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11 Intermolecular Forces Solutions to Exercises Select Properties of Liquids (section 11.3) 11.33 (a) Surface tension and viscosity are the result of intermolecular attractive forces or cohesive forces among molecules in a liquid sample. As temperature increases, the number of molecules with sufficient kinetic energy to overcome these attractive forces increases, and viscosity and surface tension decrease. (b) Surface tension and viscosity are both directly related to the strength of intermolecular attractive forces. The same attractive forces that cause surface molecules to be difficult to separate cause molecules elsewhere in the sample to resist movement relative to one another. Liquids with high surface tension have intermolecular attractive forces sufficient to produce a high viscosity as well. 11.34 (a) Cohesive forces bind molecules to each other. Adhesive forces bind molecules to surfaces. (b) The cohesive forces are hydrogen bonding among water molecules and also hydrogen bonding among cellulose molecules in the paper towel. Adhesive forces are any attractive forces between water and cellulose (the paper towel), likely also hydrogen bonding. If adhesive forces between cellulose and water weren't significant, paper towels wouldn't absorb water. (c) The shape of a meniscus depends on the strength of the cohesive forces within a liquid relative to the adhesive forces between the walls of the tube and the liquid. Adhesive forces between polar water molecules and silicates in glass (Figure 11.19) are even stronger than cohesive hydrogen-bonding forces among water molecules, so the meniscus is U-shaped (concave-upward). 11.35 (a) CHBr₃ has a higher molar mass, is more polarizable, and has stronger dispersion forces, so the surface tension is greater. (b) As temperature increases, the viscosity of the oil decreases because the average kinetic energy of the molecules increases [Solution 11.33(a)]. (c) Adhesive forces between polar water and nonpolar car wax are weak, so the large tension of water draws the liquid into the shape with the smallest surface area, a sphere. (d) Surface tension due to dispersion forces in oil is not great. Adhesive forces between nonpolar oil and nonpolar car wax are similar to cohesive forces in oil, so the oil drops spread out on the waxed hood. 11.36 (a) (b) All have bonds (N-H or O-H, respectively) capable of forming hydrogen bonds. Hydrogen bonding is the strongest intermolecular interaction between neutral molecules and leads to very strong cohesive forces in liquids. The stronger the cohesive forces in a liquid, the greater the surface tension. 11.37 (a) The three molecules have similar structures and all experience hydrogen bonding, dipole-dipole and dispersion forces. The main difference in the series is the increase in the number of carbon atoms in the alkyl chain, with a 316