# Ch. 14: Mixtures & Solutions

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Ch. 14: Mixtures & Solutions
Sec. 14.4: Colligative Properties of Solutions

Objectives Describe colligative properties.
Identify four colligative properties of solutions. Determine the boiling point elevation and the freezing point depression of a solution.

Colligative Properties of Solutions
Solutes affect the physical properties of their solvents. Physical properties of solutions that are affected by the number of particles but not the identity of dissolved solute particles are called colligative properties. Vapor pressure, boiling point, freezing point, and osmotic pressure are colligative properties.

Electrolytes and Colligative Properties
Ionic compounds are electrolytes because they dissociate in water to form a solution that conducts electric current. Electrolytes that produce many ions are strong electrolytes. Electrolytes that produce only a few ions are weak electrolytes.

Electrolytes and Colligative Properties
NaCl is a strong electrolyte. It almost completely dissociates in solution, producing Na+ and Cl- ions. NaCl Na Cl- Each mole of NaCl that is dissolved, therefore, will produce 2 moles of ions. A 1 m solution of NaCl will produce a 2 m solution of ions.

Nonelectrolytes in Aqueous Solution
If one mole of each was dissolved in a liter of water NaCl would create twice the number of pieces floating around because it ionizes. Therefore it would have more of an effect on the colligative properties of the solution.

Nonelectrolytes in Aqueous Solution
Molecular compounds in solution generally DO NOT ionize. They are nonelectrolytes. Therefore, when 1 mole of such a compound is dissolved in water, there will only be one mole of molecules in solution. A 1 m sucrose solution will contain 1 m of sucrose molecules.

Remember . . . Colligative properties depend on the number of particles in solution. We would therefore expect the compound producing more particles in solution to have a greater effect on the colligative properties.

Practice Problem Which of the following substances will have the greatest effect on the colligative properties of a solution? HCl C6H12O6 MgCl2 CuSO4 Al(NO3)3

Colligative Properties: Solutes Cause Vapor Pressure Lowering
Vapor pressure is the pressure exerted in a closed container by liquid particles that have escaped the liquid’s surface and entered the gaseous state. The particles that produce vapor pressure escape the liquid phase at its surface.

Why does adding a nonvolatile solute to a solvent lower the solvent’s vapor pressure?

Vapor Pressure Depression
When a solvent is pure, its particles occupy the entire surface area. When the solvent contains a solute, a mix of particles occupies the surface area. Fewer solvent particles are there, so fewer are able to enter the gaseous state. The greater the number of solute particles, the lower the resulting vapor pressure. When the solvent is pure (left), its particles occupy the entire surface area. However, when the solvent contains solute (right), a mix of solvent and solute particles occupy the surface area. With fewer solvent particles at the surface, fewer particles enter the gaseous state and the vapor pressure is lowered.

Colligative Properties: Solutes Cause Boiling Point Elevation
A substance boils when its vapor pressure equals atmospheric pressure. If a solution is heated to the boiling point of its solvent, because the vapor pressure of the solution is lower than the VP of the pure solvent, the VP will NOT equal the atmospheric pressure. A solution will not boil at the solvent’s boiling point. When a solute is added and the temp is raised to the boiling point of the pure solvent, the resulting vapor pressure is still less than the atmospheric pressure and the solution will not boil. The solution must be heated to a higher temperature to supply the additional kinetic energy needed to raise the vapor pressure to atmospheric pressure.

Boiling Point Elevation
The solution must be heated to a higher temperature to boil. The vapor pressure of the solution must be increased (by heating) to equal the atmospheric pressure. The temperature difference between a solution’s boiling point and a pure solvent’s boiling point is called the boiling point elevation (ΔTb). ΔTb = Tb solution - Tb solvent

Boiling Point Elevation
For nonelectrolytes, ΔTb = Kbm where: ΔTb is the boiling point elevation of the solvent. m is the molal concentration of the solution. Kb is the molal boiling point elevation constant (see Table 5, pg. 500). The units for this constant are 0C/m. Note: Every solute has a different Kb. Kb is the difference between the boiling points between a 1 m solution and a pure solvent. Like vapor pressure lowering, boiling point elevation is a colligative property. The value of the boiling point elevation is directly proportional to the solutions, solute molality, the greater the number of solute particles in the solution, the greater the boiling point elevation.

