15.3: Colligative Properties of Solutions
I. Electrolytes & Colligative Properties
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Colligative properties of solutions are those properties affected by the number of particles but not the
identity of the dissolved solute.
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Electrolytes are ionic compounds that dissociate in water to form a solution that conducts electricity. (Ex:
NaCl)
– Some molecular compounds can do this, too.
– Strong electrolytes produce many ions.
– Weak electrolytes produce few ions.
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Nonelectrolytes are usually molecular compounds that dissociate in solvents, but do not ionize. (Ex:
sucrose)
II. Vapor Pressure Lowering
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Vapor pressure is exerted by evaporated particles in a closed container of liquid.
– At constant T & P, a dynamic equilibrium between evaporating & condensing particles exists.
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The greater the number of solute particles in a solution, the lower the resulting vapor pressure.
– This is because there are fewer solvent particles at the surface to escape the liquid phase.
– Electrolytes have a greater effect than nonelectrolytes.
III. Boiling Point Elevation
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Because a solute affects a solvent’s vapor pressure, it also affects the boiling point of the solvent.
– Recall: Boiling occurs when vapor pressure equals atmospheric pressure.
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Because vapor pressure is lowered by the presence of a solute, the solution must be heated to a higher
temperature than the solvent’s normal boiling point for boiling to occur.
– More solute particles = greater b.p. elevation
IV. Freezing Point Depression
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At the freezing point, the particles of a liquid do not have enough kinetic
energy to overcome intermolecular forces and become solid.
– In a solution, solute particles interfere with intermolecular attractions.
– The freezing point of a solution is always lower than that of the pure
solvent.
– Electrolytes have a greater effect than nonelectrolytes.
V. Osmosis & Osmotic Pressure
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Osmosis is the diffusion of solvent particles across a semipermeable
membrane from an area of high concentration to an area of low.
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More solvent particles can pass from the pure side than from the solution side.
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The increasing height of the solution creates a pressure that opposes the
diffusion of solvent from the pure side—osmotic pressure.
– Depends on the number of solute particles in a given volume of
solution.
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Eventually, the rates of diffusion in both directions become the same.