Ions in Aqueous Solutions and Colligative Properties

Dissociation One formula unit of NaCl produces two ions:One formula unit of NaCl produces two ions: NaCl (s) → Na + (aq) + Cl - (aq) One mole of NaCl produces two moles of ionsOne mole of NaCl produces two moles of ions One formula unit of CaCl 2 produces three ions:One formula unit of CaCl 2 produces three ions: CaCl 2 (s) → Ca +2 (aq) + 2Cl - (aq) One mole of CaCl 2 produces three moles of ionsOne mole of CaCl 2 produces three moles of ions The separation of ions that occurs when an ionic compound dissolves

Dissociation Equations NaCl(s)  AgNO 3 (s)  MgCl 2 (s)  Na 2 SO 4 (s)  AlCl 3 (s)  Na + (aq) + Cl - (aq) Ag + (aq) + NO 3 - (aq) Mg 2+ (aq) + 2 Cl - (aq) 2 Na + (aq) + SO 4 2- (aq) Al 3+ (aq) + 3 Cl - (aq)

Precipitation Reactions Solubility Rules No compound is completely insolubleNo compound is completely insoluble Compounds of very low solubility can be considered insolubleCompounds of very low solubility can be considered insoluble Dissociation equations cannot be written for insoluble compoundsDissociation equations cannot be written for insoluble compounds

More Solubility Guidelines Oxalates are generally insoluble except for those of alkali metals. (alkaline earth oxalates are insoluble.)Oxalates are generally insoluble except for those of alkali metals. (alkaline earth oxalates are insoluble.) Chromates of silver, lead & mercury are insoluble. Those of alkali metals, calcium, magnesium & ammonium are soluble.Chromates of silver, lead & mercury are insoluble. Those of alkali metals, calcium, magnesium & ammonium are soluble.

Double displacement forming a precipitate… Pb(NO 3 ) 2 (aq) + 2KI (aq)  PbI 2 (s) + 2KNO 3 (aq) Pb 2+ (aq) + 2 NO 3 - (aq) + 2 K + (aq) +2 I - (aq)  PbI 2 (s) + 2 K + (aq) + 2 NO 3 - (aq) Pb 2+ (aq) + 2 I - (aq)  PbI 2 (s) Double displacement (ionic) equation Complete ionic equation shows compounds as aqueous ions Net ionic equation eliminates the spectator ions Includes only those compounds and ions that undergo a chemical change Spectator ions are those ions that do not take part in a chemical rxn and are found in solution both before and after the rxn- unchanged!

Ionization Ions are formed from solute compounds by the action of the solventIons are formed from solute compounds by the action of the solvent Polar water molecules are attracted to polar or charged solute particlesPolar water molecules are attracted to polar or charged solute particles Electronegative oxygen of water is attracted to electropositive portion of a solute compoundElectronegative oxygen of water is attracted to electropositive portion of a solute compound Electropositive hydrogen of water is attracted to the electronegative portion of a solute compoundElectropositive hydrogen of water is attracted to the electronegative portion of a solute compound

The Hydronium Ion H 3 O + is called the "hydronium" ionH 3 O + is called the "hydronium" ion H 2 O (l) + HCl (g) → H 3 O + (aq) + Cl - (aq)

Strong Electrolytes Any compound of which all or almost all of the dissolved compound exists as ions in an aqueous solutionAny compound of which all or almost all of the dissolved compound exists as ions in an aqueous solution All soluble ionic compounds are strong electrolytesAll soluble ionic compounds are strong electrolytes Hydrogen halides :HCl, HBr, HIHydrogen halides :HCl, HBr, HI

Weak Electrolytes A compound of which a relatively small amount of the dissolved compound exists as ions in an aqueous solutionA compound of which a relatively small amount of the dissolved compound exists as ions in an aqueous solution Examples: HF, organic acidsExamples: HF, organic acids

Colligative Properties These are properties that are dependent only on the number & concentration of solute particles.These are properties that are dependent only on the number & concentration of solute particles.

Vapor Pressure Lowering Effect of Solutes on Vapor-PressureEffect of Solutes on Vapor-Pressure Any nonvolatile solute (has little tendency to become a gas) will lower the vapor pressure of a solution, having two noticeable effects: Raising the boiling point of the solutionRaising the boiling point of the solution Lowering the freezing point of the solutionLowering the freezing point of the solution

So vapor pressure is lowered, boiling occurs at a higher temp b/c more energy is required for the vapor pressure to equal the atmospheric pressure. Solute particles take up the liquid-air surface.

