Solutions Chapter 14

Key concepts 1.Understand the solvation process at the molecular level. 2.Be able to qualitatively describe energy changes during solution formation. 3.Understand how entropy affects the solution process. 4.Know the terms solute, solvent, saturated, unsaturated, and supersaturated. 5.Know factors contributing to miscibility and immiscibility. 6.Know how pressure affects solubility of gases: Henry’s law 7.Understand the effects of colligative properties: Raoult’s Law, freezing/boiling point changes, and osmosis. 8.Know what a colloidal suspension is and how it may differ from a solution.

Solution process When intermolecular attractions between solute molecules are overcome by intermolecular attractions with solvent. solute breaks up to become dissolved within solvent. process of solvation

Solids and liquids Overcoming intermolecular forces between solvent molecules (h-bonds in water)

Solids and liquids Overcoming intermolecular forces between solute molecules/ions

Solids and liquids Solvent molecules surround and interact with solute (solvation) Solvation energy (hydration energy) = step b + step c

Solvation and heat The heat flow associated with formation of a solution is called the heat of solution (  H solution ) If solvation process is exothermic, then  H solution is ___________. If solvation process is endothermic, then  H solution is ___________. DEMO: Ammonium nitrate vs calcium chloride

Two thermodynamic principles of solution formation 1.spontaneous processes tend to be exothermic (  H solution is ?) 2.processes where entropy increases tend to occur spontaneously –solution formation favored by an increase in entropy –Solution formation may involve an endothermic process IF the entropy change outweighs the need to absorb heat.

Liquid-liquid interactions solute-solvent interactions: like dissolves like miscible: immiscible: Molecular “red rover”…

Dissolving gases in liquids Miscibility rules for liquids also apply to gas/liquid solutions. Usually, aqueous solutions of gases occur when –Hydrogen bonding is possible with the gas –The gas ionizes extensively in water –Rxn between gas and water takes place

dynamic equilibrium of solutions solvent + solute  solution saturated solution: in equilibrium with undissolved solute unsaturated solution: not enough solute available to reach equilibrium.

crystallization for crystallization to take place, two conditions must be met: 1.solution must reach saturation point 2.crystal order must be established Supersaturated solution: an unstable solution with a concentration higher than the equilibrium concentration. –Solution is beyond normal saturation point, but a crystal order has not yet established.

Temperature effects on solubility Increasing temperature may increase OR decrease solubility. Generally… Solubility of substances that undergo endothermic dissolution ________ with increasing temperature Solubility of substances that undergo exothermic dissolution ________ with increasing temperature

Dissolving gases in solution: why your pop bottle fizzes over…. The solubility of a gas is directly proportional to partial pressure of the gas. if pressure on gas increases, the solubility of the gas also increases. Henry’s law P g = k C g solubility and temperature:

Colligative properties Physical properties of solutions that depend on the total number of solutes (not type of solute) are colligative properties. We will look at several types of colligative properties.

Raoult’s Law: solutes and vapor pressure Partial pressure of a liquid solvent is dependent upon the amount of solute it contains. P solvent = X solvent P 0 solvent This form of the law applies only to non- volatile, non-ionizing solutes in ideal solutions.

Raoult’s law for two volatile liquids Two volatile liquids mutually effect each other’s partial pressures. P A = X A P 0 A P B = X B P 0 B

Deviations from ideality Positive and negative Depend on interactions between the two substances. Mixtures of polar/non-polar substances usually lead to _______ deviations of the ideal pressure. Mixtures of polar substances usually lead to ________ deviations of the ideal pressure.

Fractional distillation A process by which two volatile liquids are separated. Each time boiling and condensation takes place, the resulting liquid contains an increasing amount of the more volatile component. Fractional distillation columns are designed to facilitate this process.

Boiling point elevation/ freezing point depression  T = k f m or  T = k b m  T is change in temperature Must include molality of all solutes in solution. K f/b constants have been determined for several substances… (p 556) What is the change in freezing point temperature for a molal solution of sucrose in water? What is the change in freezing point temperature for a molal solution of K 2 CO 3 in water?

Ion association Ions in solution occasionally “stick” to each other. When this happens, the combination behaves as one particle in solution. The “effective concentration” of ions in solution depends on the amount of association that occurs.

Physical behavior of ionic solutions H2OH2O H2OH2O H2OH2OH2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O Near ideal ionic solutions H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O Non-ideal ionic solutions

van’t Hoff factor Table 14-3 (p. 561). What happens as the number of ions per formula unit increases? What will the actual  T f be for a m solution of K 2 CO 3 ?

Osmotic pressure: making pickles Height of column  pressure head from osmosis h

Calculating osmotic pressure For dilute solutions, the osmotic pressure is given by  = MRT Example: molar solution of sucrose in water at 20  C. Osmotic pressure plays a key role in biological processes.

Molecular mass of polymers A 2.30 g sample of a polymer is dissolved in 250 mL of water, resulting in an osmotic pressure of 5.40 torr. What is the molecular mass of the polymer?

Reverse osmosis If a pressure is applied to the solution side of the system, greater than the osmotic pressure (2.40 atm in our sugar example), pure liquid can be forced out of the membrane. –Everything moves in the opposite direction –Desalination of seawater—26 atm pressure required.

Colloids Colloids, or colloidal suspensions, are in an “in- between” area between homogeneous and heterogenous mixtures. Solute-like particles are dispersed in a solvent- like medium. Colloidal suspensions disperse and scatter light (Tyndall effect) Example: milk.

Hydrophilic colloids Hydrophilic colloids form between water and dispersed phases containing polar groups about their surfaces. Example: hemoglobin

Hydrophobic colloids Hydrophobic dispersed particles are not attracted to water. In order to produce a colloidal suspension, we must introduce an emulsifying agent. –How to make your own mayo…. Soaps are excellent examples of emulsifying agents.

Micelles Contain a “polar head” and a “non-polar tail”. Non-polar side attracts to non-polar substances (dirt, grease, etc). Polar side attracts to water. Micelles surround non- polar particle, suspending it in aqueous solution.