Fractional Distillation
Experiment 2: Fractional Distillation of a Mixture of Two Unknowns
Background:
Boiling is a process familiar to anyone 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 as the liquid water is converted into gaseous water, or steam. Most organic liquids will behave in a similar fashion. On heating, the temperature of the liquid increases until 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 fact that different substances have different boiling points allows us to separate them. The process of heating a substance until it is vaporized, 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 a closed container, some molecules at the surface of the liquid evaporate into the space above the liquid. If this didn’t happen, you wouldn’t be able to smell a liquid. Once vaporized, some of the molecules in the vapor condense back 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 (the liquid’s vapor pressure) rises until it reaches a certain value. When the pressure stabilizes, the rates of evaporation and condensation are equal and the system is at equilibrium. The equilibrium vapor pressure of a liquid increases with increasing temperature. As the temperature of the liquid is raised, more molecules vaporize and the equilibrium vapor pressure increases. Again, think about boiling water, it steams more and more as it gets hot, and then eventually it boils.
Background:
Boiling is a process familiar to anyone 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 as the liquid water is converted into gaseous water, or steam. Most organic liquids will behave in a similar fashion. On heating, the temperature of the liquid increases until 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 fact that different substances have different boiling points allows us to separate them. The process of heating a substance until it is vaporized, 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 a closed container, some molecules at the surface of the liquid evaporate into the space above the liquid. If this didn’t happen, you wouldn’t be able to smell a liquid. Once vaporized, some of the molecules in the vapor condense back 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 (the liquid’s vapor pressure) rises until it reaches a certain value. When the pressure stabilizes, the rates of evaporation and condensation are equal and the system is at equilibrium. The equilibrium vapor pressure of a liquid increases with increasing temperature. As the temperature of the liquid is raised, more molecules vaporize and the equilibrium vapor pressure increases. Again, think about boiling water, it steams more and more as it gets hot, and then eventually it boils.