solutions - Student version

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Information about solutions - Student version

Published on March 24, 2009

Author: amitmgupta31


Intro to Solutions : Intro to Solutions We are now going to use our knowledge of thermodynamics to examine solutions… Consider a solution of two components: 1 and 2 The Gibbs energy is a function of T, P, and the two mole numbers… At constant T and P… We can show by Euler’s theorem: (1) Differentiate: (2) (1) (2) - Divide by n1 + n2: Gibbs-Duhem Equation (constant T and P) Short Mathematics Review : Short Mathematics Review Equations will we use today: Eq 22.59 Eq 24.10 or 24.11 Eq 24.6 -OR- Gibbs-Duhem Equation = standard (1 bar) -Divide everything by n (total number of moles) Eq 24.13 Chemical Potential of Liquids : Chemical Potential of Liquids We need to know how the Gibbs energy of a liquid varies with composition in order to discuss properties of liquid mixtures (like solutions). Vapor Pressure = For vapor phase: Partial Pressure = For vapor phase: At equilibrium… At equilibrium… For liquid phase: For solution: Combine these expressions… Ideal Solutions : Ideal Solutions Two types of molecules are randomly distributed Typically, molecules are similar in size and shape Intermolecular forces in pure liquids & mixture are similar Examples: benzene & toluene, hexane and heptane (more precise thermodynamic definition coming) In ideal solutions, the partial vapor pressure of componentA is simply given by Raoult’s Law: vapor pressure of pure A mole fraction of A in solution Total VP of an ideal solution : Total VP of an ideal solution This serves to define an ideal solution if true for all values of xA The total vapor pressure of an ideal solution: 40 °C Deviations from Raoult’s Law : Deviations from Raoult’s Law Methanol, ethanol, propanolmixed with water. Which oneis which? (All show positive deviations from ideal behavior) CS2 and dimethoxymethane: Positive deviation from ideal (Raoult’s Law) behavior. trichloromethane/acetone: Negative deviation from ideal (Raoult’s Law) behavior. Raoult and Henry : Raoult and Henry Raoult’s law Henry’s law constant: The Henry’s law constant reflects the intermolecular interactions between the two components. Solutions following both Raoult’s and Henry’s Laws are called ideal-dilute solutions. Henry’s behavior: Henry’s law (Raoult’s Law) DGmix, DSmix, and DHmix for ideal solution : DGmix, DSmix, and DHmix for ideal solution Make sense? & “The Bends” : “The Bends” If a deep sea or scuba diver rises to the ocean surface too quickly, he or she can have great pain (mostly at the joints) and may double over in pain… they have “the bends”. In terms of what we’ve discussed today, brainstorm some causes of “the bends”. Temperature-Composition Diagrams : Temperature-Composition Diagrams Point a: On solution line … Point b: On vapor line … 1-propanol and 2-propanol at ambient pressure (i.e., 760 torr) Dalton’s Law How does this relate to fractional distillation? Distillation and Azeotropes : Distillation and Azeotropes One example of non-ideal solutions: Benzene and Ethanol at 1 atm Azeotrope: A mixture for which there is no change in composition upon boiling. Can you separate these compounds by distillation? Really non-ideal: immiscible mixtures : Really non-ideal: immiscible mixtures T3 > TC > T2 > T1 Deviations greater with increasing T What is this line? Temp-composition diagrams for immiscibles : Temp-composition diagrams for immiscibles Other thoughts on non-ideal solutions : Other thoughts on non-ideal solutions Vapor Pressures can often be represented empirically… for example: Activity : Activity For ideal solutions: For non-ideal solutions: Activity Activity defined as: as With definitions for vapor pressure of non-ideal solutions on Sol-15, what is a? Activity coefficient (a measure of deviation from ideality): Typical non-ideal solution : Typical non-ideal solution Chlorobenzene + 1-nitropropane at 75 °C, torr Activities must be calculated wrt standard states : Activities must be calculated wrt standard states Activity using Raoult’s law as standard state… Activity using Henry’s law as standard state… as as Using Rauolt’s Law Using Henry’s Law Gibbs Energy and Activity Coefficients : Gibbs Energy and Activity Coefficients Ideal Solutions… Non-ideal Solutions… (Derivations on pg 994) (Slide Sol-8) Activity etc with other concentration scales : Activity etc with other concentration scales Table 25.1 You need to know how to convert between mole fraction, molality and molarity! Recall colligative properties?! : Recall colligative properties?! Boiling point elevation : Boiling point elevation At equilibrium… or Use Gibbs-Helmholtz equation (see A&G-18) and chemical potential def: m T Label gas, liquid and solid lines Label melting and boiling pt Boiling pt elevation con’t : Boiling pt elevation con’t Assumptions on this page Let… Why these integrands? Osmotic Pressure : Osmotic Pressure Assume the solution is dilute… ln a1 ~ x2 and x2 ~ n2/n1 Osmotic Pressure and Molecular Weight : Osmotic Pressure and Molecular Weight It is found that 2.20 g of polymer dissolved in enough water to make 300 mL of solution has an osmotic pressure of 7.45 torr at 20 °C. Determine the molecular mass of the polymer. Why do we use osmotic pressure to find molecular weight and not one of the other colligative properties? Osmotic Pressure and Cells : Osmotic Pressure and Cells In the figure, red blood cells are placed into saline solutions. In which case (hypertonic, isotonic, or hypotonic) does the concentration of the saline solution match that of the blood cells? In which case is the saline solution more concentrated than the blood cells? Hemolysis Crenation Electrolyte Solutions : Electrolyte Solutions Electrolyte solutions deviate from ideal behavior more strongly and at lower concentrations than nonelectrolyte solutions. (Why?) Activities/activity coefficients are essential when working with electrolytes! Examples of electrolytes… NaCl, MgSO4, MgCl2, Na2SO4 or From this reaction… Also know… Therefore… or Ionic Activity, Molality, & Activity Coefficients : Ionic Activity, Molality, & Activity Coefficients We can define single-ion activity coefficients… Mean ionic activity becomes… Mean ionic molality Mean ionic activity coefficient Write out the mean ionic activity for CaCl2… Table 25.3: Activity and electrolytes : Table 25.3: Activity and electrolytes Colligative Properties of Electrolytes : Colligative Properties of Electrolytes For a strong electrolyte… v = total # of dissociated ions m = molality M1 = molar mass (in kg/mol) If you use this definition in derivation of colligative properties … Debye-Hückel Theory : Debye-Hückel Theory Debye-Hückel Theory: Assumes ions are point ions (no radii) with purely Coulombic interactions and activity coefficients depend only on the ion charges and the solvent properties. Ionic Strength For Aqueous Solutions… Validity of Debye-Hückel Theory : Validity of Debye-Hückel Theory Extended Debye-Hückel: Why are activities so important anyway?! : Why are activities so important anyway?! The activity can be thought of as “the real concentration”… anywhere concentrations are used, activities should be used instead. Some examples: Key Concepts : Key Concepts Gibbs-Duhem Equation Partial Pressure Ideal Solutions Raoult’s Law Henry’s Law Azeotropes Immiscible Solutions Activity and Activity Coefficients Collagative Properties Electrolytes (and properties) Debye-Huckel Theory Importance of Activity

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