Ions in Aqueous Solutions and Colligative Properties: ch 14

I. Dissociation: the separation of ions from each other when an ionic compounds dissolves

A. water will be attracted to and attract the ions that are in the solid phase

B. the water pulls the ions apart and into solution; each ion is pulled away from its neighbors

C. the water molecules surround each ion, called hydration, if the solvent is other than water,

this surrounding process is called solvation

D. the number of moles of particles present in the solution is greater than the number of moles of

formula units in the solid because the ions in the solid are pulled apart; each ion in solution acts independently

II. Precipitation reactions: a double displacement reaction between ionic compounds where the ions

in solution recombine to form an insoluble solid – a precipitate (also called metathesis reactions).

III. Solubility Rules: You must MEMORIZE THESE!

Salts are written in ionic form if soluble, and in molecular form if insoluble.

1. Alkali metal (group IA) compounds are soluble.
2. Salts containing NH₄⁺¹, NO₃^-1, ClO₃^-1, ClO₄^-1, and C₂H₃O₂^-1 are soluble.
3. Cl^-1, Br^-1, and I^-1 salts are soluble EXCEPT those of Ag⁺¹, Pb⁺², and Hg₂⁺².
4. SO₄^-1 salts are soluble EXCEPT those of Hg₂⁺², Pb⁺², Ca⁺², Sr⁺², and Ba⁺².
5. OH^-1 and S^-2 compounds are insoluble EXCEPT those of Group IA, NH₄⁺¹, Ca⁺², Sr⁺², and Ba⁺².
6. CO₃^-2, SO₃^-2, PO₄^-3, and SiO₃^-2 compounds are insoluble EXCEPT those of Group IA, and NH₄⁺¹.

IV. Writing Equations

1. Molecular equations: give the overall reaction stoichiometry but not necessarily the actual forms

of the reactants and products in solution.

2. Complete ionic equations: represents as ions all reactants and products that are strong

electrolytes. All substances that are strong electrolytes (substances that dissociate or ionize completely, or almost completely, in water) are represented as ions.

3. Net Ionic Equations: includes only those solution components undergoing a change. Spectator

ions are not included. Spectator ions are ions that remain unchanged during the reaction.

V. Ionization: ions are formed from solute molecules by the action of the solvent. Unlike ionic

compounds, ions are not present in molecules. They are formed in the process of dissolving in a polar solvent.

VI. Strong versus weak electrolytes

1. Strong electrolytes are those compounds that completely or almost completely ionize in solution and therefore exist as ions in solution, i.e. strong acids, strong bases, and soluble salts.
2. Weak electrolytes are those compounds that do not ionize extensively and therefore exist as a compound in solution, i.e. weak acids, weak bases, and insoluble salts.

VII. Colligative properties - properties that depend on the number of particles present in solution and

not the type of particles; vapor pressure, freezing point, boiling point, and osmotic pressure (rate of diffusion through a semipermeable membrane)

VIII. Raoult's Law - the vapor pressure of a solution is directly proportional to the mole fraction of

solvent present: P_solution = c_solvent ^. P_solvent

IX. Vapor pressure

1. the vapor pressure of a pure solvent is caused by particles at the surface gaining enough energy to

break the attraction forces and leave the surface to enter the vapor phase

2. in a solution the solute takes up space on the surface leaving less solvent particles able to enter the

vapor phase

C. the vapor pressure of a solution is lower than the vapor pressure of the pure solvent

X. Freezing point

A. the temperature where the vapor pressure of the solid equals the vapor pressure of the liquid

3. if a solute lowers the vapor pressure of a pure solvent, than the temperature where the vapor

pressure of the solid equals the vapor pressure of the liquid is lowered

4. the freezing point of a solution is lower than the freezing point of the pure solvent: freezing point

depression

XI. Boiling point

A. the temperature where the vapor pressure of the liquid equals atmospheric pressure

2. if a solute lowers the vapor pressure of a pure solvent, than it takes a higher temperature to raise

the vapor pressure to atmospheric pressure

3. the boiling point of a solution is higher than the boiling point of the pure solvent: boiling point

elevation

XII. Osmotic pressure, ∏

A. the pressure developed across a semipermeable membrane due to different diffusion rates

B. temperature dependent

XIII. Amount of freezing point and boiling point change

A. molality of the solution: m - the number of moles of solute per kilogram of solvent

B. the number of particles in the solution, the van’t Hoff factor - i

1. nonionizing solutes

a. molecules that do not dissociate, organic compounds do not ionize

b. the number of moles of particles present in the solution is the number of moles of molecules in the solid; 1 mole of solid sugar = 1 mole of wet sugar in solution

2. ionizing solutes

a. ionic compounds that dissociate into its component ions

b. the number of moles of particles present in the solution is dependent upon the number of

ions in the compound

C. molal constants - k_f and k_b (TABLE 14-2 page 438)

1. molal constants - how much the freezing or boiling point of 1 kilogram of solvent changes per

molal of nonvolatile solute particles

2. the molal freezing point constant (k_f) of water is –1.86 ºC per molal of particles

3. the molal boiling point constant (k_b) of water is 0.512 ºC per molal of particles

D. ∆FP = m^.i^.k_f and ∆BP = m^.i^.k_b

E. FP_solution = FP_solvent – ∆FP and the (–) comes from the (–) on the k_f value

BP_solution = BP_solvent + ∆BP

XIV. Amount of osmotic pressure change

A. molarity of the solution: M - the number of moles of solute per Liter of solution

B. the temperature, in Kelvin (TinK)

C. the gas constant, R = 0.0821 L^.atm/mol^.K

D. ∏ = MRT

XV. All intermolecular forces (solute-solute, solute-solvent, solvent-solvent) are not equal

1. ions and molecules interact with each other decreasing its actual effectiveness in changing the

temperature, called the activity of an ion

2. as a solution becomes more concentrated the ions interact more and become less effective in

changing the temperature