 # Ions in Aqueous Solutions and Colligative Properties

## Presentation on theme: "Ions in Aqueous Solutions and Colligative Properties"— Presentation transcript:

Ions in Aqueous Solutions and Colligative Properties
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 ionization 1 mol of NaCl = 1 mol Na+ and 1 mol Cl- NaCl (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 mol SO42– = 5 mol of solute ions

Solubility Rules for Ionic Compounds
Exceptions 1. Salts of alkali metals and ammonia Soluble Some lithium compounds 2. Nitrate salts and chlorate salts Few Exceptions 3. Sulfate salts Compounds of Pb, Ag, Hg, Ba, Sr, and Ca 4. Chloride salts 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… Using solubility rules, look at possible new pairings of cation and anion that give an insoluble substance. 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