Distillation - Boiling Points and Distillation The action...

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Boiling Points and Distillation The action of boiling is a process familiar to anybody who has cooked pasta or brewed tea. As heat is applied to a pan of water, the temperature of the water increases until it reaches 100°C (212°F). At this temperature, additional heat causes the water to bubble vigorously and transforms liquid water into gaseous water, or steam. Most organic liquids will behave in a similar fashion. On heating, the temperature of the liquid increases until a certain temperature called the boiling point is reached. Additional heating causes the liquid to vaporize accompanied by vigorous bubbling of the liquid. The boiling point of a substance is a physical property of a substance and can be useful for characterizing that substance. The process of heating a substance until it vaporizes, cooling the vapors, and collecting the condensed liquid is the basis of a commonly used purification technique called distillation . Vapor Pressure and Boiling Point If a liquid is placed in an empty, closed container, some molecules at the surface of the liquid evaporate into the empty space above the liquid. Once vaporized, some of the molecules in the vapor condense into the liquid in a competing process. As the space above the liquid becomes occupied with molecules of vaporized liquid, the pressure of the vapor above the liquid rises until it reaches a certain value. When the pressure stabilizes, the rates of evaporation and condensation are equal. The pressure of the vapor under these conditions is called the equilibrium vapor pressure . The equilibrium vapor pressure of a liquid is temperature dependent. As the temperature of the liquid is raised, more molecules vaporize and the equilibrium vapor pressure increases. The graph in Figure 11 shows the relationship between vapor pressure and temperature for three substances, dichloromethane, water, and d -limonene. Figure 11. Vapor pressure-temperature diagram for dichloromethane, water and d -limonene. The normal boiling point of a substance is defined as the temperature at which the vapor pressure of that substance equals atmospheric pressure, 760 mmHg. The normal boiling points of dichloromethane, water, and d -limonene are, respectively, 40.2°C, 100°C, and 175°C. If the barometric pressure is less than 760 mmHg, the temperature at which a substance boils will be less than the normal boiling point. When measuring boiling points, it is important to record the barometric pressure at the time of the measurement. In the chemical literature, there are several different methods of reporting boiling points with the barometric pressure. For example, the boiling point of 2,5-hexanedione at 754 mmHg is 194°C and is
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reported in the following formats: bp 754 194°C, bp 194°C/754 mmHg, bp 194°C (754 mmHg), or 194°C 754 . If a boiling point is reported without a pressure it is assumed to be the normal boiling point and the pressure
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This note was uploaded on 10/27/2010 for the course CHEM 337 taught by Professor Kim during the Fall '10 term at CSU Sacramento.

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Distillation - Boiling Points and Distillation The action...

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