CHM 112 M. Prushan Chapter 11 Solutions and Their Properties

CHM 112 M. Prushan I.Introduction Alloy -A solid solution (i.e. brass is a solution of Cu and Zn)Alloy -A solid solution (i.e. brass is a solution of Cu and Zn) A)Solution -A homogeneous mixture of two or more substances in a single phase (gas, liquid, solid) Solvent -The component that makes up the bulk of the solution.Solvent -The component that makes up the bulk of the solution. Solute -The component that dissolves in the solvent.Solute -The component that dissolves in the solvent. B)Colligatve Properties -Properties of solutions which depend on the number of solute particles in the solution and not the nature of the solute. Four Colligative PropertiesFour Colligative Properties 1)Vapor pressure lowering 2)Boiling point elevation 3)Freezing point depression 4)Osmosis

CHM 112 M. Prushan II.Units of Concentration Not useful in colligative properties because the exact amount of solvent is unknown.Not useful in colligative properties because the exact amount of solvent is unknown. The following concentration units reflect the number of solute particles per solvent molecules and are useful with colligative properties.The following concentration units reflect the number of solute particles per solvent molecules and are useful with colligative properties.

CHM 112 M. Prushan II.Units of Concentration Naturally occurring solutions are often very dilute so environmental chemists, biologists, geologists, etc. often use parts per million (ppm).Naturally occurring solutions are often very dilute so environmental chemists, biologists, geologists, etc. often use parts per million (ppm).

CHM 112 M. Prushan IV.Colligative Properties A.Vapor Pressure Lowering B.Boiling Point Elevation C.Freezing Point Depression D.Osmosis There are four colligative properties of solutions:There are four colligative properties of solutions: Colligative properties are independent of the nature of the solvent and depend only on the relative number of solute and solvent particles.Colligative properties are independent of the nature of the solvent and depend only on the relative number of solute and solvent particles.

CHM 112 M. Prushan IV.Colligative Properties The vapor pressure of the solution is lowered because the solute particles at the liquid/vapor boundary block the solvent particle from jumping into the vapor state.The vapor pressure of the solution is lowered because the solute particles at the liquid/vapor boundary block the solvent particle from jumping into the vapor state. A. Vapor Pressure Lowering Raoult’s Law P solv = X solv P° solv or  P solv = X solute P° solv

CHM 112 M. Prushan IV.Colligative Properties For nonvolatile, nonelectrolyte solvents the change in boiling point (  T bp ) is:For nonvolatile, nonelectrolyte solvents the change in boiling point (  T bp ) is: B. Boiling Point Elevation  T bp = K bp m solute K bp =boiling point elevation constant m solute =molality of solute

CHM 112 M. Prushan IV.Colligative Properties For nonvolatile, nonelectrolyte solvents the change in freezing point (  T fp ) is:For nonvolatile, nonelectrolyte solvents the change in freezing point (  T fp ) is: C. Freezing Point Depression  T fp = K fp m solute K fp =freezing point depression constant m solute =molality of solute

CHM 112 M. Prushan IV.Colligative Properties Colligative properties can be used to determine the molar mass of a solute when it is dissolved in a solvent of appreciable vapor pressure and a known K bp or K fp.Colligative properties can be used to determine the molar mass of a solute when it is dissolved in a solvent of appreciable vapor pressure and a known K bp or K fp. D. Colligative Properties and Molar Mass Determination Measure a change in VP, BP elevation, FP depression, or osmotic pressure. Solution Conc. Moles of Solute Molar Mass Use mass of solvent g solute mol solute

CHM 112 M. Prushan IV.Colligative Properties The change in VP, BP, FP, or osmotic pressure is greater than expected for electrolyte (ionic salt) solutions.The change in VP, BP, FP, or osmotic pressure is greater than expected for electrolyte (ionic salt) solutions. E. Colligative Properties of Solutions Containing Ions Predicted BP elevation of an aqueous m NaCl solution  T bp = K bp m solute (For H 2 O K bp = °C/m)  T bp, calculated = ( °C/m)(0.100 m) = °C Actual BP elevation of an aqueous m NaCl solution  T bp, measured = °C (Almost double the  T bp calculated) Colligative properties depend on the total number of solute particles in solution. Ionic compounds form ions in solution so the total number of solute particles in solution is equal to the total ions in solution. Colligative properties depend on the total number of solute particles in solution. Ionic compounds form ions in solution so the total number of solute particles in solution is equal to the total ions in solution.

