CHAPTER 4-3.pptx

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CHAPTER 4Physical Transformation of Pure SubstancesPrentice Hall 2003Chapter 13

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Some Properties of LiquidsViscosityViscosity is the resistance of a liquid to flow.A liquid flows by sliding molecules over each other.The stronger the intermolecular forces, the higher the viscosity.Surface TensionBulk molecules (those in the liquid) are equally attracted to their neighbours.

Prentice Hall 2003Chapter 11

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Surface Tension

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Some Properties of LiquidsSurface TensionSurface molecules are only attracted inwards towards the bulk molecules.Therefore, surface molecules are packed more closely than bulk molecules.Surface tension is the amount of energy required to increase the surface area of a liquid.Cohesive forces bind molecules to each other.Adhesive forces bind molecules to a surface.


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Some Properties of LiquidsSurface TensionMeniscus is the shape of the liquid surface. If adhesive forces are greater than cohesive forces, the liquid surface is attracted to its container more than the bulk molecules. Therefore, the meniscus is U-shaped (e.g. water in glass).If cohesive forces are greater than adhesive forces, the meniscus is curved downwards.Capillary Action: When a narrow glass tube is placed in water, the meniscus pulls the water up the tube.


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Phase ChangesSurface molecules are only attracted inwards towards the bulk molecules.Sublimation: solid gas. Hsub > 0 (endothermic). Vaporization: liquid gas. Hvap > 0 (endothermic).Melting or fusion: solid liquid. Hfus > 0 (endothermic).Deposition: gas solid. Hdep < 0 (exothermic). Condensation: gas liquid. Hcon < 0 (exothermic).Freezing: liquid solid. Hfre < 0 (exothermic).Prentice Hall 2003Chapter 11

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Phase Changes

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Phase ChangesCritical Temperature and PressureGases liquefied by increasing pressure at some temperature.Critical temperature: the minimum temperature for liquefaction of a gas using pressure.Critical pressure: pressure required for liquefaction.


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Vapor PressureExplaining Vapor Pressure on the Molecular LevelSome of the molecules on the surface of a liquid have enough energy to escape the attraction of the bulk liquid.These molecules move into the gas phase.As the number of molecules in the gas phase increases, some of the gas phase molecules strike the surface and return to the liquid.After some time the pressure of the gas will be constant at the vapor pressure.


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Vapor PressureExplaining Vapor Pressure on the Molecular LevelPrentice Hall 2003Chapter 11

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Vapor PressureExplaining Vapor Pressure on the Molecular LevelDynamic Equilibrium: the point when as many molecules escape the surface as strike the surface evaporation and condensation occur at equal rate.Vapor pressure is the pressure exerted when the liquid and vapor are in dynamic equilibrium.Volatility, Vapor Pressure, and TemperatureIf equilibrium is never established then the liquid evaporates.Volatile substances evaporate rapidly.Vapor pressure increases with increase in temperature.


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Vapor PressureVapor Pressure and Boiling PointLiquids boil when the external pressure equals the vapor pressure.Temperature of boiling point increases as pressure increases.


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Phase DiagramsPhase diagram: plot of pressure vs. temperature summarizing all equilibria between phases.Given a temperature and pressure, phase diagrams tell us which phase will exist.Any temperature and pressure combination not on a curve represents a single phase.


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Phase DiagramsFeatures of a phase diagram:Triple point (A): temperature and pressure at which all three phases are in equilibrium. Vapor-pressure curve (A to B): generally as pressure increases, temperature increases.Critical point (B): critical temperature and pressure for the gas (beyond this liquid and gas become indistinguishable)Melting point curve: as pressure increases, the solid phase is favored if the solid is more dense than the liquid.Normal melting point: melting point at 1 atm.Prentice Hall 2003Chapter 11

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Phase DiagramsPrentice Hall 2003Chapter 11

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Phase DiagramsThe Phase Diagrams of H2O and CO2


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Phase DiagramsThe Phase Diagrams of H2O and CO2Water:The melting point curve slopes to the left because ice is less dense than water.Triple point occurs at 0.0098C and 4.58 mmHg.Normal melting (freezing) point is 0C.Normal boiling point is 100C.Critical point is 374C and 218 atm.Prentice Hall 2003Chapter 11

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Phase DiagramsThe Phase Diagrams of H2O and CO2Carbon Dioxide:Triple point occurs at -56.4C and 5.11 atm.Normal sublimation point is -78.5C. (At 1 atm CO2 sublimes it does not melt.)Critical point occurs at 31.1C and 73 atm.


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CHAPTER 4Properties of SolutionsPrentice Hall 2003Chapter 13

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The Solution ProcessA 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.


