Chapter 13 Properties of Solutions CHEMISTRY The Central Science 9th Edition Chapter 13 Properties of Solutions David P. White Chapter 13

The Solution Process A solution is a homogeneous mixture of solute (present in smallest amount) and solvent (present in largest amount). In the process of making solutions with condensed phases, intermolecular forces become rearranged. Chapter 13

Chapter 13

Consider NaCl (solute) dissolving in water (solvent) Chapter 13

Chapter 13

Chapter 13

Energy Changes and Solution Formation There are three energy steps in forming a solution separation of solute molecules (H1) separation of solvent molecules (H2), formation of solute-solvent interactions (H3) Chapter 13

We define the enthalpy change in the solution process as Hsoln = H1 + H2 + H3 Hsoln can either be positive or negative depending on the intermolecular forces Breaking attractive intermolecular forces is always endothermic Forming attractive intermolecular forces is always exothermic Chapter 13

Chapter 13

To determine whether Hsoln is positive or negative, we consider the strengths of all solute-solute and solute-solvent interactions It is possible to have either H3 > (H1 + H2) or H3 < (H1 + H2) Chapter 13

Chapter 13

“Rule”: polar solvents dissolve polar solutes “Rule”: polar solvents dissolve polar solutes. Non-polar solvents dissolve non-polar solutes Water in octane: water has strong H-bonds. There are no attractive forces between water and octane to compensate for the H-bonds Chapter 13

Solution Formation, Spontaneity, and Disorder A spontaneous process occurs without outside intervention. (If leads to greater state of disorder, then process is spontaneous). When energy of the system decreases the process is spontaneous Some spontaneous processes do not involve the system moving to a lower energy state (e.g. an endothermic reaction) Chapter 13

Solution Formation, Spontaneity, and Disorder Chapter 13

Saturated Solutions and Solubility Dissolve: solute + solvent  solution Crystallization: solution  solute + solvent Saturation: crystallization and dissolution are in equilibrium Solubility: amount of solute required to form a saturated solution Supersaturated: a solution formed when more solute is dissolved than in a saturated solution Chapter 13

Chapter 13

Factors Affecting Solubility Solute-Solvent Interaction Polar liquids tend to dissolve in polar solvents Miscible liquids: mix in any proportions Immiscible liquids: do not mix Intermolecular forces are important The number of carbon atoms in a chain affect solubility: the more C atoms the less soluble in water Chapter 13

The number of -OH groups increases solubility in water Generalization: “like dissolves like” The more polar bonds in the molecule, the better it dissolves in a polar solvent The less polar the molecule the less it dissolves in a polar solvent and the better is dissolves in a non-polar solvent Network solids do not dissolve because the strong intermolecular forces in the solid are not re-established in any solution Chapter 13

Solute-Solvent Interaction Chapter 13

Solute-Solvent Interaction Chapter 13

the higher the pressure, the greater the solubility Pressure Effects Solubility of a gas in a liquid is a function of the pressure of the gas the higher the pressure, the greater the solubility lower the pressure, the lower the solubility Chapter 13

Pressure Effects Chapter 13

If Sg is the solubility of a gas, k is a constant, and Pg is the partial pressure of a gas, then Henry’s Law gives Chapter 13

As temperature increases, solubility of solids generally increases Temperature Effects As temperature increases, solubility of solids generally increases Sometimes, solubility decreases as temperature increases (e.g. Ce2(SO4)3) Gases get less soluble as temperature increases Chapter 13

Chapter 13

Chapter 13

Ways of Expressing Concentration Mass Percentage, ppm, and ppb Chapter 13

Ways of Expressing Concentration Mole Fraction, Molarity, and Molality Chapter 13

Chapter 13

Colligative Properties Physical Properties of a solution that depend on the number of solute particles, not the identity of solute particles. Vapor Pressure lowering Boiling point elevation Freezing point depression Osmosis – net movement of solvent molecules through a semi permeable membrane toward a solution with a greater concentration of solute particles. Chapter 13

Colligative Properties Colligative properties depend on quantity of solute molecules. (E.g. freezing point depression and melting point elevation.) Lowering Vapor Pressure Non-volatile solvents reduce the ability of the surface solvent molecules to escape the liquid. The amount the vapor pressure lowered depends on the amount of solute. Chapter 13

