperatures and pressures of a series of halogenated methanes are as follows:
(a) List the intermolecular forces that occur for each compound.
Problem 50c
The critical temperatures and pressures of a series of halogenated methanes are as follows: (c) Predict the critical temperature and pressure 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 Handbook of Chemistry and Physics, and suggest a reason for any discrepancies.
- 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
Problem 51
Problem 53
(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 intermolecular forces.
- True or false: (a) CBr4 is more volatile than CCl4. (b) CBr4 has a higher vapor pressure at the same temperature than CCl4.
Problem 54