Freezing Point Depression Solutions have lower freezing points than pure solvents.Solutions have lower freezing points than pure solvents. If the solution is aqueous, its freezing point will always be lower than 0  C.If the solution is aqueous, its freezing point will always be lower than 0  C. How much lower?How much lower? Depends on the # & concentration of solute particles.Depends on the # & concentration of solute particles. Ex: Salt on ice in the winterEx: Salt on ice in the winter

Colligative Properties Colligative properties depend on:Colligative properties depend on: number & concentration of solute particles number & concentration of solute particles Since ionic substances dissolve into multiple particles, their colligative effects are greater than those of covalent substances.Since ionic substances dissolve into multiple particles, their colligative effects are greater than those of covalent substances.

Freezing Point Depression Ionic solutes depress the freezing point more than covalent solutes. Look at their solubility rxns, note the number of particles formed:Ionic solutes depress the freezing point more than covalent solutes. Look at their solubility rxns, note the number of particles formed: Ionic: NaCl (s) + H 2 O (l)  Na + (aq) + Cl - (aq)Ionic: NaCl (s) + H 2 O (l)  Na + (aq) + Cl - (aq) Covalent: C 6 H 12 O 6 (s) + H 2 O (l)  C 6 H 12 O 6 (aq)Covalent: C 6 H 12 O 6 (s) + H 2 O (l)  C 6 H 12 O 6 (aq) 2 particles formed 1 particle formed

Molal Freezing-Point Constant for Water The freezing-point depression of the solvent in a 1- molal solution of a nonvolatile, nonelectrolyte soluteThe freezing-point depression of the solvent in a 1- molal solution of a nonvolatile, nonelectrolyte solute K f = °C/m Freezing-Point DepressionFreezing-Point Depression The difference between the freezing points of the pure solvent and a solution of a nonelectrolyte in that solventThe difference between the freezing points of the pure solvent and a solution of a nonelectrolyte in that solvent Δt f = K f m Where: m = molality Δt f = change in freezing point

What is Molality? The concentration of a solution expressed in moles of solute per kilogram of solventThe concentration of a solution expressed in moles of solute per kilogram of solvent Symbol is mSymbol is m m= moles solute mass solvent(kg)

Freezing-Point Depression Example What is the freezing-point depression of water in a solution of 17.1 grams of sucrose ( table sugar, C 12 H 22 O 11 ) in 200. grams of water. What is the actual freezing point of the solution?What is the freezing-point depression of water in a solution of 17.1 grams of sucrose ( table sugar, C 12 H 22 O 11 ) in 200. grams of water. What is the actual freezing point of the solution?

Boiling Point Elevation Solutions have higher boiling points than pure solvents.Solutions have higher boiling points than pure solvents. If the solution is aqueous, its boiling point will always be higher than 100  C.If the solution is aqueous, its boiling point will always be higher than 100  C. Boiling: Temperature at which vapor pressure equals atmospheric pressure.Boiling: Temperature at which vapor pressure equals atmospheric pressure.

Molal Boiling-Point Constant for Water The boiling point elevation of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte soluteThe boiling point elevation of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte solute K b = 0.51 °C/m Boiling-Point ElevationBoiling-Point Elevation The difference between the boiling points of the pure solvent and a solution of a nonelectrolyte in that solventThe difference between the boiling points of the pure solvent and a solution of a nonelectrolyte in that solvent Δt b = K b m Where: m = molality Δt b = change in boiling point

Boiling-Point Elevation Example What is the boiling-point elevation of a solution made from 20.1 g of a nonelectrolyte solute and g of water? The molar mass of the solute is 62.0 g.What is the boiling-point elevation of a solution made from 20.1 g of a nonelectrolyte solute and g of water? The molar mass of the solute is 62.0 g.

Freezing Point Depression and Boiling Point Elevation Constants

Osmotic Pressure Semipermeable membranesSemipermeable membranes Membranes that allow the movement of some particles while blocking the movement of othersMembranes that allow the movement of some particles while blocking the movement of others OsmosisOsmosis The movement of solvent through a semipermeable membrane from the side of lower solute concentration to the side of higher solute concentrationThe movement of solvent through a semipermeable membrane from the side of lower solute concentration to the side of higher solute concentration Osmosis occurs when two solutions of different concentration are separated by a semipermeable membraneOsmosis occurs when two solutions of different concentration are separated by a semipermeable membrane

Osmotic Pressure The external pressure that must be applied to stop osmosisThe external pressure that must be applied to stop osmosis Osmotic pressure increases with the concentration of solute particlesOsmotic pressure increases with the concentration of solute particles Osmotic pressure is not dependent on the TYPE of solute particlesOsmotic pressure is not dependent on the TYPE of solute particles

Calculated Values for Electrolyte Solutions What is the expected change in the freezing point of water in a solution of 62.5 g of barium nitrate, Ba(NO 3 ) 2, in 1.00 kg of water?What is the expected change in the freezing point of water in a solution of 62.5 g of barium nitrate, Ba(NO 3 ) 2, in 1.00 kg of water?