The Chemistry of Solutes and Solutions Chapter 15 The Chemistry of Solutes and Solutions William Henry 1774-1836. Henry’s Law. Francois Raoult 1830-1901. Raoult’s Law. 1 1 1 1

The Solution Process Na+ O H Cl- H O Solutes and solvent are components of the solution. In the process of making solutions with condensed phases, intermolecular forces become rearranged. Consider NaCl (solute) dissolving in water (solvent): the water H-bonds have to be interrupted, NaCl dissociates into Na+ and Cl-, ion-dipole forces form: Na+ … -OH2 and Cl- … +H2O. Na+ O δ- H Cl- H O δ+

The Solution Process

The Solution Process “Rule”: LIKE DISSOLVES LIKE polar solvents dissolve polar solutes. Non-polar solvents dissolve non-polar solutes. Water is polar (because it’s bent). It will therefore tend to dissolve other polar molecules or ions. For example, most salts, alcohols and sugars dissolve in water. Alcohols and sugars all contain the O-H part of a molecule which makes them polar: O X . . H

The Solution Process Most organic substances (compounds of carbon) are non-polar. That is why they in general do not dissolve in water (which is polar). C H Generally, carbon chains are non-polar (no dipole moment). Since the electronegativities of hydrogen and carbon are virtually the same, hydrocarbons are non-polar. Gasoline is non-polar, because it is a hydrocarbon.

Solute-Solvent Interactions Miscible liquids: mix in any proportions. Immiscible liquids: do not mix. Soluble liquids: mix in certain proportions. Intermolecular forces are important: water and ethanol are miscible because the broken hydrogen bonds in both pure liquids are re-established in the mixture. The number of carbon atoms in a chain affect solubility: the more C atoms the less soluble in water.

Factors Affecting Solubility Solute-Solvent Interactions ethanol ---- ethanol ethanol ---- water

Factors Affecting Solubility Solute-Solvent Interactions The number of -OH groups within a molecule increases solubility in water. Glucose – (a sugar)

Ways of Expressing Concentration All methods involve quantifying amount of solute per amount of solvent or solution. Amounts are masses, moles or liters. Qualitatively solutions are dilute or concentrated. Definitions: soln in component of mass Mass fraction of component = soln of mass total mass of component in soln 6 component of ppm = ´ 10 total mass of soln Mass % = mass fraction ´ 100

Ways of Expressing Concentration Mole Fraction, Molarity, and Molality moles of solute solute of fraction Mole = total moles of all components X M moles of solute Molality = kg of solvent m

The Composition of the Oceans Concentrations in ppm and Molarity Chloride 19350 0.55 Sodium 10760 0.47 Sulfate 2710 0.028 Magnesium 1290 0.054 Calcium 412 0.010 Potassium 400 0.010 Carbon dioxide* 106 2.3 x 10-4 Bromide 67 8.3 x 10-4 Boric acid 27 4.3 x 10-4 Strontium 7.9 9.1 x 10-5 Fluoride 1.3 7.0 x 10-5 * present as bicarbonate and carbonate

Factors Affecting Solubility Pressure Effects: solubility of a gas in a liquid is a function of the pressure of the gas as given by: Henry’s Law: Cg is the solubility of gas, Pg the partial pressure, k = Henry’s law constant. Carbonated beverages are bottled under PCO2 > 1 atm. As the bottle is opened, PCO2 decreases and the solubility of CO2 decreases. Therefore, bubbles of CO2 escape from solution.

Temperature effects dissolving gases dissolving solids opening pop bottles thermal pollution in lakes dissolving solids sugar dissolves better in warm water

Lowering the Vapor Pressure Non-volatile solutes reduce the ability of the surface solvent molecules to escape the liquid. Therefore, vapor pressure is lowered. The amount of vapor pressure lowering depends on the amount of solute (this is a colligative property)

Colligative Properties Colligative properties depend on number of (nonvolatile) solute molecules. Some colligative properties: Vapor pressure lowering Boiling point elevation Freezing point depression Osmotic pressure

Colligative Properties Raoult’s Law (used for vapor pressure lowering) PA is the vapor pressure of A in a solution PA is the vapor pressure of pure A A is the mole fraction of A in solution,

Colligative Properties Boiling-Point Elevation

Colligative Properties Boiling-Point Elevation At 1 atm (normal boiling point of pure liquid) there is a lower vapor pressure of the solution. Therefore, a higher temperature is required to reach a vapor pressure of 1 atm for the solution (Tb). Molal boiling-point-elevation constant, Kb, expresses how much Tb changes with molality, m: Kb depends only on solvent

Colligative Properties Freezing-Point Depression The solution freezes at a lower temperature (Tf) than the pure solvent. Decrease in freezing point (Tf) is directly proportional to molality (Kf is the molal freezing-point-depression constant): Kf depends only on solvent

Figure: 13-T04 ΔTb =Kbm ΔTf=Kfm

Colligative Properties Can be used to determine molecular mass of solute. Problem: Soln of 5.00 g lauryl alcohol (solute) dissolved in 100 g benzene (solvent) freezes at 4.1oC. Calculate molar mass of lauryl alcohol. Use ΔTf= Kfm; ΔTf = 4.1oC ΔTf=5.5-4.1=1.4oC Kf = 5.12oC/m, fp = 5.5° for benzene m= ΔTf/Kf = 1.4oC/(5.12oC/m) = 0.27 m molality is mol solute/kg solvent

OSMOSIS Water Semipermeable membrane more concentrated solution less concentrated solution H2O H2O Particles of solute H2O Particles of solute H2O Water more concentrated less concentrated

Osmotic pressure causes a difference in levels of water Figure: 13-23

Figure: 13-24

Colligative Properties Osmosis Osmotic pressure, , is the pressure required to stop osmosis: Isotonic solutions: two solutions with the same  separated by a semipermeable membrane. Hypotonic solutions: a solution of lower  (less concentrated) than a hypertonic solution (more concentrated). Red blood cell walls are semipermeable membranes.

Salty food causes retention Colligative Properties Salty food causes retention of water and swelling of tissues (edema) Osmosis Crenation and Hemolysis: (Cell in hypertonic solution) (Cell in hypotonic solution) Crenation (shrivels up) Hemolysis (bursts)

TOTAL NUMBER OF PARTICLES Colligative properties depend on total numbers of particles, and salts give more than 1 mole of particles per mole of compound. A 1.0 m solution of NaCl provides 1.0 mol of Na+ and 1.0 mol of Cl- Thus, 1.0 mol of NaCl provides 2.0 mol of ions (particles) The colligative property is enhanced by a factor of 2. Other examples: 1.0 mol of Na2SO4 provides 3.0 mol of ions. 1.0 mol of (NH4)3PO4 provides 4.0 mol of ions. Covalent molecules like sucrose or ethylene glycol do not ionize.

Colloids Colloids are huge molecules which create suspensions with properties between those of mixtures and solutions. Examples: smoke, fog, mayonnaise, marshmallows, emulsions, colored glass. The Tyndall effect is used to characterize liquid colloids in water: a beam of light through a colloidal suspension can be seen from the side (because of the scattered light); a beam of light through a true solution is not visible from the side.