Solutions and their Solutions and their propertiesproperties

Part I:Part I:What’s a solution: The process of dissolvingWhat’s a solution: The process of dissolving

Factors affecting solubilityFactors affecting solubilityWays of expressing concentrationWays of expressing concentration

Part II:Part II:Colligative propertiesColligative properties

Learning objectivesLearning 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 Perform concentration calculations using

molarity, molality, mole fraction and percent by molarity, molality, mole fraction and percent by mass and volumemass and volume

Mixtures are a fact of lifeMixtures are a fact of life

• We learn about matter by studying pure We learn about matter by studying pure substancessubstances

• In nature, most things are mixtures – In nature, most things are mixtures – nothing is “pure”:nothing is “pure”:– Air– Water– Soil– Living systems

Solution or colloid?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 In a solution, the particle size is on the molecular

scale (< 1 nm)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?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 forcesReview of intermolecular forces

Name of forceName of force OriginOrigin StrengthStrength

Ion-dipoleIon-dipole Between ions and moleculesBetween ions and molecules Quite strong (10 – 50 kJ/mol)Quite strong (10 – 50 kJ/mol)

Dipole-dipoleDipole-dipole Between permanent dipolesBetween permanent dipoles Weak (3 – 4 kJ/mol)Weak (3 – 4 kJ/mol)

Hydrogen bondsHydrogen bonds Polar bonds with H and (O,N)Polar bonds with H and (O,N) Quite strong (10 – 40 kJ/mol)Quite strong (10 – 40 kJ/mol)

London dispersion forcesLondon dispersion forces Fluctuating dipoles in non-polar Fluctuating dipoles in non-polar bondsbonds

Weak (1 – 10 kJ/mol)Weak (1 – 10 kJ/mol)

Ion - dipoleIon - dipole

• Characteristic of interactions in solutions Characteristic of interactions in solutions of ionic compounds in polar solventsof ionic compounds in polar solvents– Negative ion with the positive dipole end– Positive ion with the negative dipole end

Dipole - dipoleDipole - dipole

• Important attractive force in polar Important attractive force in polar substancessubstances

• Strength of the order of 3 – 4 kJ/mol Strength of the order of 3 – 4 kJ/mol (compared with 200 – 400 kJ/mol for (compared with 200 – 400 kJ/mol for covalent bonds)covalent bonds)

Dipole force manifested in Dipole force manifested in boiling points:boiling points:

• Nonpolar substances have much lower boiling Nonpolar substances have much lower boiling pointspoints– Acetone (polar) 56ºC butane (nonpolar) -0.5ºC

• Boiling point increases with dipole strengthBoiling point increases with dipole strength

London callingLondon calling

• Even molecules with no net dipole moment attract each other.Even molecules with no net dipole moment attract each other.• Electrons are not static but mobile: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 polarizableEffect 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 For small molecules, dispersion forces are weaker than other inter-molecular forces. For large molecules this is not true. Large molecules are solids forces. For large molecules this is not true. Large molecules are solids because of dispersion forcesbecause of dispersion forces

Solute – solvent interactionsSolute – solvent interactions

• The stronger the interactions between The stronger the interactions between solute and solvent, the greater the solute and solvent, the greater the solubilitysolubility– Dispersion forces, dipolar interactions and

liquids– Ion-dipole interactions and ionic compounds

in water

Dissolving an ionic compoundDissolving an ionic compound

• Attractive forces between solute and Attractive forces between solute and solvent compensate attractive forces solvent compensate attractive forces between solute – solute and solvent - between solute – solute and solvent - solventsolvent

Solution on the atomic scaleSolution on the atomic scale

• The ions held tightly in the crystal lattice The ions held tightly in the crystal lattice become solvated by water moleculesbecome solvated by water molecules

• Ion-ion interactions are replaced by ion-Ion-ion interactions are replaced by ion-dipole interactions in the solutiondipole interactions in the solution

Profits and lossesProfits 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 entropySolutions, disorder and entropy

• Substances dissolve with either gain or loss of Substances dissolve with either gain or loss of enthalpyenthalpy

• Dissolving usually results in an increase in Dissolving usually results in an increase in entropyentropy

• Exothermic processes Exothermic processes tendtend to occur to occur spontaneouslyspontaneously

• Processes that result in increased entropy Processes that result in increased entropy tendtend to occur spontaneouslyto occur spontaneously

Strong interactions complicate Strong interactions complicate predictions about solubilitypredictions 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 meSuper-saturate me

• SaturatedSaturated solution is in equilibrium with solution is in equilibrium with undissolved solute and cannot contain any undissolved solute and cannot contain any more solutemore solute

SupersaturationSupersaturation: a state of the solution : a state of the solution where it contains more solute than allowed where it contains more solute than allowed by saturation. Non-equilibrium conditionby saturation. Non-equilibrium condition

• Crystallization and rainfall depend on Crystallization and rainfall depend on supersaturationsupersaturation

Temperature and solubilityTemperature and solubility

• Solubility is amount of solute present in solution Solubility is amount of solute present in solution at saturation pointat saturation point

• Solubility varies widely from compound to Solubility varies widely from compound to compoundcompound

• Solubility varies with TSolubility varies with T– Most substances increase with T (KNO3) although the

dependence can vary– Some are unaffected by T (NaCl)– A few decrease with T (Na2SO4)

• Results can be explained by sign of Results can be explained by sign of ΔΔH solution H solution and Le Chatelier’s principle (later)and Le Chatelier’s principle (later)

Gases always decrease solubility Gases always decrease solubility with temperature with temperature

• Solubility of gases always decreases as Solubility of gases always decreases as temperature increasestemperature increases

Solubility and pressureSolubility and pressure

• For solutions of solids in liquids and liquids in For solutions of solids in liquids and liquids in liquids, pressure has almost no effect.liquids, pressure has almost no effect.

• For gases in liquids:For gases in liquids:• Henry’s Law Henry’s Law operatesoperates

Solubility = Solubility = k Pk P• Solubility of gas depends on partial pressure Solubility of gas depends on partial pressure

above solutionabove solution• kk is function of gas for given solvent is function of gas for given solvent

Henry’s law and equilibriumHenry’s law and equilibrium

• Amount of gas in solution is in equilibrium with Amount of gas in solution is in equilibrium with gas above solutiongas above solution

• When P is increased, density of molecules When P is increased, density of molecules above solution increasesabove solution increases

• More molecules enter solution to restore More molecules enter solution to restore equilibriumequilibrium

Concentration: ways of Concentration: ways of expressing it expressing it

• MolarityMolarity

• Mole fractionMole fraction

• Mass percent/volume percentMass percent/volume percent

• MolalityMolality

Molarity (M)Molarity (M)

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 Amount of solvent requires knowledge of

densitydensity

Moles solute (mol)

Vol solution (L)

Mole Fraction (X)Mole Fraction (X)

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

Moles solute (mol)

Moles solution (mol)

Mass percent (mass %)Mass percent (mass %)

Mass percent = Mass percent =

• For very dilute solutions:For very dilute solutions:Parts per million (ppm) = Parts per million (ppm) =

• Independent of temperatureIndependent of temperature• Need to work with masses rather than volumesNeed to work with masses rather than volumes

Mass solute (g)x 100%

Mass solution (g)

6Mass solute (g)x 10

Mass solution (g)

Molality (m)Molality (m)

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

Moles solute (mol)

Mass solvent (kg)