Colligative Properties
description
Transcript of Colligative Properties
Colligative Properties
Vapor Pressure
Freezing and Boiling Points
Osmotic Pressure
Objectives
SWBAT Explain why boiling point can be
elevated or freezing point can be depressed.
Use ΔTb = i Kbm to solve boiling point or freezing point problems.
Drill
We will finish the Ksp problem from yesterday.
Website to Try
www.chemprofessor.com/colligative.htm
Definitions
Po = vapor pressure of the pure solventP = vapor pressure of the solvent in a solution
Boiling point - The temperature at which the vapor pressure of a liquid is equal to the pressure on the liquid.
Normal (Standard) Boiling Point – The temperature at which the vapor pressure of a liquid is equal to standard pressure (1.00 atm = 760 mmHg = 760 torr = 101.325 kPa)
Vapor Pressure Reduction
Nonvolatile Substance- a substance that doesn’t have a tendency to vaporize (i.e. sugar)
When a nonvolatile substance is added to a pure substance, solute molecules block the solvent molecules from evaporating
However, the solvent molecules can still change from gas to liquid
Vapor Pressure Reduction
Because more molecules are leaving the gas state than entering it, there are less molecules of gas
This in turn lowers the pressure of the gas, known as vapor pressure reduction
The reduction of vapor pressure does not depend on the solute used
Vapor Pressure and IMF
“If the liquid is part of a solution, the non-volatile solute molecules act to disrupt the evaporation process by attracting the solvent molecules, since they have intermolecular forces at least as strong as the liquid molecules, they impede evaporation. The presence of the solute does not affect the condensation process, because, since it is non-volatile, it is not present in the vapor phase.”
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Boiling Point Elevation
The boiling point of any substance is the temperature where the vapor pressure is equal to the atmospheric pressure (usually)
Because the vapor pressure is reduced when a nonvolatile substance is added, a higher temperature is required to make the two pressures match
(new boiling point) – (original boiling point) = the boiling point elevation This is shown as ΔTb
Boiling Point Elevation
Solute particles weaken IMF in the solvent.
Adding a Solute
There are two reasons why the addition of a solute raises the boiling point of a solution
(i.e. why the vapor pressure of the solvent in a solution is lower than the vapor pressure of a pure solvent):
Reason #1
1. The solute particles occupy space at the surface. This consequently slows the rate at which the solvent molecules in the liquid phase can escape into the gas phase
Reason #2
2. P < Po
P = Xsolvent Po
Raoult's Law - The vapor pressure of the solvent above a solution is equal to the product of the mole fraction of the solvent and the vapor pressure of the pure solvent:
The solute particles introduce a new set of attractive forces with the solvent molecules.
The vapor pressure of the “solvent with the solute” is lower than the vapor pressure of the pure solvent.
In order to raise the vapor pressure of the “solvent with the solute” to atmospheric pressure, the temperature must be raised.
Effect of Adding Solute to a Pure Solvent
http://library.thinkquest.org/C006669/media/Chem/img/antiBP.gif
Boiling Point Elevation
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Calculating ΔTb
ΔTb = i Kbm m is the molality of the solution Kb is a constant that is different for every solvent i is the Van’t Hoff factor, which is the number of
particles into which the solute dissociates
Common Kb’s are located in Figure 15-25 in your book
Freezing Point Depression
Freezing Point Depression
is the change in temperature to where the solid vapor pressure equals the liquid vapor pressure
ΔTf = i Kfm
Freezing Point Depression
Freezing Point Depression
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Definition
Osmotic pressure is the pressure that must be applied to a solution to prevent the inward flow of water across a semipermeable membrane.
Π = iMRTi is the dimensionless van 't Hoff factor
M is the molarity
R=0.08206 L · atm · mol-1 · K-1 is the gas constant
T is the thermodynamic (absolute) temperature
Thermodynamic (absolute) temperature is the temperature measured or calculated on an absolute scale.
Osmotic Pressure
When two solutions of different concentrations are separated by a semi-permeable membrane, osmosis occurs to result in the distribution of solute molecules
What happens if you have a u-shaped tube, with a membrane in the middle, and different concentrations on either side?
If, assuming that the two sides have different concentrations, what happens to this solution?
The solution will actually become uneven. This will continue until the concentrations are equal, or the osmotic pressure is reached
Osmotic Pressure
Osmotic pressure is the pressure required to stop osmosis in the previous example
It is generally represented as the Greek letter pi
Osmotic pressure plays very important roles in the chemistry of the body
Osmosis Equation
π = i M R T
http://www.brynmawr.edu/Acads/Chem/Chem103lc/colligative_lecture.html
Try this Osmotic Pressure website
http://web.fccj.org/~ethall/2046/ch11/op.htm
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Osmosis in Real Life
http://www.chrisrepetsky.com/wp-content/uploads/2009/10/553px-Osmotic_pressure_on_blood_cells_diagram.svg.png