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Chapter 13 Properties of SolutionsCHEMISTRY The Central Science 9th Edition Chapter 13 Properties of Solutions David P. White Prentice Hall © 2003 Chapter 13
The Solution Process A solution is a homogeneous mixture of solute (present in smallest amount) and solvent (present in largest amount). Solutes and solvent are components of the solution. In the process of making solutions with condensed phases, intermolecular forces become rearranged. Prentice Hall © 2003 Chapter 13
The Solution Process Prentice Hall © 2003 Chapter 13
The Solution Process 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. We say the ions are solvated by water. If water is the solvent, we say the ions are hydrated. Prentice Hall © 2003 Chapter 13
The Solution Process
The Solution Process Energy Changes and Solution FormationThere are three energy steps in forming a solution: separation of solute molecules (H1), separation of solvent molecules (H2), and formation of solute-solvent interactions (H3). 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. Prentice Hall © 2003 Chapter 13
Prentice Hall © 2003 Chapter 13
The Solution Process Energy Changes and Solution FormationBreaking attractive intermolecular forces is always endothermic. Forming attractive intermolecular forces is always exothermic. Prentice Hall © 2003 Chapter 13
The Solution Process Energy Changes and Solution FormationTo determine whether Hsoln is positive or negative, we consider the strengths of all solute-solute and solute-solvent interactions: H1 and H2 are both positive. H3 is always negative. It is possible to have either H3 > (H1 + H2) or H3 < (H1 + H2). Prentice Hall © 2003 Chapter 13
The Solution Process Energy Changes and Solution Formation Examples:NaOH added to water has Hsoln = kJ/mol. NH4NO3 added to water has Hsoln = kJ/mol. “Rule”: polar solvents dissolve polar solutes. Non-polar solvents dissolve non-polar solutes. Why? If Hsoln is too endothermic a solution will not form. NaCl in gasoline: the ion-dipole forces are weak because gasoline is non-polar. Therefore, the ion-dipole forces do not compensate for the separation of ions. Prentice Hall © 2003 Chapter 13
The Solution Process Energy Changes and Solution FormationWater in octane: water has strong H-bonds. There are no attractive forces between water and octane to compensate for the H-bonds. Prentice Hall © 2003 Chapter 13
The Solution Process Solution Formation, Spontaneity, and DisorderA spontaneous process occurs without outside intervention. When energy of the system decreases (e.g. dropping a book and allowing it to fall to a lower potential energy), the process is spontaneous. Some spontaneous processes do not involve the system moving to a lower energy state (e.g. an endothermic reaction). Prentice Hall © 2003 Chapter 13
The Solution Process Solution Formation, Spontaneity, and DisorderIf the process leads to a greater state of disorder, then the process is spontaneous. Example: a mixture of CCl4 and C6H14 is less ordered than the two separate liquids. Therefore, they spontaneously mix even though Hsoln is very close to zero. There are solutions that form by physical processes and those by chemical processes. Prentice Hall © 2003 Chapter 13
The Solution Process Solution Formation, Spontaneity, and DisorderPrentice Hall © 2003 Chapter 13
Ni(s) + 2HCl(aq) NiCl2(aq) + H2(g).The Solution Process Solution Formation and Chemical Reactions Example: a mixture of CCl4 and C6H14 is less ordered Consider: Ni(s) + 2HCl(aq) NiCl2(aq) + H2(g). Note the chemical form of the substance being dissolved has changed (Ni NiCl2). When all the water is removed from the solution, no Ni is found only NiCl2·6H2O. Therefore, Ni dissolution in HCl is a chemical process. Prentice Hall © 2003 Chapter 13
NaCl(s) + H2O (l) Na+(aq) + Cl-(aq).The Solution Process Solution Formation and Chemical Reactions Example: NaCl(s) + H2O (l) Na+(aq) + Cl-(aq). When the water is removed from the solution, NaCl is found. Therefore, NaCl dissolution is a physical process. Prentice Hall © 2003 Chapter 13
Saturated Solutions and SolubilityDissolve: 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. Prentice Hall © 2003 Chapter 13
Prentice Hall © 2003 Chapter 13
Factors Affecting SolubilitySolute-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: 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. Prentice Hall © 2003 Chapter 13
