4hMANVPliqsoln - Vapor Pressure of Liquids & Solutions...

Info iconThis preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon
Vapor Pressure of Liquids & Solutions Revised 03/01/10 1 VAPOR PRESSURE OF LIQUIDS & SOLUTIONS Adapted from "Chemistry with Computers" Vernier Software, Portland OR, 1997 OBJECTIVES For you, the student, to write…. SAFETY Wear safety goggles and lab apron at all times in lab. Ethanol is flammable; make sure there are no sparks or flames in lab. Ethanol fumes can irritate eyes and lungs; avoid contact with fumes. 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
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Vapor Pressure of Liquids & Solutions Revised 03/01/10 2 the natural log of the vapor pressure (ln P) versus inverse Kelvin temperature (1/T), the heat of vaporization ( H vap ) will be determined from the slope of the best fit line using the Clausius Clapeyron equation (eqn 1): (1) ln P = (– 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). (2) P solution = P solvent solvent (P solvent = vapor pressure of the pure solvent , solvent = mole fraction of solvent in solution) Solutions containing volatile solutes follow a modified version of Raoult’s Law (eqn 3). (3) P solution = P solvent solvent + P solute solute (P solute = vapor pressure of the pure solute , solute = mole fraction of solute in solution) What is a volatile solute? A volatile solute is a liquid that evaporates easily. It is a chemical with a high vapor pressure under standard conditions (25 C, 1 atm). Typically, solids can be classified as nonvolatile. Liquids, however, range in volatility. A good indicator of volatility is boiling point – the higher the boiling point, the less volatile a substance will be. 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 ( 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 ( H solution is large & negative). If repulsive forces are present the deviation will be
Background image of page 2
Vapor Pressure of Liquids & Solutions Revised 03/01/10 3 positive
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 4
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 10

4hMANVPliqsoln - Vapor Pressure of Liquids & Solutions...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online