+

Chapter 13

Ions in Aqueous Solutions and Colligative Properties

+Compounds in Aqueous Solutions

When a compound that is made of ions dissolves in water, the ions separate from one another.

This separation of ions that occurs when an ionic compound dissolves is called dissociation.

The opposite reaction is called ionizationNaCl (s) H2O Na+ (aq) + Cl –

(aq)

+Remember: Coefficients in a Balanced Equation = Number of moles! Problem:

Write the equation for the dissolution of aluminum sulfate, Al2(SO4)3, in water. How many moles of aluminum ions and sulfate ions are produced by dissolving 1 mol of aluminum sulfate? What is the total number of moles of ions produced by dissolving 1 mol of aluminum sulfate?

Answer:

Al2(SO4)3 (s) H2O 2Al 3+ (aq) + 3SO42–

(aq)

2 mol Al 3+ + 3 mol SO42– = 5 mol of

solute ions

+ Solubility Rules for Ionic Compounds

Compounds Solubility Exceptions1. Salts of alkali metals and ammonia

Soluble Some lithium compounds

2. Nitrate salts and chlorate salts

Soluble Few Exceptions

3. Sulfate salts Soluble Compounds of Pb, Ag, Hg, Ba, Sr, and Ca

4. Chloride salts Soluble Compounds of Ag and some compounds of Hg and Pb

5. Carbonates, phosphates, chromates, sulfides, and hydroxides

Most are insoluble Compounds of the alkali metals and of ammonia

+Net Ionic Equations

Write a complete ionic equation that shows dissolved ionic compounds as their free ions.

Eliminate ions that do not participate in the reaction by canceling ions that appear on both sides of the equation. These are called spectator ions.

Ions that are not directly involved in a reaction are called spectator ions.

Rewrite the equation, leaving out the canceled spectator ions.

Balance the atoms and the charges of the ions.

A net ionic equation indicates only those particles that actually take part in the reaction.

+Net Ionic Equations

Cd2+ (aq) +2NO3- (aq) + 2NH4

+ (aq) + S2- (aq) CdS (s) + 2NO3- (aq) +

2NH4+ (aq)

**Get rid of spectator ions

** Find the solid and then the ions that make up that solid. Those are the only molecules that are used in the net ionic equations.

Cd2+ (aq) + S2- (aq) CdS (s)

+Practice Problem

Write a balanced net ionic equation for the following reaction:

Pb(s) + AgNO3 (aq) Ag (s) + Pb(NO3)2 (aq)

Answer:

1. The nitrate ion is the spectator ion.

2. The number of atoms balance, but the charges on

the ions do not balance.

3. Place a coefficient 2 in front of Ag+ (aq) to balance

the charges.

4. A coefficient of 2 in front of Ag (s) rebalances the

atoms.

5. Pb(s) + 2Ag+ (aq) 2Ag (s) + Pb2+ (aq) is the balanced net ionic equation

+Practice Using Net Ionic Equations

& Solubility Rules

Identify the precipitate formed and write the net ionic equation for the reaction of aqueous potassium carbonate with aqueous strontium chloride.

1. Write the formula for the beginning compounds (reactants). Look up the symbols and balance the charges.

2. Double-displacement

a) list the products- change partners

..cation listed first

b) balance charges in the products

c) balance the equation (count the atoms)

+Continued…

3. Using solubility rules, look at possible new pairings of cation and anion that give an insoluble substance.

4. Dissociation: separate the ions

Include symbol, charge, and state (aq)

Do not separate the solid

5. Eliminate the spectator ions and write the net ionic equation.

+Answer

1. Reactants as dissociated free ions

2K+ (aq) + CO32- (aq) + Sr2+ (aq) + 2Cl- (aq)

Charges must be balanced to equal 0.

2. Of the two possible combinations, KCl is soluble (Rules 1 and 4) and SrCO3 is insoluble (Rule 5)

3. The net ionic equation must be balanced for the number of atoms of each element and the charges on the ions.

Sr2+ (aq) + CO32- (aq) SrCO3 (s)

+Hydronium Ion

The H3O+ ion is known as the hydronium ion. The reaction of the H+ ion to form the hydronium ion produces much of the energy needed to ionize a molecular solute.

H2O(l) + HCl (g) H3O+ +Cl- (aq)

+Electrolytes

Strong Electrolytes:

Any compound whose dilute aqueous solutions conduct electricity well; this is due to the presence of all or almost all of the dissolved compound in the form of ions.

Hydriodic acid HI

Hydrobromic acid HBr

Hydrochloric acid HCl

Sulfuric acid H2SO4

Weak Electrolytes:

Any compound whose dilute aqueous solutions conduct electricity poorly; this is due to the presence of a small amount of the dissolved compound in the form of ions.

Acetic acid (CH3COOH)

Carbonic acid (H2CO3)

Amonnia (NH3)

Sodium Hydroxide (NaOH)

+Colligative Properties of Solutions

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

We will discuss: Vapor-Pressure Lowering Freezing-Point Depression Boiling-Point Elevation Osmotic Pressure Relationship between Colligative Properties

and Electrolytes

+Vapor-Pressure Lowering

Nonvolatile Substance- A substance that has little tendency to become a gas under existing conditions.

Adding a nonvolatile solute to a solvent always lowers the vapor pressure.

+Freezing-Point Depression

Freezing-Point Depression(Δtf)- the difference between the freezing points of the pure solvent and a solution of a non-electrolyte in that solvent, and it is directly proportional to the molal concentration of the solution

Δtf = Kfm

Molal freezing point constant (Kf)- the freezing point depression of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte solute.

Molality (m) – mol solute/kg of solvent

+Freezing-Point Depression Con’t

+Practice Problem

A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of -0.23°C. What is the molal concentration of the solution?

m= / Kf

Δtf = f.p of solution – f.p of pure solvent = -0.23°C – 0.00°C = -0.23°C

m= 0.12m

Δtf

+Boiling-Point Elevation

Boiling-Point Elevation- the difference between the boiling points of the pure solvent and a solution of a non-electrolyte in that solvent, and it is directly proportional to the molal concentration of the solution

Δtb= Kbm

*Molal freezing point constant (Kb)

Example - antifreeze

+Osmotic Pressure

Osmosis- The movement of solvent through a semipermeable membrane from the side of lower solute concentration to the side of higher solute concentration

Semipermeable membrane- allows the passage of some particles while blocking the passage of others

Osmotic Pressure- the external pressure that must be applied to stop osmosis Because osmotic pressure is

dependent on the concentration of solute particles and not on the type of solute particles, it is a colligative property

The greater the concentration of a solution, the greater the osmotic pressure