Colligative properties of solutions The Effects of Solutes on Solvents.

Colligative properties of solutionsThe Effects of Solutes on Solvents

Objectives• When you complete this presentation, you will be

able too define a colligative propertyo explain how colligative properties are dependent

on the number of particles in solutiono describe the effects on the numbers of particles

in a solution on the vapor pressures, freezing points, boiling points, and osmotic pressures of those solutions

Introduction• The physical properties of a solution differ from

the properties of the pure solvent.o Salt added to water decreases the freezing point

of the solution.o Copper(II) sulfate added to water changes the

color.• When a property depends on the number of

particles of solute in a solution, that is a colligative property.

Introduction• Colligative properties depend only on the number

of solute particles, not on identity of the solute.• Different solutes produce different numbers of

solute particles when they are dissolved.• Ionic compounds produce more particles per mol

of solute dissolved than do molecular compounds.

Introduction• For example:• NaCl produces two mols of particles for each

mol of salt dissolved:»1 mol of Na+

and»1 mol of Cl-

• Glucose, C6H12O6, produces one mol of particles for each mol of glucose dissolved.

Introduction• There are many different kinds of colligative

properties.• Important colligative properties of solutions that

we will study include:• vapor-pressure lowering• freezing-point depression• boiling-point elevation• osmotic pressure increase

Vapor-pressure lowering• Vapor pressure is the pressure exerted by a vapor

that is in equilibrium with its liquid in a closed system.

• When we add a solute to pure liquid, we reduce the number of particles of liquid that are available to be converted to vapor.

pure liquid solution

Vapor-pressure lowering• In the pure liquid, there are 8 solvent particles

available to go into vapor.• In the solution, there are only 6 solvent particles

available to go into vapor.

pure liquid solution

Vapor-pressure lowering• If we have more solute, there are even fewer

solvent particles available.

pure liquid solution

Vapor-pressure lowering• If we have fewer solute, there are more solvent

particles available.

pure liquid solution

Vapor-pressure lowering• The decrease in vapor pressure is proportional to

the number of particles in solution.• Adding one mol of NaCl produces twice the vapor

pressure lowering of adding one mol of glucose.

pure liquid solution

Freezing-point depression• Adding a solute to a solvent lowers the freezing

point of the solution.• The lowering is proportional to the number of

particles of the solute added to the solvent.• 1 mole of glucose added to 1 kg of water results

in a fp lowering of 1.86°C.• 1 mole of NaCl added to 1 kg of water results in

a fp lowering of 3.72°C.

Freezing-point depression• This is used for -• keeping the roads ice-free in the winter

•home-made ice cream

•adding antifreeze to your car

Boiling-Point elevation• Adding a solute to a solvent also raises the boiling

point of the solution.• The elevation is also proportional to the number

of particles of the solute added to the solvent.• 1 mole of glucose added to 1 kg of water results

in a bp elevation of 0.51°C.• 1 mole of NaCl added to 1 kg of water results in

a bp elevation of 1.02°C.

Boiling-Point elevation• This is used for -• adding to your car’s cooling system for the

summer

Osmotic Pressure

• Osmosis is the spontaneous movement of solvent molecules through a partially permeable membrane into an area of higher solute concentration.

• This process is used extensively in biological systems.o Biological membranes are semipermeable.

Osmotic Pressure

In an osmotic cell, we have two solutions of different concentrations divided be a semipermeable membrane.

Osmotic Pressure

Pure water will move through the semipermeable membrane from the cell on the right into the cell on the left.

Osmotic Pressure

This causes the water level on the left to rise and the water level on the right to fall.

Osmotic Pressure

This causes the water level on the left to rise and the water level on the right to fall.

Osmotic Pressure

At a certain point, the pressure from the height of the water on the left will cause water to flow back into the cell on the right and the system will come to an equilibrium.

Osmotic Pressure

The difference in liquid levels between the two cells is a measure of the osmotic pressure of the system.

Osmotic Pressure

A system with a greater number of solute particles in one cell will have a greater osmotic pressure.

Osmotic Pressure

A system with a greater number of solute particles in one cell will have a greater osmotic pressure.

Osmotic Pressure

If we raise the solution level of the left cell only, there will be more pressure to move pure water to the right than there is pressure to move water to the left.

Osmotic Pressure

If we raise the solution level of the left cell only, there will be more pressure to move pure water to the right than pressure to move water to the left.

Osmotic Pressure

Water will move from the left cell to the right cell until the equilibrium is reestablished.

Osmotic Pressure

This is called reverse osmosis and is used around the world to purify water.

Osmotic Pressure

Membranes are fragile and expensive, so the process is used only where other purification systems will not work.

Summary• The physical properties of a solution differ from

the properties of the pure solvent.• When a property depends on the number of

particles of solute in a solution, that is a colligative property.

• Colligative properties depend only on the number of solute particles, not on identity of the solute.

• Ionic compounds produce more particles per mol of solute dissolved than do molecular compounds.

Summary• Important colligative properties of solutions

include vapor-pressure lowering, freezing-point depression, and boiling-point elevation.

• When we add a solute to pure liquid we -o reduce the vapor pressure that is exerted over

the surface of the liquid.o lower the freezing point of the liquid.o raise the boiling point of the liquid.o increase the osmotic pressure of the liquid.