Published on February 11, 2009
Solubility and Distribution Phenomena : Solubility and Distribution Phenomena Terminology : Terminology Solution - A homogeneous mixture of two or more substances Not a suspension or colloid Solvent - The component of the solution that has the same physical state as the final solution The larger component of the solution Solute - Any component that is dissolved in the solvent The smaller component of the solution Saturated solution - A solution that contains the maximum amount of solute at the prevailing condition (T) Supersaturated solution - A solution that contains the more than the maximum amount of solute at the prevailing condition (T) This is a temporary unstable situation that lead to either precipitation or separation into layers Slide 3: Solutions are homogeneous mixtures which contain small atoms, ions, or molecules. Types of solutions : Types of solutions Solution of gases (Air) homogeneous mixture nitrogen, oxygen, argon and other things Solution of solids (Alloy-bronze) homogeneous mixture of copper and tin (etc) Solution of liquids (Gasoline) homogeneous mixture of various hydrocarbons (octane, etc) Aqueous solutions – most common type of solution Gas in water (natural water containing O2: carbonated beverage) Liquid in water (various alcohols, antifreezes) Solid in water (salt water, countless reagent solutions) Slide 5: 12.1 A solution is a homogenous mixture of 2 or more substances The solute is(are) the substance(s) present in the smaller amount(s) The solvent is the substance present in the larger amount Slide 6: A saturated solution contains the maximum amount of a solute that will dissolve in a given solvent at a specific temperature. An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a specific temperature. A supersaturated solution contains more solute than is present in a saturated solution at a specific temperature. Sodium acetate crystals rapidly form when a seed crystal is added to a supersaturated solution of sodium acetate. 12.1 Slide 7: 12.2 Three types of interactions in the solution process: solvent-solvent interaction solute-solute interaction solvent-solute interaction DHsoln = DH1 + DH2 + DH3 Slide 8: Two substances with similar intermolecular forces are likely to be soluble in each other. non-polar molecules are soluble in non-polar solvents CCl4 in C6H6 polar molecules are soluble in polar solvents C2H5OH in H2O ionic compounds are more soluble in polar solvents NaCl in H2O or NH3 (l) 12.2 Different Kinds of Solutions : Different Kinds of Solutions Like Dissolves in Likepolar-in-polar or nonpolar-in-nonpolar : Like Dissolves in Likepolar-in-polar or nonpolar-in-nonpolar Both entropy and enthalpy need to be considered Solubility : Solubility Polar/polar or ion/polar Strong solvent-solute interactions can dominate. Ion-ion interactions in solute may dominate and make solute insoluble. Non-polar/non polar Solvent-solvent, solute-solute-and solute-solvent are relatively weak. Entropy of mixing becomes important. Non-polar/polar Polar-polar interactions hold solvent together. Molecular Interactions : Molecular Interactions Ionic interactions Dipole-dipole interactions Induced dipoles Hydrogen bond + - - + + - + - + - + - Solvent - Solute Interactions : Solvent - Solute Interactions Polar solvents (e.g., water): (1) owing to high dielectric constant reduce ionic attraction between oppositely charged ions in crystals; (2) break covalent bonds of strong electrolytes resulting in dissociation; (3) solvate molecules through dipole interaction forces (particularly, hydrogen bond). Nonpolar solvents (e.g. hydrocarbons): dissolve nonpolar compounds through induced dipole interactions. Semipolar solvents (ketones, alcohols): can induce polarity in nonpolar solvent molecules (co-solvents). Solvent Polarity : Solvent Polarity Liquid-Liquid Systems : Liquid-Liquid Systems Miscibility: polar and semipolar solvents (e.g. water and alcohol, glycerin and alcohol, alcohol and acetone) are completely miscible in all proportions. Partial miscibility: With certain proportions two liquid layers are formed (e.g., water-ether, phenol-water) Miscibility is characterized by the phase diagram. A 100% A 100% B 100% C B C Miscible LPS Effects on Solubility : Effects on Solubility Pressure: increases solubility of gases (Henry’s law: PB = PBo. ?A xB)Temperature: solubilty of most gases decreases, solubility of liquids and solids may be affected both waysCo-solvents: increase solubility by changing the solvent polarity.Salting out: elaboration of gases from solutions by electrolytes (NaCl) and highly polar