Factors Affecting SolubilitySolute-Solvent Interaction The number of -OH groups within a molecule 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. Prentice Hall © 2003 Chapter 13
Factors Affecting SolubilitySolute-Solvent Interaction Prentice Hall © 2003 Chapter 13
Factors Affecting SolubilitySolute-Solvent Interaction Prentice Hall © 2003 Chapter 13
Factors Affecting SolubilitySolute-Solvent Interaction Network solids do not dissolve because the strong intermolecular forces in the solid are not re-established in any solution. Pressure Effects Solubility of a gas in a liquid is a function of the pressure of the gas. Prentice Hall © 2003 Chapter 13
Factors Affecting SolubilityPressure Effects Prentice Hall © 2003 Chapter 13
Factors Affecting SolubilityPressure Effects The higher the pressure, the more molecules of gas are close to the solvent and the greater the chance of a gas molecule striking the surface and entering the solution. Therefore, the higher the pressure, the greater the solubility. The lower the pressure, the fewer molecules of gas are close to the solvent and the lower the solubility. 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: Prentice Hall © 2003 Chapter 13
Factors Affecting SolubilityPressure Effects Carbonated beverages are bottled with a partial pressure of CO2 > 1 atm. As the bottle is opened, the partial pressure of CO2 decreases and the solubility of CO2 decreases. Therefore, bubbles of CO2 escape from solution. Prentice Hall © 2003 Chapter 13
Factors Affecting SolubilityTemperature Effects Experience tells us that sugar dissolves better in warm water than cold. As temperature increases, solubility of solids generally increases. Sometimes, solubility decreases as temperature increases (e.g. Ce2(SO4)3). Prentice Hall © 2003 Chapter 13
Factors Affecting SolubilityTemperature Effects Experience tells us that carbonated beverages go flat as they get warm. Therefore, gases get less soluble as temperature increases. Thermal pollution: if lakes get too warm, CO2 and O2 become less soluble and are not available for plants or animals. Prentice Hall © 2003 Chapter 13
Ways of Expressing ConcentrationMass Percentage, ppm, and ppb All methods involve quantifying amount of solute per amount of solvent (or solution). Generally amounts or measures are masses, moles or liters. Qualitatively solutions are dilute or concentrated. Definitions: Prentice Hall © 2003 Chapter 13
Ways of Expressing ConcentrationMass Percentage, ppm, and ppb Parts per million (ppm) can be expressed as 1 mg of solute per kilogram of solution. If the density of the solution is 1g/mL, then 1 ppm = 1 mg solute per liter of solution. Parts per billion (ppb) are 1 g of solute per kilogram of solution. Prentice Hall © 2003 Chapter 13
Ways of Expressing ConcentrationMass Percentage, ppm, and ppb Mole Fraction, Molarity, and Molality Recall mass can be converted to moles using the molar mass. Prentice Hall © 2003 Chapter 13
Ways of Expressing ConcentrationMole Fraction, Molarity, and Molality We define Converting between molarity (M) and molality (m) requires density. Prentice Hall © 2003 Chapter 13
Colligative PropertiesColligative 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. Therefore, vapor pressure is lowered. The amount of vapor pressure lowering depends on the amount of solute. Prentice Hall © 2003 Chapter 13
Colligative PropertiesLowering Vapor Pressure Prentice Hall © 2003 Chapter 13
Colligative PropertiesLowering Vapor Pressure Raoult’s Law: PA is the vapor pressure with solute, PA is the vapor pressure without solvent, and A is the mole fraction of A, then Recall Dalton’s Law: Prentice Hall © 2003 Chapter 13
Colligative PropertiesLowering 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. 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. Prentice Hall © 2003 Chapter 13
Colligative PropertiesBoiling-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 teach a vapor pressure of 1 atm for the solution (Tb). Molal boiling-point-elevation constant, Kb, expresses how much Tb changes with molality, m: Prentice Hall © 2003 Chapter 13
Colligative PropertiesFreezing Point Depression At 1 atm (normal boiling point of pure liquid) there is no depression by definition When a solution freezes, almost pure solvent is formed first. Therefore, the sublimation curve for the pure solvent is the same as for the solution. Therefore, the triple point occurs at a lower temperature because of the lower vapor pressure for the solution. Prentice Hall © 2003 Chapter 13
