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11 Intermolecular Forces Solutions to Exercises corresponding increase in chain length, molecular weight, and strength of dispersion forces. The boiling points, surface tension, and viscosities all increase because the strength of dispersion forces increases. (b) Ethylene glycol has an -OH group at both ends of the molecule. This greatly increases the possibilities for hydrogen bonding, so the overall intermolecular attractive forces are greater and the viscosity of ethylene glycol is much greater. (c) Water has the highest surface tension but lowest viscosity because it is the smallest molecule in the series. Since water molecules are small, they approach each other closely and form many strong hydrogen bonds. There is no hydrocarbon chain to disrupt hydrogen bond formation or to inhibit their attraction to molecules in the interior of the drop. Water molecules at the surface of a drop are missing a few hydrogen bonds and are strongly pulled into the center of the drop, resulting in high surface tension. The absence of an alkyl chain also means the molecules can move around each other easily, resulting in the low viscosity. 11.38 (a) For molecules with similar shapes, viscosity usually decreases with decreasing molecular weight. Since n-pentane has one fewer carbon atom and a shorter chain than n-hexane, the molecules are slightly more free to move around each other and n-pentane will have the smaller viscosity. (b) At 270 K (-3 °C), both neopentane and n-pentane will be liquids. According to Figure 11.6, neopentane is roughly spherical, while n-pentane is cylindrical or rod-shaped. The spherical neopentane has weaker dispersion forces and the molecules are more free to tumble, so it will have the smaller viscosity. Phase Changes (Section 11.4) 11.39 (a) melting, endothermic (b) evaporation or vaporization, endothermic (c) deposition, exothermic (d) condensation, exothermic 11.40 (a) condensation, exothermic (b) sublimation, endothermic (c) vaporization (evaporation), endothermic (d) freezing, exothermic 11.41 The heat energy required to increase the kinetic energy of molecules enough to melt the solid does not produce a large separation of molecules. The specific order is disrupted, but the molecules remain close together. On the other hand, when a liquid is vaporized, the intermolecular forces which maintain close molecular contacts must be overcome. Because molecules are being separated, the energy requirement is higher than for melting. 317