nonelectrolytes (glucose) The “Solution Process” = Dissolving : The “Solution Process” = Dissolving Intermolecular forces between solvent molecules (solvent-solvent attractions) have to be disrupted Interparticle forces between solute particles (solute-solute attractions) have to be disrupted New interactions between the solute particles and the solvent molecules (solute-solvent attractions) have to occur The strengths of the new attractions have to be equivalent to or stronger than solvent-solvent and solute-solute attractions Intermolecular Forces : Intermolecular Forces Intermolecular Forces - Forces of attraction between molecules in the liquid, solid or mixed state (solutions) A special category of interparticle forces Types of attractions in liquids and solids London Forces - Instantaneous dipole-dipole attractions that exist between all molecules, nonpolar as well as polar. Only intermolecular attraction in nonpolar substances . Dipole-dipole Forces - Electrostatic attractions between the positive end of one polar molecule and the negative end of another polar molecule Present in polar substances Hydrogen Bonding - Dipole-dipole attractions between a hydrogen atom covalently bonded to N, O or F atom on one molecule and a N, O or F atom on another molecule an especially strong dipole- dipole attraction Types of attractions in solutions Later when discussing solutions The “hydrogen bond” : The “hydrogen bond” H- Bond d+ d+ d+ d- d+ d- Gases are more soluble at low temperature : Gases are more soluble at low temperature Henry’s Law: Solubility Proportional to Pressure solubility = kP : Henry’s Law: Solubility Proportional to Pressure solubility = kP Slide 22: Solubility - The amount of solute that will dissolve in a given quantity of solvent at a given temperature. Saturated Solution - A solution that contains an amount of solute that is equal to its solubility. The dissolved solute in the solution is in dynamic equilibrium with the un- dissolved solute (the precipitate). Dissolved solute Precipitate The rate of dissolving is equal to the rate of precipitation. This is a dynamic equilibrium. Slide 23: Unsaturated Solution - A solution that contains an amount of solute that is less than its solubility. All of the solute present is dissolved in an unsaturated solution. Supersaturated Solution - A solution that contains an amount of solute that is more than its solubility. This is a “metastable” state (“without stability”). If disturbed in anyway, the excess solute will precipitate out of solution and a saturated solution will result. Vapor-Pressure Lowering of Solutions: Raoult’s Law : Vapor-Pressure Lowering of Solutions: Raoult’s Law Raoult’s Law: Psoln = PsolvxXsolv Non–volatile solute: vapor pressure decreases upon addition of solute. Linear for dilute solutions Vapor pressure lowering : ?P = Po ? P = Po(1?Xsolv) E.g. Determine vapor pressure lowering when 5.00 g of sucrose added to 100.0 g of H2O. FM(sucrose) = 342.3 g/mol. The vapor pressure of water at 25°C is 23.8 mmHg. E.g. 2 Determine the mass of sucrose dissolved in 100.0 g of water if the vapor pressure was 20.0 mmHg. Slide 25: Raoult’s Law: The vapor pressure of a solution that con- tains a non-volatile solute is directly proportional to the mole fraction of the solvent in the solution. PA ? ?A PA = ?APA o “Ideal solutions” containing more than one volatile component also obey Raoult’s Law. PA = ?APA PB = ?BPB o o Ptotal = PA + PB = ?APA + ?BPB o o Factors Affecting Solubility : Factors Affecting Solubility Solute-Solvent Interactions types of solute/solvent -- polar or non-polar like dissolves like pairs of liquids which are soluble in all proportions are miscible pairs which are not soluble are immiscible Pressure solubility of gases increase with increasing pressure increasing mass Slide 28: Henry’s Law Cg = kPg Cg is solubility of the gas, Pg is partial pressure of gas, k is a constant Temperature generally, solubility of solids increases with increasing temperature solubility of gases decreases with increasing temperature Colligative Properties : Colligative Properties Properties which depend on the quantity of solute: Vapor Pressure Lowering addition of a nonvolatile solute lowers the vapor pressure of the solvent extent of lowering depends on concentration of solute, described by Raoult’s Law Raoult’s Law Psolvent = XsolventP°solvent Solubility of Gases in Liquids : Solubility of Gases in Liquids Factors affecting the Solubility of Gases in Liquids : Factors affecting the Solubility of Gases in Liquids The solubility of a gas in a liquid is the concentration of the