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The Solution ProcessPrentice Hall 2003Chapter 13

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The Solution ProcessEnergy Changes and Solution FormationThere are three energy steps in forming a solution:separation of solute molecules (H1),separation of solvent molecules (H2), formation of solute-solvent interactions (H3).


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The Solution ProcessEnergy Changes and Solution FormationWe 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.


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The Solution ProcessEnergy Changes and Solution FormationBreaking attractive intermolecular forces is always endothermic.Forming attractive intermolecular forces is always exothermic.


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The Solution ProcessEnergy 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).


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The Solution ProcessEnergy Changes and Solution FormationExamples: NaOH added to water has Hsoln = -44.48 kJ/mol.NH4NO3 added to water has Hsoln = + 26.4 kJ/mol.Rule: polar solvents dissolve polar solutes. Non-polar solvents dissolve non-polar solutesIf 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.


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The Solution ProcessEnergy 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.


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The Solution ProcessSolution 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).


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The Solution ProcessSolution 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.


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The Solution ProcessSolution Formation, Spontaneity, and DisorderPrentice Hall 2003Chapter 13

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The Solution ProcessSolution Formation and Chemical ReactionsExample: a mixture of CCl4 and C6H14 is less orderedConsider: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 NiCl26H2O. Therefore, Ni dissolution in HCl is a chemical process.


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The Solution ProcessSolution Formation and Chemical ReactionsExample: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.


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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.


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Factors Affecting SolubilitySolute-Solvent InteractionPolar 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.


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Factors Affecting SolubilitySolute-Solvent InteractionThe 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.


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Factors Affecting SolubilitySolute-Solvent Interaction


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Factors Affecting SolubilitySolute-Solvent Interaction


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Factors Affecting SolubilitySolute-Solvent InteractionNetwork solids do not dissolve because the strong intermolecular forces in the solid are not re-established in any solution.Pressure EffectsSolubility of a gas in a liquid is a function of the pressure of the gas.


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Factors Affecting SolubilityPressure Effects


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Factors Affecting SolubilityPressure EffectsThe 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 Henrys Law gives:


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Factors Affecting SolubilityPressure EffectsCarbonated 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 2003Chapter 13

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Factors Affecting SolubilityTemperature EffectsExperience 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).


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Factors Affecting SolubilityTemperature EffectsExperience 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.


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Ways of Expressing ConcentrationMass Percentage, ppm, and ppbAll 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:


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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.


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Ways of Expressing ConcentrationMass Percentage, ppm, and ppb

Mole Fraction, Molarity, and MolalityRecall mass can be converted to moles using the molar mass.


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Ways of Expressing ConcentrationMole Fraction, Molarity, and MolalityWe define

Converting between molarity (M) and molality (m) requires density.


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Colligative PropertiesColligative properties depend on quantity of solute molecules. (E.g. freezing point depression and melting point elevation.)Lowering Vapor PressureNon-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.


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Colligative PropertiesLowering Vapor PressurePrentice Hall 2003Chapter 13

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Colligative PropertiesLowering Vapor PressureRaoults 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 Daltons Law:


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Colligative PropertiesLowering Vapor PressureIdeal solution: one that obeys Raoults law.Raoults law breaks down when the solvent-solvent and solute-solute intermolecular forces are greater than solute-solvent intermolecular forces.Boiling-Point ElevationGoal: interpret the phase diagram for a solution.Non-volatile solute lowers the vapor pressure.Therefore the triple point - critical point curve is loweredPrentice Hall 2003Chapter 13.

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Colligative PropertiesBoiling-Point ElevationAt 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 2003Chapter 13

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Colligative PropertiesFreezing Point DepressionAt 1 atm (normal boiling point of pure liquid) there is no depression by definitionWhen 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.


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Colligative PropertiesFreezing Point DepressionThe 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):


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Colligative PropertiesFreezing Point Depression


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Colligative PropertiesOsmosisSemipermeable 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.Eventually the pressure difference between the arms stops osmosis.


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Colligative PropertiesOsmosisPrentice Hall 2003Chapter 13

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Colligative PropertiesOsmosisOsmotic pressure, , is the pressure required to stop osmosis:

Isotonic solutions: two solutions with the same separated by a semipermeable membrane.Prentice Hall 2003Chapter 13

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Colligative PropertiesOsmosisHypotonic solutions: a solution of lower than a hypertonic solution.Osmosis is spontaneous.Red blood cells are surrounded by semipermeable membranes.


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Colligative PropertiesOsmosisCrenation: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.


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Colligative PropertiesOsmosisPrentice Hall 2003Chapter 13

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Colligative PropertiesOsmosisHemolysis: 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.


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Colligative PropertiesOsmosisCucumber 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).


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


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