lab 4 - Separation of Liquids by Fractional Distillation...

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Separation of Liquids by Fractional Distillation and Analysis of Separation Efficacy by Constructing a Volume vs. Temperature Graph and Gas-Liquid Chromatography Mayank Kumar March 2, 2007 Methods and Background The purpose of the experiment was to separate a 1:1 mixture of ethyl acetate and butyl acetate by fractional distillation and determine the separation efficiency by constructing a volume of distillate versus head temperature graph and analyzing the distillates through gas-liquid chromatography. Given a system with a liquid in a container, the rate at which molecules in the gas phase reenter the liquid will eventually become equal to the rate at which they escape from the liquid, resulting in a dynamic equilibrium. The molecules in the gas phase will collide with the walls of the container, producing pressure referred to as the equilibrium vapor pressure, which varies directly with temperature. When the equilibrium vapor pressure equals the atmospheric pressure (760 Torr or 1 atm), the rate of evaporation of the liquid increases significantly and bubbles form; thus it has reached its boiling point. When considering mixtures of liquids, Raoult’s law is used to determine the partial pressure of each liquid, as it states that the partial pressure is equal to the product of the equilibrium vapor pressure and the mole fraction. Dalton’s law of partial pressures then dictates that the partial pressures of the liquids must sum up to the total pressure of the liquid. Boiling points are affected by intermolecular forces; molecules with greater attractive forces have higher boiling points because it takes more energy to break the bonds between the molecules. Although ethyl acetate (EtOAc) and butyl acetate (BuOAc) have similar polarities due to the presence of ester functional groups, BuOAc is a larger, more massive molecule, thus the Van der Waals forces between its molecules will be greater, leading to stronger intermolecular bonds and a higher temperature required to cause BuOAc to boil (boiling point). Fractional distillation is used to isolate individual pure liquid components from a mixture of two or more volatile substances whose boiling points differ by no less than 20-30 °C. Given a mixture of two volatile components, it can be determined that the number of molecules of each component in a given volume of the vapor above the mixture will be proportional to the partial vapor pressures of the respective components. Since the partial vapor pressures of the components depend on the composition of the mixture according to Raoult’s law, and the
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mixture reaches its boiling point when the sum of the partial vapor pressures of the components is equal to the atmospheric pressure according to Dalton’s law, it can be reasoned that the boiling point of the mixture is determined by its composition. From the fact that volatility governs ease of vaporization, it can be determined that at any given temperature, the vapor phase contains a higher proportion of the more volatile component than does the boiling mixture at the same
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lab 4 - Separation of Liquids by Fractional Distillation...

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