Vapor Pressure of Liquids & Solutions - VAPOR PRESSURE OF LIQUIDS SOLUTIONS Adapted from"Chemistry with Computers Vernier Software Portland OR 1997

Vapor Pressure of Liquids & Solutions - VAPOR PRESSURE...

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VAPOR PRESSURE OF LIQUIDS & SOLUTIONS Adapted from "Chemistry with Computers" Vernier Software, Portland OR, 1997 INTRODUCTION When a liquid is added to a closed system (a sealed Erlenmeyer flask, Figure 1), a net flow of molecules into the gas phase will occur until the rate of evaporation is equal to the rate of condensation. At this point, the vapor pressure of the liquid is equal to the partial pressure of its vapor in the flask. Figure 1 In this experiment, pure ethanol or a solution containing ethanol will be injected into a sealed Erlenmeyer flask. A computer-interfaced pressure sensor will be used to measure changes in the total pressure in the flask as temperature of the surrounding water bath is increased. By plotting the natural log of the vapor pressure (ln P) versus inverse Kelvin temperature (1/T), the heat of vaporization ( Δ H vap ) will be determined from the slope of the best fit line using the Clausius Clapeyron equation (eqn 1): (1) ln P = (– Δ H vap / R) (1/T) + C (R = 8.3145 J/mol·K & C = constant) A colligative property of solutions is the lowering of the vapor pressure of a solvent in solution in comparison to a solvent by itself. Solutions containing nonvolatile solutes follow Raoult’s Law (eqn 2). 1
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(2) P solution = P ° solvent Χ solvent (P ° solvent = vapor pressure of the pure solvent , Χ solvent = mole fraction of solvent in solution) Solutions that follow Raoult’s Law are ideal . Intermolecular forces between solvent and solute in such solutions are very weak. Solution formation results in a negligible change in enthalpy ( Δ H solution ~ 0). The vapor pressure of nonideal solutions deviates from Raoult’s Law. The direction of the deviation is dependent on the type of intermolecular forces present between solute and solvent. If attractive forces are present, the deviation will be negative : the actual vapor pressure will be less than that predicted by Raoult’s Law. Solution formation is exothermic ( Δ H solution is large & negative). If repulsive forces are present, the deviation will be positive : the actual vapor pressure will be greater than that predicted by Raoult’s Law. Solution formation is endothermic ( Δ H solution is large & positive). SAFETY PRECAUTIONS Wear safety goggles and a lab coat at all times in the lab. Ethanol and acetone are flammable; make sure there are no sparks or flames in the lab. Ethanol and acetone fumes can irritate eyes and lungs; avoid contact with fumes. IN-LAB SAFETY MOMENT EXPOSURE ROUTES 1. Your TA will split you into six groups with each group assigned one of the following chemicals: ethyl acetate, heptane, acetic anhydride, copper sulfate, borax, or crystal violet. 2. Look up the SDS on SigmaAldrich or another chemical company and take three minutes to answer the following questions in your ELN.
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