Colligative Properties of Solutions

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# Colligative Properties of Solutions - PowerPoint PPT Presentation

Colligative Properties of Solutions. Step 1 Breaking up the solute into individual components (expanding the solute) Step 2 Overcoming the intermolecular forces in the solvent to make room for the solute (expanding the solvent) Step 3

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### Colligative Properties of Solutions

Step 1

Breaking up the solute into individual components (expanding the solute)

Step 2

Overcoming the intermolecular forces in the solvent to make room for the solute (expanding the solvent)

Step 3

Allowing the solute and solvent to interact to form the solution

Forming Solutions
• Step 1 and 2 are endothermic
• Step 3 often is exothermic
• Delta Hsoln = deltaH1 + deltaH2 + deltaH3
• + delta H means?
• - delta H means?
Enthalpy of hydration (heat of hydration)

DeltaHhyd combines deltaH2 and delta H3

Example: For NaCl

deltaHsoln = 786 kJ/mol – 783 kJ/mol

= 3 kJ/mol

we must supply a small amount of energy

Why does it happen so readily?

Processes run to disorder (ping pong demo)

Structural Effects:

Polar vs Nonpolar

Pressure Effects

Henry’s Law

P = kC

P pressure of gaseous solute

C solution

K constant

The amount of a gas dissolved in a solution is directly proportional to the pressure of the gas above the solution

Solubility
Lake Nyos Tragedy

With excess CO2 on bottom.

August 21, 1986 A cloud of gas

Boiled from Lake Nyos, killing

2000 people and many animals.

It was CO2 !

A nonvolatile solute lowers the vapor pressure of a solvent

RAOULT’S LAW

Psoln = XsolventPosolvent

P vapor pressure of solution

X mole fraction of solvent

Po vapor pressure of pure solvent

Vapor Pressure
Because changes of state depend on vapor pressure, the presence of a solute affects the

BOILING POINT

FREEZING POINT

OSMOTIC PRESSURE

So, what is a colligative property anyway?
Colligative properties are grouped together because they depend only on the number and not on the identity of particles in an ideal solutionColligative Properties Continued
Used to characterize the nature of a solute after it is dissolved in a solvent

Used to determine the molar masses of substances

Why Should We Care?

Properties of Solutions

• Colligative Properties
• Boiling point elevation occurs as a result of lowering the vapor pressure of a liquid containing a nonvolatile solute.
• In order to reach any vapor pressure the temperature of the solution must be higher than the temperature of the pure liquid.
• In this Figure, the red line corresponds to the vapor pressure of pure benzene vs. temperature whereas the blue line is the vapor pressure of a 2.0 m solution of a nonvolatile solute in benzene.
• The increase in boiling temperature is proportional to the concentration of the solute:
• Dtbp=Kbpm where Dtbp is the increasein the boiling temperature of the solvent
• Kbp is the boiling point elevation constant for the solvent.
• m is the molality of solute in the solution.

Properties of Solutions

• Freezing point depression also results from a lowering of the vapor pressure of a liquid containing a nonvolatile solute.
• The decrease in freezing temperature is proportional to the concentration of solute:
• Dtfp=Kfpm where Dtfp is the decrease in the boiling temperature of solvent
• Kfp is the freezing point depression constant for the solvent.
• m is the molality of the solute in the solution.
• Calculating molar masses from colligative properties

Change in vapor pressure, boiling point elevation, or freezing point depression or osmotic pressure

Use mass

of solvent

Solution concentration

Molar mass

Moles of solute

Properties of Solutions

• Calculating molar masses from colligative properties
• Example: 0.640 g azulene (empirical formula C5H4) is dissolved in 99.0 g benzene. The boiling point of the solution is 80.23 oC. What is the molecular formula of azulene?

Properties of Solutions

• Colligative Properties
• Freezing point depression
• For solutions of ionic compounds, the molality will be an integer multiple of the compound’s molality.
• For NaCl, there are 2 moles of particles per mole NaCl, so a 1 molal solution of NaCl is 2 molal in particles
• For CaCl2, there are 3 moles of particles per mole CaCl2, so a 1 molal solution of CaCl2 is 3 molal in particles
• The expected effect on freezing temperature and boiling temperature for a 1 molal solution of NaCl or CaCl2will be 2 or 3 times that of a 1 molal solution of a nondissociating solid.
• A 1.00 m solution of NaCl should freeze at 2x(-1.86) oC.
• Experimentally it is found that the freezing point of solutions of ionic solids is not quite as low as predicted.
• This results because ion pairs are formed in solution because of ion-ion interactions, reducing the expected molality of the solute.

Properties of Solutions

• Colligative Properties
• Freezing point depression
• The van’t Hoff factor, i, is a measure of the dissociation of an ionic compound
• Van’t Hoff factors for several substances at 25 oC

Properties of Solutions

• Colligative Properties
• Osmosis is the transport of solvent molecules through a semipermiable membrane from a solution of high solvent concentration to a solution of lower solvent concentration.
• Solvent molecules move from the solution of lower soluteconcentration to the solution of higher solute concentration in an attempt to dilute the solution with higher solute concentration.
• Osmosis will cease when the pressure resulting from transport of solvent to the solution of higher solute concentration prevents further transport of solvent through the membrane.
• The osmotic pressure, , is related to the concentration of the solution, and the absolute temperature:
• This is analogous to the ideal gas law, where P~ and n/V~c

where c is the molarity of the solution

• Isotonic solutions have the same osmotic pressure.
• A hypotonic solution has lower osmotic pressure than another solution.
• A hypertonic solution has higher osmotic pressure than another solution.

Properties of Solutions

• Colligative Properties
• Osmosis
• Example: Lysozyme is an enyme tha breaks bacterial cell walls. A solution containing 0.150 g in 210 mL of solution has an osmotic pressure of 0.953 torr at 25 oC. What is the molecular weight of of lysozyme?

Properties of Solutions

Colloids are suspensions of particles that have a size between approximately 10 Å and 2,000Å dispersed in another medium.

• Colloidal particles can be made up of an agglomeration of many small particles or molecules or could be a single, large molecule.

Properties of Solutions

• Colloids
• Hydrophilic colloids are stabilized as a result of having polar or ionic groups on their surfaces which are water loving.
• The strong attractive interactions with water keeps the large particles in suspension.
• Hydrophobic colloids are stabilized by adsorption of ions on the particle surfaces.
• Adsorption is the binding of a substance to the surface of bulk matter.
• The adsorbed ions interact with water to keep the particles suspended.
• The charges on the surfaces of different particles are of the same sign which keeps them apart and prevents agglomerating and precipitating.
• Soaps and detergents consist of a long hydrophobic tail and a hydrophilic end.
• The hydrophobic tail inserts into oil or other hydrophobic materials and causes the particles to be suspended in water as the hydrophilic end will be on the outside of the particle

-

+

Properties of Solutions

• Colloids
• Soaps and detergents behave as emulsifying agents causing oils to be suspended in aqueous dispersing medium.
• Removal of colloidal particles from suspension
• Colloidal particles in dispersed in liquids stabilized by adsorption of ions can be coagulated by adding an inert electrolyte - HNO3 - and heating the suspension.
• Solid particles suspended in air can be precipitated by producing a surface charge on the particles and attracting them to an oppositely charged electrode.