Lowering Vapor Pressure Chapter 13

Raoult’s Law: PA is the vapor pressure with solution, PA is the vapor pressure of pure solvent, and A is the mole fraction of solvent, then Chapter 13

Lowering Vapor Pressure Ideal solution: one that obeys Raoult’s law. Raoult’s law breaks down when the solvent-solvent and solute-solute intermolecular forces are greater than solute-solvent intermolecular forces. Chapter 13

Boiling-Point Elevation Goal: interpret the phase diagram for a solution. Non-volatile solute lowers the vapor pressure. Therefore the triple point - critical point curve is lowered. Chapter 13

Chapter 13

Boiling-Point Elevation Molal boiling-point-elevation constant, Kb, expresses how much Tb changes with molality, m: Chapter 13

Freezing Point Depression Decrease in freezing point (Tf) is directly proportional to molality (Kf is the molal freezing-point-depression constant): Chapter 13

Freezing Point Depression Chapter 13

Chapter 13

Eventually, the pressure difference between the arms stops osmosis. Chapter 13

Osmotic pressure, , is the pressure required to stop osmosis: Chapter 13

Hypotonic solutions: a solution of lower  than a hypertonic solution. Isotonic solutions: two solutions with the same  separated by a semipermeable membrane. Hypotonic solutions: a solution of lower  than a hypertonic solution. Osmosis is spontaneous. Red blood cells are surrounded by semipermeable membranes. Chapter 13

Crenation: red blood cells placed in hypertonic solution (relative to intracellular solution); there is a lower solute concentration in the cell than the surrounding tissue; osmosis occurs and water passes through the membrane out of the cell. The cell shrivels up. Chapter 13

Osmosis Chapter 13

Hemolysis: red blood cells placed in a hypotonic solution; there is a higher solute concentration in the cell; osmosis occurs and water moves into the cell. The cell bursts. To prevent crenation or hemolysis, IV (intravenous) solutions must be isotonic. Chapter 13

Salty food causes retention of water and swelling of tissues (edema). Cucumber placed in NaCl solution loses water to shrivel up and become a pickle. Limp carrot placed in water becomes firm because water enters via osmosis. Salty food causes retention of water and swelling of tissues (edema). Water moves into plants through osmosis. Salt added to meat or sugar to fruit prevents bacterial infection (a bacterium placed on the salt will lose water through osmosis and die). Chapter 13

Active transport is not spontaneous. Active transport is the movement of nutrients and waste material through a biological system. Active transport is not spontaneous. Chapter 13

Colloids Colloids are suspensions in which the suspended particles are larger than molecules but too small to drop out of the suspension due to gravity. Particle size: 10 to 2000 Å. There are several types of colloid: aerosol (gas + liquid or solid, e.g. fog and smoke), foam (liquid + gas, e.g. whipped cream), emulsion (liquid + liquid, e.g. milk), sol (liquid + solid, e.g. paint) Chapter 13

Tyndall effect: ability of a Colloid to scatter light Tyndall effect: ability of a Colloid to scatter light. The beam of light can be seen through the colloid. Chapter 13

Chapter 13

“Water loving” colloids: hydrophilic. “Water hating” colloids: hydrophobic. Molecules arrange themselves so that hydrophobic portions are oriented towards each other Typical hydrophilic groups are polar (containing C-O, O-H, N-H bonds) or charged Hydrophobic colloids need to be stabilized in water Adsorption: when something sticks to a surface we say that it is adsorbed Chapter 13

Chapter 13

Hydrophilic and Hydrophobic Colloids Chapter 13

Colloids Chapter 13

Hydrophilic and Hydrophobic Colloids Most dirt stains on people and clothing are oil-based. Soaps are molecules with long hydrophobic tails and hydrophilic heads that remove dirt by stabilizing the colloid in water. Bile excretes substances like sodium stereate that forms an emulsion with fats in our small intestine. Emulsifying agents help form an emulsion. Chapter 13

Colloid particles are too small to be separated by physical means (e.g. filtration) Colloid particles are coagulated (enlarged) until they can be removed by filtration Chapter 13

End of Chapter 13 Properties of Solutions