Solutions Chapter 11

Why do solutions form? Energy lowering? Increase in disorder? (enthalpy) (entropy) ∆Hsln: separate solute ; ∆H1 endothermic separate solvent ; ∆H2 endothermic solute-solvent ; ∆H3 exothermic interactions

∆Hsln= ∆H1 + ∆H2 + ∆H3 ∆Hhydration ∆H2 + ∆H3 No s`eparation of solute b/c H2O just surrounds molecule

Solution A mixture of two substances that has a uniform compostition Can be Solid – liquid (where solid dissolves) Solid –Solid Liquid – liquid Gas –gas Liquid - gas

3 Factors Affecting Solubility Structure Polar Solvent (H2O) Gaseous Solute More LDF (more e-) = more soluble More polar….more soluble Liquid Solute: More polar = more soluble Solid Solute: Ionic Lower LE = more soluble Covalent solids – polar= more soluble

Non-Polar solvents (CCl4, C6H6- benzene, C10H8-toluene) Organic compounds – alcohols C-H bond is non-polar Not soluble (hydrophobic) Alcohols C---------C-O-H O-H is polar….soluble More O-H bonds = more soluble Non-Polar solvents (CCl4, C6H6- benzene, C10H8-toluene) Non polar solutes

Pressure Gas-gas solution Gas – liquid No effect (Boyle’s Law) Greater the pressure, the greater the solubility of a gas Henry’s Law- the amount of gas dissolved is directly porportional to pressure Has limitations: closely obeyed for dilute slns. Of gases that do not react or dissociate O2 and H2O – obey HCl and H2O –No SO2 and H2O-

Temperature- Gas- Gas sln Gas-liquid sln Solid liquid solution No affect Gas-liquid sln Increase temp = decrease solubility Solid liquid solution Depends on ∆Hsln Hard to predict so must be tested in lab

Reminder of solution Concentrations Molarity – 5M “five molar” Moles solute/L solution Mass Percent Mass solute/mass solution Mole fraction (usually used with gases) Moles substance/total moles of mixture Molality = 5m “five molal” Moles solute/kg solvent

Colligative Properties Colligative properties are properties of a solution that depend only on the number of solute particles not the identity of the solute. Freezing Point Depression Boiling point elevation Vapor Pressure lowering Osmotic pressure

Vapor Pressure Lowering The addition of nonvolatile solute lowers the vapor pressure of the liquid solvent.  PA = XAPA Ex 1: The vapor pressure of water at 30C is 31.8 mm Hg. What is the vapor pressure of a solution prepared by dissolving 396 g of sucrose (C12H22O11) in 624 g of water?

PA = XAPA PB = XBPB PA + PB = Ptotal Two or more volatile components: PA = XAPA PB = XBPB PA + PB = Ptotal   (Raoult’s Law) Ex 2: The vapor pressures of ethanol (C2H5OH) and methanol (CH3OH) are 44 mm Hg and 94 mm Hg at 20C. A solution is prepared by mixing 30.0 g of methanol with 40.0 g of ethanol. What is the vapor pressure of the solution and what is the mole fraction of methanol in the vapor?

Ideal Solution The vapor pressure of the mixture is = the sum of the vapor pressures of each substance Raoult’s Law Positive deviation adhesive forces b/t A and B weaker than cohesive forces between A-A and B-B Solute-solvent – no attraction ….more evaporation….observed pressure > calculated Negative deviation adhesive forces between A and B are stronger than the cohesive forces of A-A to B-B solute-solvent attraction, less evaporation, lower V.P. observed pressure < calculated

Boiling Point Elevation The addition of a nonvolatile solute raises the boiling point of the solvent. Tb = kb m Freezing Point Depression The addition of a nonvolatile solute lowers the freezing point of the solvent. Tf = kf m

Ex 3: What are the freezing point and boiling point of a solution prepared by dissolving 651 g of ethylene glycol, CH2(OH)CH2(OH), in 2505 g of water?   Ex 4: A solution of 0.85 g of an organic compound in 100.0 g of benzene has a freezing point of 5.16C. Determine the molar mass of the solute.

Osmotic Pressure  = MRT Osmosis is the flow of solvent through a semi-permeable membrane. The osmotic pressure is the pressure required to just stop this osmosis.    = MRT Dilute solutions (small concentrations) can produce large, easily measured osmotic pressures 

Reverse Osmosis Semi –permeable membrane only allows water through.

Hypotonic with respect to urea--- flows with the concentration gradient Remove water and urea