Colligative Properties properties of solutions, packet page 7 You might need to take additional notes as well

Colligative Properties Properties of solutions (compared to the pure solvent) Depend upon the number of solute molecules or ions (concentration), but NOT upon the identity of the solute. Include freezing point depression, boiling point elevation, vapor pressure, and osmotic pressure.

You should be able to: 1. Define the CP 2. How does concentration of the solute affect the CP? 3. Explain the mechanism of how the solute affects the CP. 4. Draw a molecular picture illustrating the effect of the solute on the solvent.

How do you determine how many particles there are? *Covalent compounds do NOT ionize so they stay as one particle. *Ionic compounds will dissociate into ions. Add the total resulting number of ions: NaCl  Na+ + Cl- Na2SO4  2Na+ + SO42- (NH4)3PO4  3NH4+ + PO43- Notice that polyatomic ions do not break apart further.

What is vapor pressure? Vapor pressure is the pressure produced when vapor particles above a liquid collide with the container walls Some molecules from the liquid phase will vaporize if The molecules are moving fast enough/sufficient energy The molecules are at the surface Vapor pressure increases with temperature

The vapor pressure of a solution is lower than the vapor pressure of the pure solvent. Solvent molecules are attracted to the solute particles, so fewer particles escape to the gas phase  fewer particles means lower pressure. Solute particles also reduce the number of solvent particles at the surface, making vaporation more difficult.

What is boiling point? What is a boiling point? The temperature at which a liquid becomes a gas When does boiling happen? When the vapor pressure above a solution equals the atmospheric pressure Vapor pressure increases as temperature increases, because molecules are moving faster and changing to gas phase

Boiling point elevation (BPE) The boiling point of a solution is higher than the boiling point of the pure solvent. Solvent molecules are attracted to the solute particles, and that attraction (IMF) requires energy to break. Solute particles also reduce the number of solvent particles at the surface, making evaporation more difficult.

Boiling point elevation is caused by lowered vapor pressure Solutes block solvent particles at the surface from escaping, which reduces vapor pressure More kinetic energy is required for solvent particles to evaporate, to raise the vapor pressure to atmospheric pressure (boil) More energy needed means higher temperature to boil

Air pressure at different altitudes affects the boiling point of water If water boils at a lower temperature, why do food packages recommend increased cooking times?

Freezing point depression (FPD) When a solute is present, the orderly pattern that the pure solvent normally forms becomes disrupted. More kinetic energy must be lost from the solution for it to solidify (freeze). This lowers the freezing point of the solution.

Solutes interfere with solid structure pattern during freezing

Some animals adapted to utilize freezing point depression Rainbow smelt survive winter under ice by producing anti-freeze proteins and glycerol (non-electrolyte) Spring peeper frogs can increase blood [glucose] by massive breakdown of liver glycogen to survive temporary freezes

Osmosis Osmosis is the net flow of solvent molecules from higher solvent concentration to lower solvent concentration. Net flow stops once the mass of liquid on the higher side exerts a pressure equal to the flow. This pressure is known as osmotic pressure.

Osmotic pressure When the concentrations of solutes are different on either side of a semi-permeable membrane, water is pressured to flow across the membrane to try to equalize the concentrations. [Note: they will never be equal.]

Osmotic pressure example Consider two beakers. One contains pure water, and the other contains water and sugar (red). If these two beakers were connected by a tube that contained a semi-permeable membrane, water molecules would flow from the side where [water] is higher to the side where [water] is lower. This flow of water, from the higher [water] lower [water], results in a change in volume of the two sides. The side that also has sugar has a larger volume.

Osmotic pressure in dialysis

As the solute concentration increases, more solvent will flow into the solution, increasing the osmotic pressure More liquid on one side means more pressure

Osmosis in blood cells A cell membrane is a semi-permeable membrane. Water can flow in and out of the cell based on the solutes in its environment.

Electrolytes Compounds that conduct an electric current in aqueous solution or in a molten (liquid) state. An electrolyte solution contains charged particles (ions) which can move. Any salt (ionic compound) dissolved in water is an electrolyte NaCl, KI, Mg(NO3)2, etc.

The degree to which a compound conducts depends on ionization Strong electrolytes—a large portion of the solute exists as ions, e.g. aqueous solutions of most ionic compounds (ionic) Weak electrolytes—these are solutions in which only a small portion of the solute exists as ions (WEAK acids and bases) Non-electrolytes—compounds that do NOT conduct electricity either when molten or in aqueous solution (covalent, organic, gases, distilled water)