11.55 Using the vapor-pressure curves in Figure 11.25, (a) estimate the boiling point of ethanol at an external pressure of 200 torr, (b) estimate the external pressure at which ethanol will boil at 60 °C, (c) estimate the boiling point of diethyl ether at 400 torr, (d) estimate the external pressure at which diethyl ether will boil at 40 °C. 11 F Go Figure Volatility, Vapor Pressure, and Temperature When vaporization occurs in an open container, as when water evaporates from a bowl, the vapor moves away from the liquid. Little, if any, is recaptured at the surface of the liquid. Equilibrium What is the vapor pressure of ethylene glycol at its normal boiling point? ture where COr never occurs, and the continues to form until the liquid evap gas ization Pvers Using subs orates to dryness. Substances with high vapor pressure (such as vapor gasoline) evaporate more quickly than substances with low vapor 34.6 °C 800 760 78.3 °C 100 °C pressure (such as motor oil). Liquids that evaporate readily are said to be volatile. Normal boiling point Hot water evaporates more quickly than cold water because vapor pressure increases with increasing temperature. To see why this statement is true, we begin with the fact that the molecules of a liquid move at various speeds. Figure 11.24 shows the distribution of kinetic energies of the molecules at the surface of a liquid at two tem- peratures. (The curves are like those shown for gases in Section 10.7) As the temperature is increased, the molecules move more energeti- cally and more of them can break free from their neighbors and enter the gas phase, increasing the vapor pressure. Figure 11.25 depicts the variation in vapor pressure with tem- perature for four common substances that differ greatly in volatility. Note that the vapor pressure in all cases increases nonlinearly with increasing temperature. The weaker the intermolecular forces in the liquid, the more easily molecules can escape and, therefore, the higher the vapor pressure at a given temperature. 600 Diethyl ether tic Ethyl alcohol (ethanol) ga 400 Water 200 Ethylene glycol 0 0 20 40 60 80 100 Temperature (C) Figure 11.25 Vapor pressure for four liquids as a function of mperature. I I Vapor pressure (torr)