AQUEOUS SOLUTIONS HOLT MODERN CHEMISTRY.

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Presentation transcript:

AQUEOUS SOLUTIONS HOLT MODERN CHEMISTRY

DISSOCIATION Dissociation is separation of ions that occurs when an ionic compound is solvated - Ionic compounds have 100% dissociation Ionization is the process of forming ions from a substance that was not ionic already Substances that already contain ions (i.e. ionic compounds) merely release their ions as the crystal dissociates.

DISSOCIATION Write the equation for the dissolution of the following: Al2(SO4)3 Al2(SO4)3  2 Al3+ + 3SO42- Na2S Na2S 2 Na+ + S2- Ba(NO3)2 Ba(NO3)2  Ba2+ + 2 NO3-

IONIZATION Process in which ions are formed from solute molecules by the action of the solvent Substances that are neutral molecules (i.e. molecular/covalent compounds) must react with water to produce ions HYDRONIUM ION – H3O+ formed from the hydration of H+

Strong vs. Weak Electrolytes STRONG ELECTROLYTE Any compound of which all or almost all of the dissolved compound exist as ions in an aqueous solution EX: NaCl, HCl

Strong vs. Weak Electrolytes A compound of which a relatively small amount of the dissolved compound exists as ions in an aqueous solution EX: HF, CH3COOH

COLLIGATIVE PROPERTIES Properties that depend on the concentration of solute particles but not on their identities Concentration is given in molality Vapor-pressure lowering Freezing-point depression Boiling-point elevation Osmotic pressure

Vapor Pressure Lowering Nonvolatile Substance Has little tendency to become gas under existing conditions Vapor pressure can be thought of as a measure of the tendency of molecules to escape from a liquid

Vapor Pressure Lowering Nonvolatile solute changes the boiling and freezing points of solvents Addition of nonvolatile solute lowers the concentration of solvent molecules at the surface of the liquid This lowers the tendency of the solvent molecules to leave the solution and enter the vapor phase Thus the vapor pressure of the solution is lower than the vapor pressure of the pure solvent Vapor Pressure Lowering animation

Vapor Pressure Lowering When vapor pressure is lowered, the solution remains in liquid over a larger temperature range, causing LOWER FREEZING POINT and HIGHER BOILING POINT Boiling Point elevation animation

Freezing-Point Depression (Nonelectrolytes) MOLAL FREEZING-POINT CONSTANT (Kf) The freezing-point depression of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte solute Kf H2O: –1.86 °C/m See Table 14.2, page 438 FREEZING-POINT DEPRESSION Difference between the freezing points of the pure solvent and a solution of a nonelectrolyte in that solvent; directly proportional to the molal concentration of the solution Δtf = Freezing-pt. Depression Kf = Molal Freezing-pt constant (°C/m) m = molality (mol/kg)

F-pt Depression PROBLEMS A solution consists of 10.3 g of glucose, C6H12O6, dissolved in 250. g of water. What is the freezing point depression of the solution? In a laboratory experiment, the freezing point of an aqueous solution of glucose is found to be -0.325°C. What is the molal concentration of the solution? If 0.500 m of a nonelectrolyte solute are dissolved in 500.0 g of ether, what is the freezing point of the solution?

Boiling-Point Elevation (Nonelectrolytes) MOLAL BOILING-POINT CONSTANT (Kb) The boiling-point elevation of the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte solute Kb H2O: 0.51 °C/m See Table 14.2, page 438 BOILING-POINT ELEVATION Difference between the boiling points of the pure solvent and a solution of a nonelectrolyte in that solvent; directly proportional to the molal concentration of the solution Δtb = boiling-point elevation Kb = molal boiling point constant (°C/m) m = molality (mol/kg)

B-Pt Elevation PROBLEMS A solution contains 150.0 g sucrose, C12H22O11, dissolved in 500.0 g of water. What is a) the boiling point elevation and b) boiling point of the solution? If the boiling point elevation of an aqueous solution containing a nonvolatile electrolyte is 1.02°C, what is the molality of the solution? The boiling point of an aqueous solution containing a nonvolatile electrolyte is 100.75°C. What is a) the boiling-point elevation and b) the molality of the solution?

Electrolytes & Colligative properties Electrolytes have a greater effect on the freezing and boiling points of solvents than do nonelectrolytes Electrolytes cause changes in in colligative properties proportional to the total molality of all dissolved particles (# of ions produced) instead of formula units

Electrolytes & Colligative properties i = # of ions per formula unit (Electrolytes) Δtb = boiling-point elevation Δtf = boiling-point elevation Kb = molal b-pt constant (°C/m) Kf = molal f-pt constant (°C/m) m = molality (mol/kg) SOLUTION Ba(NO3)2 (s)  Ba2+(aq) + 2NO3-(aq) Produces 3 ions (1 barium and 2 nitrate ions); i = 3 Δtf = iKfm Δtf = (3)(-1.86°C/m)(0.5 m) Δtf = -2.79°C tf = 0°C + (-2.79°C) = -2.79°C What is the freezing point of a 0.5 m aqueous solution of Ba(NO3)2 ?

Electrolytes & Colligative properties What is the expected freezing-point depression for a solution that contains 2.0 mol of magnesium sulfate dissolved in 1.0 kg of water? What is the expected boiling-point elevation of water for a solution that contains 150 g of calcium chloride in 1.0 kg of water? The freezing-point of an aqueous solution of sodium chloride is -20.0°C. What is the molality of the solution?

APPLICATIONS

Osmotic Pressure SEMIPERMEABLE MEMBRANE OSMOSIS OSMOTIC PRESSURE Allow the movement of some particles while blocking the movement of others Allows only solvent to pass through OSMOSIS The movement of solvent through a semipermeable membrane from the side of lower solute concentration to the side of higher solute concentration Solvent has a net flow towards more concentrated solution Solvent tends to dilute the solution and make it more disorganized OSMOTIC PRESSURE The external pressure that must be applied to stop osmosis

Osmotic Pressure

Osmotic Pressure HYPERTONIC SOLUTION Cells Shrink! Regulation of osmosis is vital to life of a cell because cell membranes are semipermeable Hypertonic solution – more concentrated solution Cells lose water and shrink when placed in a solution of higher concentration (PICKLES) HYPERTONIC SOLUTION Cells Shrink!

Osmotic Pressure Hypotonic solution – less concentrated solution Cells gain water and swell when placed in a solution of lower concentration (turgor pressure in plants) HYPOTONIC SOLUTION Cells Swell & Burst!

Osmotic Pressure In vertebrates, blood and lymph protect the cells from swelling and shrinking because they have the same concentration as the inside of the cell Isotonic solution – same concentration

Tonicity

REVERSE OSMOSIS/DIALYSIS Pressure is applied to the concentrated solution to reverse the flow of water DESALINATION OF SALTWATER Membrane is permeable to water but not to ions DIALYSIS Occurs at the walls of most plant and animal cells Membrane allows transfer of both solvent and small solute molecules and ions

Kidney Dialysis Animation of the kidney dialysis process Used in artificial Kidney Machines to purify blood The resulting dialysis moves waste molecules into dialyzing solution and cleanses the blood Animation of the kidney dialysis process

KIDNEY DIALYSIS