dissolved gas when it is at equilibrium with some of the undissolved gas above the solution. This solubility is dependant on Temperature, Pressure, Salts present, Chemical reactions, and Micelles solubilization. Effect of Pressure : Effect of Pressure Deals with Henry's Law in that the concentration of dissolved gas is directly proportional to the partial pressure of the atmosphere at a constant temperature. If pressure is increased then the solubility increases. C2 a P C2 = sP Slide 34: Solubility of Gases Pressure Solubility (g /100 g of H20) O2 N2 He Henry’s Law Slide 35: Applications Preparation of carbonated beverages, beer, Champagne etc. Determination of solubility of gases in liquid. Limitation Henry’s law is strictly applicable, only when When the temperature is maintained constant When the gas does not involve in chemical reaction with the solvent When the gas is slightly soluble in liquid Effect of Temperature : Effect of Temperature Increase in temperature decreases the solubility of the gases, This is due to Ability of the gas to expand at higher temperatures Increase in pressure at the elevated temperatures Therefore the lower temperature is preferred for storage of these solutions. Applications Dissolved gases are removed by heating the solution Distilled water for parenteral use is maintained at 80 0C. Handling of solutions One has to be very careful while handling the solutions of Ammonia, liquid bromine and chlorine Effect of the Presence of Salt : Effect of the Presence of Salt Gases are released from a liquid by an introduction of a salt. (Salting out) This is due to greater affinity of the electrolytes towards the water molecules resulting in weakening of the gas solvent interactions. Therefore if a salt is added then the solubility of the gas decreases. Effect of Chemical Reactions : Effect of Chemical Reactions The solubility of a gas will increase if the gas reacts with the solvent. For these solutions Henry’s law is not applicable. E.g. Ammonia and Carbon dioxide solutions. Application Preparation of reagents such as Hydrochloric acid, sulphuric acid and nitric acid Effect of Micellar Solubilization : Effect of Micellar Solubilization Putting the gas molecules inside micelles increases the solubility of the gas. Micelles are used to increase the solubility of a non-polar solute (gas) in a polar solvent. An example of this in the body is the lungs. The lungs allow the non-polar air to be transported through the polar water to the tissues. This is accomplished by a micellar transport system. Solubility of Liquids in Liquids : Solubility of Liquids in Liquids It is very common for two or more liquids to be mixed together in a pharmacy to make a solution, therefore the pharmacist needs to know what liquids can be mixed together without precipitation occurring. Miscibility of Liquids : Miscibility of Liquids Liquid-liquid systems are divided into two categories depending on the solubility of one substance in the other. The categories are complete miscibility and partial miscibility. Miscibility is the common solubilities of the components in liquid-liquid systems. Complete Miscibility Partial Miscibility Complete Miscibility Complete miscibility is when the substances mix in all proportions. Examples of these are alcohol and water, alcohol and acetone, and benzene and carbon tetrachloride. Partial Miscibility Partial miscibility is when the substances only mix partially. When mixed there are two layers formed each layer containing some of both liquids. Examples are phenol and water. The Influence of Foreign Substances on Solubility : The Influence of Foreign Substances on Solubility The influence of a foreign substance on a liquid-liquid system is similar to the idea of a three component system in the phase rule we studied earlier. Remember that if the new substance is soluble in both of the other ones then they will all be soluble in each other. This is called blending. An example of this is when succinic acid is added to the water-phenol mixture , succinic acid is soluble or completely miscible in each water and phenol therefore it causes a blending of the liquids making the mixture one phase. The Dielectric Constant and Solubility : The Dielectric Constant and Solubility As we have already discussed the dielectric constant of a substance effects the solubility of that substance. If needed to see General Mechanisms of Solvent-Solute Interactions. It says that the polarity is dependent on the dielectric constant. Also remember that LIKE DISSOLVES LIKE. This figure shows how the dielectric constant is related to solubility.