Chapter 11 Part 2 - Chemistry Student Notes PDF

Chapter 11: Part 2 CHEM 0120  Vapor Pressure of Solutions The vapor pressure of a solvent above a solution is lower than the vapor pressure of the pure solvent. The solute particles replace some of the solvent molecules at the surface....

Chapter 11: Part 2 CHEM 0120  Vapor Pressure of Solutions The vapor pressure of a solvent above a solution is lower than the vapor pressure of the pure solvent. The solute particles replace some of the solvent molecules at the surface. 2  Vapor-pressure lowering Vapor-pressure lowering (of a solvent): a colligative property equal to the vapor pressure of the pure solvent minus the vapor pressure of the solution ° ∆𝑷 = 𝑷𝐬𝐨𝐥𝐯𝐞𝐧𝐭 − 𝑷𝐬𝐨𝐥𝐮𝐭𝐢𝐨𝐧 3  Raoult’s Law and Vapor Pressure Raoult’s Law: the partial pressure of solvent, PA, over a solution equals the vapor pressure of the pure solvent, P°A, times the mole fraction of solvent, XA, in the solution. 𝑃𝐴 = 𝑃𝐴° 𝑋𝐴 Because the mole fraction is always less than 1, the vapor pressure of the solvent in solution will always be less than the vapor pressure of the pure solvent. ∆𝑃 = 𝑃𝐴° 𝑋𝐵 4  Ideal vs Nonideal solution In ideal solutions, the made solute–solvent interactions are equal to the sum of the broken solute–solute and solvent– solvent interactions. Ideal solutions follow Raoult’s law. Effectively, the solute is diluting the solvent. If the solute–solvent interactions are stronger or weaker than the broken interactions, the solution is nonideal. 5  Phase Diagrams A graphical way to summarize the conditions under which the different states of a substance are stable  Colligative Properties Related to Vapor Pressure Lowering Vapor Pressure lowering occurs at all temperatures. The result is the temperature required to boil the solution is higher than the boiling point of the pure solvent. Another result is the temperature required to freeze the solution is lower than the freezing point of the pure solvent. 7  Boiling Point Elevation Boiling-point elevation (ΔTb): a colligative property of a solution equal to the boiling point of the solution minus the boiling point of the pure solvent. It is directly proportional to the molal concentration of the solution. ∆𝑇𝑏 = 𝐾𝑏 𝑐𝑚 The proportionality constant, Kb, is called the boiling- point-elevation constant It depends only on the solvent. 8  Freezing-Point Depression Freezing-Point Depression (ΔTf): a colligative property of a solution equal to the freezing point of the pure solvent minus the freezing point of the solution. It is directly proportional to the molal concentration of the solution. ∆𝑇𝑓 = 𝐾𝑓 𝑐𝑚 The proportionality constant, Kf, is called the freezing-point- depression constant It depends only on the solvent. 9 Table of Constants 10  Osmosis Osmosis: the phenomenon of solvent flow through a semipermeable membrane to equalize the solute concentrations on both sides of the membrane The flow of solvent from a solution of low concentration into a solution of high concentration. The semipermeable membrane allows solvent, but not solute, to flow through it. 11  Osmotic Pressure Osmotic pressure: a colligative property of a solution equal to the pressure that, when applied to the solution, stops osmosis. Osmotic pressure (π) is directly proportional to the molarity (M) of the solute particles. 𝜋 = 𝑀𝑅𝑇 R: gas constant T: temperature 12  Electrolyte Solutions Electrolytes undergo dissociation when dissolved in water. The van’t Hoff factor (i) accounts for this effect. actual number of particles in solution after dissociation i number of formulas units initially dissolved in solution ∆Tf = iKfm ∆Tb = iKbm π = iMRT 13  Electrolyte Solutions The van’t Hoff factor (i) is 1 for all nonelectrolytes: H2O C12H22O11(s) C12H22O11(aq) For strong electrolytes i should be equal to the number of ions: NaCl(s) H2O Na+(aq) + Cl–(aq) Na2SO4(s) H2O 2Na+(aq) + SO42–(aq) 14  Electrolyte Solutions The van’t Hoff factor (i) is usually smaller than predicted due to the formation of ion pairs. An ion pair is made up of one or more cations and one or more anions held together by electrostatic forces. 15  Colloids Colloid: it is a dispersion of particles of one substance (the dispersed phase) throughout another substance or solution (the continuous phase) Will appear homogeneous, but they are larger particles of one substance dispersed throughout another substance A colloid is not the same as a true solution. A colloid has the ability to scatter light at a detectable level. All gases and liquids scatter light, however the scattering from a true solution or pure substance is quite small and typically not detectable. Dispersed particles are larger than normal molecules (~1×103 pm - 2×105 pm) 16  Tyndall Effect Tyndall effect: the scattering of light by colloidal-size particles Which vessel contains the true solution and which contains the colloid? 17  Characterization of Colloids Characterized according to the state of the dispersed phase and of the continuous phase Some examples of types of colloids: Aerosol: Liquid droplets or solid particles dispersed throughout a gas Foam: A gas dispersed in a liquid. 18  Water as the Continuous Phase Colloids with water as the continuous phase are either: Hydrophilic colloids Hydrophobic colloids Hydrophilic colloid: a colloid in which there is a strong attraction between the dispersed phase and the continuous phase Hydrophobic colloid: a colloid in which there is a lack of attraction between the dispersed phase and the continuous phase 19  Association colloids Micelle: a colloidal-sized particle formed in water by the association of molecules or ions that each have a hydrophobic end and a hydrophilic end Association colloid: a colloid in which the dispersed phase consists of micelles Example: Soap Hydrophilic ends are on the outside of the micelle facing the water Hydrophobic ends point inward toward one another 20