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

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.

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.

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.

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.

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

1 -What is the boiling point of a 0.029 m aqueous solution of sucrose? (Kb water = 0.512 0C/m)

2 -The BPE of an aqueous solution of a nonvolatile, nonelectrolyte is 1.12 0C. What is the solution molality?

3 -How many grams of sucrose (C12H22O11) must be dissolved in 125 g of ethanol to raise the boiling point by 4 C0? (Kb ethanol = 1.22 0C/m)

Colligative Properties: Solutes Cause Freezing Point Depression

• The freezing point of a solution is always lower than that of a pure solvent.

• Why?

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,

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 Problems1 -What is the freezing point of a 0.029 m aqueous

solution of sucrose? (Kf water = 1.86 0C/m)

2 -The FPD of an aqueous solution of a nonvolatile, nonelectrolyte is 4.65 0C. What is the solution molality?

3 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 = 1.99 0C/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.

Osmotic Pressure Elevation

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.