Colligative Properties 5y581q

Colligative Properties  Properties determined by the number of particles (concentration) in solution rather than the type of particles  Vapor pressure  Freezing point  Boiling point  Osmotic pressure Raoult’s Law  Vapor pressure of a solution varies directly with the mole fraction of solvent multiplied by the vapor pressure of pure solvent Psolu = Xsolvent Psolvent Xsolvent = Mol solvent_____ Mol solute + mol solvent

Ex. Calculate the vapor pressure of a 1.00 m solution of a nonvolatile solute in water at 50 oC. Vapor pressure of water at 50oC is 0.122 atm. m = m solute (change to mol) kg solvent 1.00 m = 1 mol solute 1 kg solvent Kg H2O x 1000 g x 1 mol H2O_= 55.49 mol H2O 1 kg 18.02 g H2O X H2O = 55. 49 = 0.9822 1 + 55.49

(Solve for mol solvent)

(Solve for X solvent)

Psolu = (0.9822) (0.122 atm) Psolu = 0.12 atm

(Solve for Vapor pressure)

Boiling Point  The temperature at which the vapor pressure of the liquid phase equals atmospheric pressure Boiling-Point Elevation  The difference in temperature between the boiling point of a solution and the boiling point of the pure solvent Boiling Point Elevation ( Tb) Tb = Tb solu – Tb solvent Tb = Kbm

Ex. What is the boiling point of an aqueous solution containing 364g of sucrose (C 12H22O11) in 250g of water? (FW = 342.5g/mol; to water = 1000C; kb water = 0.5120C, kg) mol m = mol solute = 364g x 1 mol kg kg solvent 342.5g_ 250g x 1 kg_ Tb = (0.5120C, kg) (4.25 mol) 1000g mol kg m = 4.25 mol

Tb = 2.1760C 2.1760C = Tb solu - 1000C

2.1760C + 1000C = Tb solu 102.1760C = Tb solu

Freezing-Point Depression Freezing Point  The difference in temperature between the freezing point of a solution and the freezing point of the pure solvent (water) Freezing-Point Depression ( Tf) Tf = Tf solvent – Tf solu Tf = Kfm

Ex. What is the freezing point of the solution containing 364g of sucrose (C 12H22O11) in 250g of water? (FW = 342.5g/mol; Tf water = 00C; Kf water = 1.860C, kg) mol Tf = ( 1.860C, kg) (4.25 mol) mol kg Tf = 7.9050C 7.9050C = 00C - Tf solu 7.9050C - 00C = - Tf solu -1 (7.9050C = - Tf solu) - 7.9050C = Tf solu

Osmotic Pressure  The pressure needed to prevent the flow of solvent from dilute solution to a more concentrated solution Colloids Suspension  Particles will settle Solution:  Homogeneous  <1nm particle size  Does not separate  es through filter paper

Solution  Mix, uniform all through out Colloid: Suspension:  Homogeneous  Heterogeneous  1-100 nm particle  >100nm particle size size  Separate/settle  Does not separate  Doesn’t through  es through filter filter paper paper

Unique properties of colloids  Tyndall effect  Scattering of light  Ex. ray of light  

Brownian movement Particles are moving in rapid random zigzag motion

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Electric charge A dispersed colloidal particle can adsorb (not go in) electrically charged particles on its surface Prevents coagulation (clumping together)/precipitation of dispersed colloidal particles

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Adsorptive Property

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Surfaces of solids and liquids can adsorb molecules from fluid phases (liquids and gases) onto their surface

Adsorptive Property:  Chemisorption  Fairly strong, resembling chemical bonds and difficult to break  

Physisorption Weak, involves on Van der Waals forces and the adsorbed material can usually be removed again

Classification of colloids: Components: dispersed particles (solute) and dispersing medium (solvent)  Can be classified according to:  Affinity between phases  Phase Classification according to Affinity between phases:  Lyophilic  solvent – loving  have a strong attraction for the molecules of the dispersing medium   

Lyophobic Solvent – fearing Dispersed phase are actually immiscible with the solvent under normal conditions

Classification according to Phase:  Aerosols  Suspensions of liquid or solid particle in gas  

Foams Colloidal dispersion of gas bubbles in liquids (liquid foams) or solid (solid foams)

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Sols A solid dispersed in either a solid or liquid

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Gels A colloidal system in which the dispersed phase consists of fibrous, interwoven particles called fibrils which exert a marked effect on the physical properties of the dispersing medium

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Emulsions Colloidal dispersion of liquid in either liquid or solid The 2 components of an emulsion are usually immiscible and require an addition of emulsifying agent or peptizing agent to form stable emulsion If the dispersed particles in an emulsion are relatively large, they still tend to separate from the dispersion medium. This could be prevented by the process called homogenization.

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Removal of Colloidal Particles  Colloid particles are coagulated (enlarged) until they can be removed by filtration

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Methods of coagulation: o Heating (colloid particles move and are attracted to each other when they collide); o Adding an electrolyte (neutralize the surface charges on the colloid particles)