Chapter 13: Ions in Aqueous Solutions and Colligative

Properties

Section 1: Compounds in Aqueous Solutions

• Standard 6.e.: Students know the relationship between the molality of a solute in a solution and the solution’s depressed freezing point or elevated boiling point.

• Objective: We will write dissolution equations, predict if a precipitate will form and if a precipitate forms, write a net ionic equation.

Dissociation• When an ionic compound dissolves in water,

the ions separate.

• To find how many moles of ions are produced, we write a balanced dissociation equation and look at the coefficients in front of the ions.

NaCl Na+ + Cl-

1 mol of Sodium Ion and 1 mol of Chloride Ion

• These are like decomposition reactions.

Example 1• Write the equation for the dissolution of

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

Al2(SO4)3 2Al3+ + 3SO42-

2 mol Al3+ and 3 mol SO42-

Total moles = 2 + 3 = 5 moles

Example 2

• Do the same thing as the last example, expect now you are dissolving 2 mols of Al2(SO4)3.

2Al2(SO4)3 4Al3+ + 6SO42-

4 mol Al3+ and 6 mol SO42-

Total moles = 4 + 6 = 10 moles

Precipitation Reactions

GENERAL SOLUBILITY GUIDELINES

1. Sodium, potassium, and ammonium compounds are soluble in water.

2. Nitrates, acetates, and chlorates are soluble.

3. Most chlorides are soluble, except those of silver, mercury (I) and lead. Lead (II) chloride is soluble in hot water.

Precipitation Reactions

GENERAL SOLUBILITY GUIDELINES CONT…

4. Most sulfates are soluble, except those of barium, strontium, lead, calcium, and mercury.

5. Most carbonates, phosphates, and silicates are insoluble, except those of sodium, potassium, and ammonium.

6. Most sulfides are insoluble, except those of calcium, strontium, sodium, potassium, and ammonium.

Example 3• Look at the solubility chart to determine if

the following are Soluble or Insoluble?

– Sodium Carbonate

– Calcium Phosphate

– Cadmium Nitrate

– Ammonium Sulfide

Example 3• Look at the solubility chart to determine if

the following are Soluble or Insoluble?

– Sodium Carbonate Soluble

– Calcium Phosphate

– Cadmium Nitrate

– Ammonium Sulfide

Example 3• Look at the solubility chart to determine if

the following are Soluble or Insoluble?

– Sodium Carbonate Soluble

– Calcium Phosphate Insoluble

– Cadmium Nitrate

– Ammonium Sulfide

Example 3• Look at the solubility chart to determine if

the following are Soluble or Insoluble?

– Sodium Carbonate Soluble

– Calcium Phosphate Insoluble

– Cadmium Nitrate Soluble

– Ammonium Sulfide

Example 3• Look at the solubility chart to determine if

the following are Soluble or Insoluble?

– Sodium Carbonate Soluble

– Calcium Phosphate Insoluble

– Cadmium Nitrate Soluble

– Ammonium Sulfide Soluble

Example 4• Will a precipitate form when solutions of

cadmium nitrate and ammonium sulfide are combined?

– Step 1: Determine if the compounds are soluble, if soluble, continue to step 2.

Compounds are both soluble…so we continue to step 2.

Example 4• Will a precipitate form when solutions of

cadmium nitrate and ammonium sulfide are combined?

– Step 2: Write double-displacement reaction between the two compounds.

(NH4)2S + Cd(NO3)2 CdS + 2NH4NO3

Example 4• Will a precipitate form when solutions of

cadmium nitrate and ammonium sulfide are combined?

(NH4)2S + Cd(NO3)2 CdS + 2NH4NO3

– Step 3: Determine if the newly formed compounds are soluble. If one is insoluble, then it is a precipitate.

CdS or Cadmium Sulfide is insoluble,

so it is the precipitate.

Net Ionic Equations• Includes only those compounds and ions that

undergo a chemical change in a reaction in an aqueous solution.

• Basically, if the ions are part of a soluble product, they don’t end up in the final equation, only the ions for the precipitate that is formed, remain in the equation.

