Chapter 13

Ions in aqueous SolutionsAnd

Colligative Properties

Compounds in Aqueous Solution

• Dissociation– The separation of ions that occurs when an ionic

compound dissolves – H2O

NaCl (s) Na+ (aq) + Cl- (aq) H2O

MgCl2 (s) Mg2+ (aq) + 2 Cl- (aq)

• Assuming 100% dissociation1 mole NaCl in water -> 1 mole Na+ and 1 mole Cl-

=2 mole of solute ions

1 mole CaCl2 -> 1 mole of Ca and 2 mol of Cl- = 3 mole of solute ions

•Do practice page 436

• Remember must represent the facts when writing an equation

• Some substances do not remain soluble in solution.

• Use table 437 and 860 to determine solubility

Net Ionic Equations

• Show the compounds and ions that undergo a chemical change in a reaction in an aq soln.

– To create an ionic equation, must convert a balanced chemical equation into an ionic eq.

• ALL soluble ionic compounds are shown as dissociated ions in soln.

Ionic equations

• 2HCl (aq) + Na2O (aq) -> 2NaCl (aq) + H20 (l)

• 2H+ (aq) + 2Cl- (aq) + 2Na+ (aq) + O2- (aq) 2 Na+ (aq)

+ 2 Cl- (aq) + H20 (l)

• Spectator Ions– Ions that do not take part in a chemical reaction

and are found in solution both before and after the reaction

• To convert an ionic equation into a net ionic equation, remove the spectator ions from both sides of the equation– The ions and compounds left after the spectator

ions are removed is called the net ionic equation

• See practice page 440

Ionization

• Molecular compounds can also form ions in solution

• Ionization means creating ions where there were none

H2O

• HCl (g) - H+ (aq) + Cl- (aq)

Hydronium Ion

• Molecular compounds often contain Hydrogen bonded by a polar covalent bond.

• Some of the compounds ionize in an aq. Solution to release H+ , the H+ ion has such a high reactivity that it attaches itself to water to form H3O+ or the hydronium ion

• HCl (g) + H2O H3O+ (aq) + Cl- (aq)

Electrolytes

• Strong electrolytes– Any compound whose dilute aqueous solutions

conduct electricity well– Caused by all (almost all) solute existing as ions in

solution• Weak electrolytes– Any compound whose dilute aqueous solutions

conduct electricity poorly– Caused by very small amounts of solute existing as ions

in solution

• Do section review page 443

• Homework: page 458 #2, 8-13

Colligative Properties of Soln.

• Properties that depend on the concentration of solute particles but not on their identity

• These include: freezing point, boiling point, vapor pressure, osmotic pressure,

Raoult's Law and Vapor Pressure Lowering

• Bp and Fp of soln differ from those of pure solvents

• When Nonvolatile solutes (a substance that has little tendency to become a gas under existing conditions) added to a liquid to form a solution, the vapor pressure above that solution decreases.

• Liquid molecules at the surface of a liquid can escape to the gas phase when they have a sufficient amount of energy to break free of the liquid's intermolecular forces.

• That vaporization process is reversible. Gaseous molecules coming into contact with the surface of a liquid can be trapped by intermolecular forces in the liquid.

• Eventually the rate of escape will equal the rate of capture to establish a constant, equilibrium vapor pressure above the pure liquid.

• If add a nonvolatile solute to that liquid, the amount of surface area available for the escaping solvent molecules is reduced because some of that area is occupied by solute particles.

• the solvent molecules will have a lower probability to escape the solution than the pure solvent.

The Vapor Pressure of a Solution is Lower than that of the Pure Solvent

• The French chemist Francois Raoult discovered the law that mathematically describes the vapor pressure lowering phenomenon.

• Raoult's law states:– the vapor pressure of an ideal solution is dependent on

the vapor pressure of each chemical component and the mole fraction of the component present in the solution.

– Once the components in the solution have reached equilibrium, the total vapor pressure p of the solution is:

• and the individual vapor pressure for each component is:

• Where p* is the vapor pressure of the pure component x is the mole fraction of the component in solution

• Solutions that obey Raoult's law are called ideal solutions because they behave exactly as we would predict.

• Solutions that show a deviation from Raoult's law are called non-ideal solutions because they deviate from the expected behavior.

• Very few solutions actually approach ideality, but Raoult's law for the ideal solution is a good enough approximation for the non- ideal solutions

Freezing Point Depression• describes the phenomenon that the freezing point of a liquid (a

solvent) is depressed when another compound is added, meaning that a solution has a lower freezing point than a pure solvent. This happens whenever a solute is added to a pure solvent, such as water.

• The phenomenon may be observed in sea water, which due to its salt content remains liquid at temperatures below 0°C, the freezing point of pure water.

• The freezing point depression happens both when the solute is an electrolyte, such as various salts, and a nonelectrolyte

ΔTf = Kf · mB

• ΔTf, the freezing point depression, is defined as Tf (pure solvent) − Tf (solution), the difference between the freezing point of the pure solvent and the solution. It is defined to assume positive values when the freezing point depression takes place.

• Kf, the cryoscopic constant, which is dependent on the properties of the solvent. It can be calculated as Kf = RTm

2M/ΔHf, where R is the gas constant, Tm is the melting point of the pure solvent in kelvin, M is the molar mass of the solvent, and ΔHf is the heat of fusion per mole of the solvent. (LOOK THESE UP)

• mB is the molality of the solution, van 't Hoff factor i as mB = msolute · i. The factor i accounts for the number of individual particles (typically ions) formed by a compound in solution.

• Examples: – i = 1 for sugar in water – i = 2 for sodium chloride in water, due to dissociation of NaCl into Na+ and Cl- – i = 3 for calcium chloride in water, due to dissociation of CaCl2 into Ca2+ and 2 Cl- – i = 2 for hydrogen chloride in water, due to complete dissociation of HCl into H+ and Cl-

• Kf of nonelectrolytes is -1.86 *C/m– *C is celsius degree – m is mol solute/kg solvent

• Kf of electrolytes may be looked up – See page 448 for examples

Boiling Point Elevation

• Nonvolatile solutes elevate the bp of the solvent

• Molal boiling point constant is the boiling point elevation of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte solute– Water is .51 *C/m

ΔTb = Kb · mB

• ΔTb, the boiling point elevation, is defined as Tb (solution) - Tb (pure solvent). • Kb, the ebullioscopic constant, which is dependent on the properties of the solvent. It can

be calculated as Kb = RTb2M/ΔHv, where R is the gas constant, and Tb is the boiling

temperature of the pure solvent, M is the molar mass of the solvent, and ΔHv is the heat of vaporization per mole of the solvent.

• mB is the molality of the solution, calculated by taking dissociation into account since the boiling point elevation is a colligative property, dependent on the number of particles in solution. This is most easily done by using the van 't Hoff factor i as mB = msolute · i. The factor i accounts for the number of individual particles (typically ions) formed by a compound in solution.

• Examples: – i = 1 for sugar in water – i = 2 for sodium chloride in water, due to the full dissociation of NaCl into Na+ and Cl- – i = 3 for calcium chloride in water, due to dissociation of CaCl2 into Ca2+ and 2Cl-

• Do practice page 450 and 451