Colligative properties of solutions The Effects of Solutes on Solvents

Objectives When you complete this presentation, you will be able to o define a colligative property o explain how colligative properties are dependent on the number of particles in solution o 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, C 6 H 12 O 6, 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 liquidsolution

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 liquidsolution

Vapor-pressure lowering If we have more solute, there are even fewer solvent particles available. pure liquidsolution

Vapor-pressure lowering If we have fewer solute, there are more solvent particles available. pure liquidsolution

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 liquidsolution

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.