• The ions that do not take part in the chemical reaction are called spectator ions.

Example 5

• Write the net ionic equation for the production of ammonium nitrate and cadmium sulfide.

2NH4+ + 2NO3

- + Cd2+ + S2-

CdS + 2NO3- + 2NH4

+

• If ions show up on both sides of the equation, cross them out and rewrite the equation without them.

Cd2+ + S2- CdS

Ionization• Ions are formed from solute molecules by the

action of the solvent.

• Different from dissociation because it involves molecular compounds rather than ionic compounds.

• In order for ions to form, the strength of the bond within the solute molecule must be weaker than the attractive forces of the solvent molecules.

The Hydronium Ion H3O+

• When a compound ionizes in a solution and releases a H+ ion, it binds to the H2O and forms H3O+.

H2O + HCl H3O+ + Cl-

Electrolytes and Nonelectrolytes• Electrolyte: a compound that conducts an

electric current when it is in an aqueous solution or in the molten (liquid) state.

• All ionic compounds are electrolytes because they dissociate into ions.

• Nonelectrolyte: a compound that does not conduct an electric current in either aqueous solution or the molten state.

• Strong Electrolyte: nearly all the ionic compound exists as separate ions.

• Weak Electrolyte: only a fraction of the ionic compound exists as separate ions.

• Standard 6.e.: Students know the relationship between the molality of a solute in a solution and the solution’s depressed freezing point or elevated boiling point.

• Objective: We will write dissolution equations, predict if a precipitate will form and if a precipitate forms, write a net ionic equation.

Homework

• Ch 13.1 page 458 # 2-7

Section 2: Colligative Properties of Solutions

• Standard 6.e.: Students know the relationship between the molality of a solute in a solution and the solution’s depressed freezing point or elevated boiling point.

• Objective: We will list the four colligative properties, and calculate freezing-point depression and boiling-point elevation.

Vapor-Pressure Lowering

• The addition of a nonvolatile substance will raise the boiling point and lower the freezing point.

• This has to do with vapor pressure of the solvent.

• As the number of solute particles increase, the proportion of solvent molecules decreases.

Freezing-Point Depression. • When 1 mol of a nonelectrolyte solution is

dissolved in 1 kg of water, the freezing point is -1.86°C instead of 0.0°C.

• If 2 mols are dissolved, it is 2 x -1.86°C.

• This is called the molal freezing-point constant (Kf) and changes for different solvents.

• Freezing-point depression Δtf is the difference between the two freezing points. Changes according to concentration.

Example 6

• What is the freezing-point depression of water in a solution of 20.54 g of sucrose, C12H22O11, in 200 g of water? What is the actual freezing point of the solution?

• Find molality of the sugar solution.

• Multiply the Kf (of water) by the molality.

• Take normal freezing point 0°C + Δtf.

Boiling-Point Elevation• When 1 mol of a nonelectrolyte solution is

dissolved in 1 kg of water, the boiling point is 100.51°C instead of 100.0°C. An increase of 0.51°C

• This is called the molal boiling-point constant (Kb) and changes for different solvents.

• Boiling-point elevation Δtb is the difference between the two boiling points. Changes according to concentration.

Example 7

• What is the boiling-point elevation of a solution made from 20.1 g of a nonelectrolyte solute and 400.0 grams of water? The molar mass of the solute is 62.0 g/mol.

Osmotic Pressure

• A semipermeable membrane allows only water molecules to pass through during osmosis. This can cause an increase in volume on one side of the membrane.

• Osmotic pressure is the external pressure that must be applied to stop osmosis.

• The higher the concentration of a solution, the greater the osmotic pressure.

Electrolytes & Colligative Properties

• When electrolytes are dissolved in a solvent, the effects are greater than that of nonelectrolytes. This is because more moles of solute particles are formed when the compounds dissolve.

• Standard 6.e.: Students know the relationship between the molality of a solute in a solution and the solution’s depressed freezing point or elevated boiling point.

• Objective: We will list the four colligative properties, and calculate freezing-point depression and boiling-point elevation.

Homework

Ch 13.2 pg 456 #1-4 and 458 #14 and 16