Colligative PropertiesFreezing Point Depression The melting-point (freezing-point) curve is a vertical line from the triple point. 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): Prentice Hall © 2003 Chapter 13
Colligative PropertiesFreezing Point Depression Prentice Hall © 2003 Chapter 13
Colligative PropertiesOsmosis Semipermeable membrane: permits passage of some components of a solution. Example: cell membranes and cellophane. Osmosis: the movement of a solvent from low solute concentration to high solute concentration. There is movement in both directions across a semipermeable membrane. As solvent moves across the membrane, the fluid levels in the arms becomes uneven. Prentice Hall © 2003 Chapter 13
Colligative PropertiesOsmosis Eventually the pressure difference between the arms stops osmosis. Prentice Hall © 2003 Chapter 13
Colligative PropertiesOsmosis Osmotic pressure, , is the pressure required to stop osmosis: Isotonic solutions: two solutions with the same separated by a semipermeable membrane. Prentice Hall © 2003 Chapter 13
Colligative PropertiesOsmosis Hypotonic solutions: a solution of lower than a hypertonic solution. Osmosis is spontaneous. Red blood cells are surrounded by semipermeable membranes. Prentice Hall © 2003 Chapter 13
Colligative PropertiesOsmosis 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. Prentice Hall © 2003 Chapter 13
Colligative PropertiesOsmosis Prentice Hall © 2003 Chapter 13
Colligative PropertiesOsmosis 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. Prentice Hall © 2003 Chapter 13
Colligative PropertiesOsmosis 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). Prentice Hall © 2003 Chapter 13
Colligative PropertiesOsmosis Active transport is the movement of nutrients and waste material through a biological system. Active transport is not spontaneous. Prentice Hall © 2003 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), Prentice Hall © 2003 Chapter 13
Colloids solid foam (solid + gas, e.g. marshmallow), solid emulsion (solid + liquid, e.g. butter), solid sol (solid + solid, e.g. ruby glass). Tyndall effect: ability of a Colloid to scatter light. The beam of light can be seen through the colloid. Prentice Hall © 2003 Chapter 13
Colloids Prentice Hall © 2003 Chapter 13
Colloids Hydrophilic and Hydrophobic ColloidsFocus on colloids in water. “Water loving” colloids: hydrophilic. “Water hating” colloids: hydrophobic. Molecules arrange themselves so that hydrophobic portions are oriented towards each other. If a large hydrophobic macromolecule (giant molecule) needs to exist in water (e.g. in a cell), hydrophobic molecules embed themselves into the macromolecule leaving the hydrophilic ends to interact with water. Prentice Hall © 2003 Chapter 13
Colloids Hydrophilic and Hydrophobic ColloidsTypical 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. If ions are adsorbed onto the surface of a colloid, the colloids appears hydrophilic and is stabilized in water. Consider a small drop of oil in water. Add to the water sodium stearate. Prentice Hall © 2003 Chapter 13
Colloids Hydrophilic and Hydrophobic Colloids Prentice Hall © 2003Chapter 13
Colloids Hydrophilic and Hydrophobic ColloidsSodium stearate has a long hydrophobic tail (CH3(CH2)16-) and a small hydrophobic head (-CO2-Na+). The hydrophobic tail can be absorbed into the oil drop, leaving the hydrophilic head on the surface. The hydrophilic heads then interact with the water and the oil drop is stabilized in water. Prentice Hall © 2003 Chapter 13
Colloids Hydrophilic and Hydrophobic ColloidsMost 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. Prentice Hall © 2003 Chapter 13
Colloids Removal of Colloidal ParticlesColloid 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. Methods of coagulation: heating (colloid particles move and are attracted to each other when they collide); adding an electrolyte (neutralize the surface charges on the colloid particles). Prentice Hall © 2003 Chapter 13
Colloids Removal of Colloidal ParticlesDialysis: using a semipermeable membranes separate ions from colloidal particles. Prentice Hall © 2003 Chapter 13
End of Chapter 13 Properties of SolutionsPrentice Hall © 2003 Chapter 13
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