Boiling Point Elevation
Since boiling point elevation is a colligative property, it is dependent on the number of particles the solute forms in solution. Therefore, for electrolyte solutions, the particle molality (or # particles in the solute times m) must be used instead of m in the equation: ΔTb = Kbm The greater the number of solute particles in the solution, the greater the BPE.

Practice Problems -What is the boiling point of a m aqueous solution of sucrose? (Kb water = C/m) -The BPE of an aqueous solution of a nonvolatile, nonelectrolyte is C. What is the solution molality? -How many grams of sucrose (C12H22O11) must be dissolved in 125 g of ethanol to raise the boiling point by 4 C0? (Kb ethanol = C/m)

Colligative Properties: Solutes Cause Freezing Point Depression
The freezing point of a solution is always lower than that of a pure solvent. Why? Antifreeze is a substance added to a solvent, such as water, to lower its freezing point. Antifreeze is typically added to water in the cooling system of an internal-combustion engine so that it can be cooled below the freezing point of pure water without freezing. Ethylene glycol is the most widely used automotive cooling-system antifreeze, although methanol, ethanol, isopropyl alcohol, and propylene glycol are also used. In automotive windshield-washer fluids, an alcohol (e.g., methanol) is usually added to keep the mixture from freezing; it also acts as a solvent to help clean the glass. Antifreeze is toxic to humans and animals. Waste antifreeze contains heavy metals such as lead, cadmium, and chromium in high enough levels to potentially make it a regulated hazardous waste, so most states strictly regulate antifreeze disposal. Dumping antifreeze can cause serious water quality problems and might harm people, pets, or wildlife.

Freezing Point Depression
Normally at the solvent’s freezing point, solvent particles do not have the KE to overcome interparticle forces. Therefore, an organized crystal forms. (The solvent freezes.) Added solute particles interfere with these forces and, so, the formation of the solid at the normal freezing point. Additional energy must be removed (the temperature must be lowered further) before the solution will freeze.

Freezing Point Depression
The temperature difference between a solution’s freezing point and a pure solvent’s freezing point is called the freezing point depression (ΔTf). ΔTf = Tf solution - Tf solvent (absolute value)

Freezing Point Depression
For nonelectrolytes, D Tf = Kfm (see Table 6, pg. 502 for Kf values) For electrolyte solutions, the particle molality (or # particles in the solute times m) must be used instead of m in the above equation. The greater the number of solute particles in the solution, the greater the FPD.

Practice Problems -What is the freezing point of a m aqueous solution of sucrose? (Kf water = C/m) -The FPD of an aqueous solution of a nonvolatile, nonelectrolyte is C. What is the solution molality? How many grams of sucrose (C12H22O11) must be dissolved in 500 g of ethanol to lower the freezing point by 2.5 C0? (Kf ethanol = C/m)

Colligative Properties: Solutes Cause Osmotic Pressure Elevation
Osmosis is the diffusion of water (solvent) particles across a semipermeable membrane from an area of higher solvent concentration to an area of lower solvent concentration. Semipermeable membranes are barriers with tiny pores that allow some particles to cross but not others.

Osmotic Pressure Elevation
Normally, water flows from your intestine into your tissues. However, drinking sea water results in a reversal in the direction of water flow and, so, the dehydration of body tissues. Drinking seawater promotes dehydration because seawater is a thirsty solution. As the seawater flows through the stomach and intestine it draws water out of bodily tissues. Solutes in solutions will always flow from a region of higher concentration to a region of lower concentration. Seawater has a lower concentration of water than pure water has (because of the salt!), so water molecules will migrate toward a sample of seawater. If the seawater is in a person's stomach or intestines, water will move toward the seawater from the body's tissues, resulting in dehydration.

Osmotic Pressure Elevation
An aqueous solution (green) is separated from pure water by a membrane permeable to H2O molecules but not to solute particles. A net flow of water through the semipermeable membrane dilutes the green solution somewhat and causes the liquid level in the tube to rise. The liquid column now exerts a downward pressure (a hydrostatic pressure) that helps push H2O molecules through the membrane and into the beaker. When the flow of H2O through the membrane is the same in both directions, there is no further change, and the hydrostatic pressure at that point is called the osmotic pressure,

Osmotic Pressure Elevation
The flow of additional solvent molecules to the solution side of a membrane creates pressure. This increase in pressure is called osmotic pressure. Osmotic pressure is defined as the amount of additional pressure caused by water molecules that moved into the concentrated solution. Osmotic Pressure depends upon the number of solute particles in a given volume of solution. The greater the number of solute particles in the solution, the greater the osmotic pressure.

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