Solutions and their properties Part I: What’s a solution: The process of dissolving Factors affecting solubility Ways of expressing concentration Part II: Colligative properties

Learning objectives Describe a solutionDescribe a solution Describe factors influencing solubilityDescribe factors influencing solubility Predict solubility based on molecular propertiesPredict solubility based on molecular properties Describe effect of temperature on solubilityDescribe effect of temperature on solubility Predict solubility of gases using Henry’s lawPredict solubility of gases using Henry’s law Perform concentration calculations using molarity, molality, mole fraction and percent by mass and volumePerform concentration calculations using molarity, molality, mole fraction and percent by mass and volume

Mixtures are a fact of life We learn about matter by studying pure substancesWe learn about matter by studying pure substances In nature, most things are mixtures – nothing is “pure”:In nature, most things are mixtures – nothing is “pure”: –Air –Water –Soil –Living systems

Solution or colloid? Both contain a solid dispersed in a liquidBoth contain a solid dispersed in a liquid In a solution, the particle size is on the molecular scale (< 1 nm)In a solution, the particle size is on the molecular scale (< 1 nm) In a colloid, the particles are larger (2 – 500 nm)In a colloid, the particles are larger (2 – 500 nm) Colloids scatter light – milk, fogColloids scatter light – milk, fog

Why do solutions form? Things tend to get mixed up (entropy)Things tend to get mixed up (entropy) Without any interactions between molecules, gases always mixWithout any interactions between molecules, gases always mix Intermolecular forces complicate mattersIntermolecular forces complicate matters –May improve mixing if forces in mixture are stronger –May oppose mixing if forces in pure substances are stroner Three intermolecular forces:Three intermolecular forces: –Solvent – solvent –Solute – solute –Solvent – solute Like dissolves like:Like dissolves like: –Polar solvents dissolve polar solutes –Nonpolar solvents dissolve nonpolar solutes

Review of intermolecular forces Name of force OriginStrength Ion-dipole Between ions and molecules Quite strong (10 – 50 kJ/mol) Dipole-dipole Between permanent dipoles Weak (3 – 4 kJ/mol) Hydrogen bonds Polar bonds with H and (O,N) Quite strong (10 – 40 kJ/mol) London dispersion forces Fluctuating dipoles in non-polar bonds Weak (1 – 10 kJ/mol)

Ion - dipole Characteristic of interactions in solutions of ionic compounds in polar solventsCharacteristic of interactions in solutions of ionic compounds in polar solvents –Negative ion with the positive dipole end –Positive ion with the negative dipole end

Dipole - dipole Important attractive force in polar substancesImportant attractive force in polar substances Strength of the order of 3 – 4 kJ/mol (compared with 200 – 400 kJ/mol for covalent bonds)Strength of the order of 3 – 4 kJ/mol (compared with 200 – 400 kJ/mol for covalent bonds)

Dipole force manifested in boiling points: Nonpolar substances have much lower boiling points – Acetone (polar) 56ºC butane (nonpolar) -0.5ºC Boiling point increases with dipole strength

London calling Even molecules with no net dipole moment attract each other. Electrons are not static but mobile: –Fluctuation creates dipole in one molecule which induces dipole in another molecule Effect increases with atomic number – as atom becomes more polarizable –Boiling increases with molar mass For small molecules, dispersion forces are weaker than other inter- molecular forces. For large molecules this is not true. Large molecules are solids because of dispersion forces

Solute – solvent interactions The stronger the interactions between solute and solvent, the greater the solubilityThe stronger the interactions between solute and solvent, the greater the solubility –Dispersion forces, dipolar interactions and liquids –Ion-dipole interactions and ionic compounds in water

Dissolving an ionic compound Attractive forces between solute and solvent compensate attractive forces between solute – solute and solvent - solventAttractive forces between solute and solvent compensate attractive forces between solute – solute and solvent - solvent