CHM 112 M. Prushan IV.Colligative Properties E. Colligative Properties of Solutions Containing Ions NaCl(s)  Na + (aq) + Cl - (aq) m van’t Hoff factor (i) So for ionic solutions: m m total

CHM 112 M. Prushan IV.Colligative Properties E. Colligative Properties of Solutions Containing Ions NaCl(s)  Na + (aq) + Cl - (aq) Predicting van’t Hoff factors 1 particle + 1 particle = 2 particles i predicted = 2 Na 2 SO 4 (s)  2 Na + (aq) + SO 4 2- (aq) 2 particles + 1 particle = 3 particles i predicted = 3

CHM 112 Summer 2007 M. Prushan IV.Colligative Properties F. Osmosis and Osmotic Pressure Osmosis -The movement of solvent molecules through a semipermeable membrane from a region of low solute concentration to a region of high solute concentration.

CHM 112 M. Prushan IV.Colligative Properties F. Osmosis and Osmotic Pressure Osmotic Pressure -The pressure created by a column of solution for a system in equilibrium.  osmotic pressure M = molar conc. (mol/L) R = T = Temperature (K) L atm mol K

CHM 112 M. Prushan IV.Colligative Properties F. Osmosis and Osmotic Pressure Reverse Osmosis is Used for Water Purification

CHM 112 M. Prushan Solutions Solutions are homogeneous mixtures of two or more pure substances. In a solution, the solute is dispersed uniformly throughout the solvent.

CHM 112 M. Prushan Solutions How does a solid dissolve into a liquid? What ‘drives’ the dissolution process? What are the energetics of dissolution?

CHM 112 M. Prushan How Does a Solution Form? 1.Solvent molecules attracted to surface ions. 2.Each ion is surrounded by solvent molecules. 3.Enthalpy (  H) changes with each interaction broken or formed. Ionic solid dissolving in water

CHM 112 M. Prushan How Does a Solution Form? 1.Solvent molecules attracted to surface ions. 2.Each ion is surrounded by solvent molecules. 3.Enthalpy (  H) changes with each interaction broken or formed.

CHM 112 M. Prushan How Does a Solution Form The ions are solvated (surrounded by solvent). If the solvent is water, the ions are hydrated. The intermolecular force here is ion- dipole.

CHM 112 M. Prushan Dissolution vs reaction Dissolution is a physical change—you can get back the original solute by evaporating the solvent. If you can’t, the substance didn’t dissolve, it reacted. Ni(s) + HCl(aq)NiCl 2 (aq) + H 2 (g) NiCl 2 (s) dry

CHM 112 M. Prushan Degree of saturation Saturated solution  Solvent holds as much solute as is possible at that temperature.  Undissolved solid remains in flask.  Dissolved solute is in dynamic equilibrium with solid solute particles.

CHM 112 M. Prushan Degree of saturation Unsaturated Solution  Less than the maximum amount of solute for that temperature is dissolved in the solvent.  No solid remains in flask.

CHM 112 M. Prushan Degree of saturation Supersaturated  Solvent holds more solute than is normally possible at that temperature.  These solutions are unstable; crystallization can often be stimulated by adding a “seed crystal” or scratching the side of the flask.

CHM 112 M. Prushan Degree of saturation Unsaturated, Saturated or Supersaturated?  How much solute can be dissolved in a solution?

CHM 112 M. Prushan Factors Affecting Solubility Chemists use the axiom “like dissolves like”:  Polar substances tend to dissolve in polar solvents.  Nonpolar substances tend to dissolve in nonpolar solvents.

CHM 112 M. Prushan Factors Affecting Solubility The stronger the intermolecular attractions between solute and solvent, the more likely the solute will dissolve. Example: ethanol in water Ethanol = CH 3 CH 2 OH Intermolecular forces = H-bonds; dipole-dipole; dispersion Ions in water also have ion-dipole forces.

CHM 112 M. Prushan Factors Affecting Solubility Glucose (which has hydrogen bonding) is very soluble in water. Cyclohexane (which only has dispersion forces) is not water- soluble.

CHM 112 Summer 2007 M. Prushan Factors Affecting Solubility Vitamin A is soluble in nonpolar compounds (like fats). Vitamin C is soluble in water.

CHM 112 Summer 2007 M. Prushan Which vitamin is water-soluble and which is fat-soluble?

CHM 112 M. Prushan Gases in Solution In general, the solubility of gases in water increases with increasing mass. Why? Larger molecules have stronger dispersion forces.

CHM 112 M. Prushan Gases in Solution

CHM 112 M. Prushan Gases in Solution The solubility of liquids and solids does not change appreciably with pressure. But, the solubility of a gas in a liquid is directly proportional to its pressure. Increasing pressure above solution forces more gas to dissolve.

CHM 112 M. Prushan Henry’s Law S g = kP g where S g is the solubility of the gas; k is the Henry’s law constant for that gas in that solvent; P g is the partial pressure of the gas above the liquid.

CHM 112 M. Prushan Temperature Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.

CHM 112 M. Prushan Temperature The opposite is true of gases. Higher temperature drives gases out of solution.  Carbonated soft drinks are more “bubbly” if stored in the refrigerator.  Warm lakes have less O 2 dissolved in them than cool lakes.