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, Al 2 (SO 4 ) 3, in water. How many moles of Al ions and SO 4 ions are produced by dissolving 1 mol of Al 2 (SO 4 ) 3 ? What is the total number of moles of ions produced? Al 2 (SO 4 ) 3  2Al SO mol Al 3+ and 3 mol SO 4 2- Total moles = = 5 moles

Example 2 Do the same thing as the last example, expect now you are dissolving 2 mols of Al 2 (SO 4 ) 3. 2Al 2 (SO 4 ) 3  4Al SO mol Al 3+ and 6 mol SO 4 2- Total moles = = 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 PhosphateInsoluble –Cadmium Nitrate –Ammonium Sulfide

Example 3 Look at the solubility chart to determine if the following are Soluble or Insoluble? –Sodium Carbonate Soluble –Calcium PhosphateInsoluble –Cadmium NitrateSoluble –Ammonium Sulfide

Example 3 Look at the solubility chart to determine if the following are Soluble or Insoluble? –Sodium Carbonate Soluble –Calcium PhosphateInsoluble –Cadmium NitrateSoluble –Ammonium SulfideSoluble

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. (NH 4 ) 2 S + Cd(NO 3 ) 2  CdS + 2NH 4 NO 3

Example 4 Will a precipitate form when solutions of cadmium nitrate and ammonium sulfide are combined? (NH 4 ) 2 S + Cd(NO 3 ) 2  CdS + 2NH 4 NO 3 –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. 2NH NO Cd 2+ + S 2-  CdS + 2NO NH 4 + If ions show up on both sides of the equation, cross them out and rewrite the equation without them. Cd 2+ + S 2-  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 H 3 O + When a compound ionizes in a solution and releases a H + ion, it binds to the H 2 O and forms H 3 O +. H 2 O + HCl  H 3 O + + 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 (K f ) and changes for different solvents. Freezing-point depression Δt f 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 g of sucrose, C 12 H 22 O 11, in 200 g of water? What is the actual freezing point of the solution? Find molality of the sugar solution. Multiply the K f (of water) by the molality. Take normal freezing point 0°C + Δt f.

Boiling-Point Elevation When 1 mol of a nonelectrolyte solution is dissolved in 1 kg of water, the boiling point is °C instead of 100.0°C. An increase of 0.51°C This is called the molal boiling-point constant (K b ) and changes for different solvents. Boiling-point elevation Δt b 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 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