Solution on the atomic scale The ions held tightly in the crystal lattice become solvated by water moleculesThe ions held tightly in the crystal lattice become solvated by water molecules Ion-ion interactions are replaced by ion- dipole interactions in the solutionIon-ion interactions are replaced by ion- dipole interactions in the solution

Profits and losses Energy costsEnergy costs –Solvent – solvent interactions –Solute – solute interactions (lattice energy) High lattice energy → low solubility Energy gainEnergy gain –Solvent – solute interactions Small, more highly charged ions have stronger interactions

Solutions, disorder and entropy Substances dissolve with either gain or loss of enthalpySubstances dissolve with either gain or loss of enthalpy Dissolving usually results in an increase in entropyDissolving usually results in an increase in entropy Exothermic processes tend to occur spontaneouslyExothermic processes tend to occur spontaneously Processes that result in increased entropy tend to occur spontaneouslyProcesses that result in increased entropy tend to occur spontaneously

Strong interactions complicate predictions about solubility Weak interactions:Weak interactions: –Gases mix completely over all compositions Moderate interactions:Moderate interactions: –Liquids exhibit limits on range of miscibility Strong interactions:Strong interactions: –Solubility of ionic compounds varies –Some ionic compounds are completely insoluble

Super-saturate me Saturated solution is in equilibrium with undissolved solute and cannot contain any more soluteSaturated solution is in equilibrium with undissolved solute and cannot contain any more solute Supersaturation: a state of the solution where it contains more solute than allowed by saturation. Non-equilibrium condition Crystallization and rainfall depend on supersaturationCrystallization and rainfall depend on supersaturation

Temperature and solubility Solubility is amount of solute present in solution at saturation pointSolubility is amount of solute present in solution at saturation point Solubility varies widely from compound to compoundSolubility varies widely from compound to compound Solubility varies with TSolubility varies with T –Most substances increase with T (KNO 3 ) although the dependence can vary –Some are unaffected by T (NaCl) –A few decrease with T (Na 2 SO 4 ) Results can be explained by sign of ΔH solution and Le Chatelier’s principle (later)Results can be explained by sign of ΔH solution and Le Chatelier’s principle (later)

Gases always decrease solubility with temperature Solubility of gases always decreases as temperature increasesSolubility of gases always decreases as temperature increases

Solubility and pressure For solutions of solids in liquids and liquids in liquids, pressure has almost no effect.For solutions of solids in liquids and liquids in liquids, pressure has almost no effect. For gases in liquids:For gases in liquids: Henry’s Law operates Henry’s Law operates Solubility = k P Solubility of gas depends on partial pressure above solutionSolubility of gas depends on partial pressure above solution k is function of gas for given solventk is function of gas for given solvent

Henry’s law and equilibrium Amount of gas in solution is in equilibrium with gas above solutionAmount of gas in solution is in equilibrium with gas above solution When P is increased, density of molecules above solution increasesWhen P is increased, density of molecules above solution increases More molecules enter solution to restore equilibriumMore molecules enter solution to restore equilibrium

Concentration: ways of expressing it MolarityMolarity Mole fractionMole fraction Mass percent/volume percentMass percent/volume percent MolalityMolality

Molarity (M) Molarity (M) = Stoichiometry calculations are easyStoichiometry calculations are easy Amounts of solution required are volumetricAmounts of solution required are volumetric Concentration varies with TConcentration varies with T Amount of solvent requires knowledge of densityAmount of solvent requires knowledge of density

Mole Fraction (X) Mole fraction (X) = DimensionlessDimensionless Independent of temperatureIndependent of temperature Used for gas mixtures and solid solutionsUsed for gas mixtures and solid solutions

Mass percent (mass %) Mass percent = For very dilute solutions:For very dilute solutions: Parts per million (ppm) = Independent of temperatureIndependent of temperature Need to work with masses rather than volumesNeed to work with masses rather than volumes

Molality (m) Molality (m) = Used in calculations of colligative propertiesUsed in calculations of colligative properties Independent of temperatureIndependent of temperature Need to know mass of solutionNeed to know mass of solution Need to know density to convert to molarityNeed to know density to convert to molarity