Colligative Properties
description
Transcript of Colligative Properties
Colligative Properties• Physical properties affected by dissolved
solute particles
• Type of solute doesn’t matter.
• Number of particles does matter.
• Colligative means “depending on the collection”
Colligative Properties
• Occur with nonvolatile solutes – ones that have little tendency to become a gas
• 4 colligative properties: – Vapor pressure lowering– Boiling point elevation– Freezing point depression– Osmotic pressure
Colligative Properties• Electrolytes are molecules that ionize or
fall apart when in solution; nonelectrolytes do not fall apart or ionize
• Electrolytes will have a greater impact on the 4 colligative properties
• Why? – Sugar (C12H22O12) remains as one molecule
when dissolved.– Salts like MgCl2 dissociate in water so one
mole of MgCl2 actually forms 3 moles of ions.
Vapor Pressure Lowering• Pressure in a CLOSED container exerted
by the particles of liquid that have escaped and become gas particles
• Greater number of escaped particles = greater vapor pressure
• The addition of a solute LOWERS the vapor pressure – WHY?
• The solute molecules get in the way of the solvent molecules trying to escape at the surface of the liquid.
Boiling Point Elevation• A liquid boils when it’s particles can escape
as a gas (or when the vapor pressure is equal to the atmospheric pressure)
• Ex: salt in water when cooking pasta antifreeze in radiators
• When a solute is added, the boiling point is driven higher – WHY?
• The solute molecules get in the way of the solvent molecules trying to escape at the surface of the liquid.
Freezing Point Depression• A liquid freezes when its particles do not have
enough energy to overcome the intermolecular forces between them.
• So the molecules are pulled closer together and take on a defined shape.
• When a solute is added, the freezing point is driven lower – WHY?
• The solute molecules get in the way and make it harder for the solvent molecules to move closer and become a solid
• Ex: salt & sand on icy roads ethylene glycol on icy airplanes
Calculating b.p. elevation and f.p. depression
• Because the type of solute doesn’t matter, you only need to know the amount of solute
• Elevation & depression happen at a constant rate depending on the amount of solute added (number of moles)
∆ Tb = Kbm (where Kb & Kf are the constants)∆ Tf = Kfm
Try It• If you have .625 m aqueous solution with a
nonvolatile, nonelectrolyte solute, what would the boiling point and freezing point be?
Try It• Calculate the freezing point and boiling
point of a solution that contains 105.4 g NaCl dissolved in 750.3 ml H2O.
Osmotic Pressure• Osmosis is a natural process where the
solvent flows across a membrane – always flows from where there is more solvent to where there is less solvent.
• Adding solute to one side of the membrane will cause more solvent to flow that way.
• This increases the osmotic pressure on that side of the membrane.
• Ex: kidney dialysisuptake of nutrients by cells