1. Prepared by PhD Halina Falfushynska Lecture 8. Colligative properties of solutions

2. A space-filling model of the water molecule.

4. GENERAL PROPERTIES OF SOLUTIONS 1. A solution is a homogeneous mixture of two or more components. 2. It has variable composition. 3. The dissolved solute is molecular or ionic in size. 4. A solution may be either colored or colorless nut is generally transparent. 5. The solute remains uniformly distributed throughout the solution and will not settle out through time. 6. The solute can be separated from the solvent by physical methods.

5. Polar water molecules interact with the positive and negative ions of a salt, assisting with the dissolving process.

7. Electrical Conductivity of Ionic Solutions

8. Electrical Conductivity

10. Comparison of a Concentrated and Dilute Solution

11. Comparison of an Unsaturated and Saturated Solution

13. Molarity (Concentration of Solutions)= M M = = Moles of Solute Moles Liters of Solution L solute = material dissolved into the solvent In air (gas), Nitrogen is the solvent and oxygen, carbon dioxide, etc. are the solutes. In sea water (liquid), Water is the solvent, and salt, magnesium chloride, etc. are the solutes. In brass, Copper is the solvent (90%), and Zinc is the solute(10%)

14. MOLALITY Molality = moles of solute per kg of solvent m = n solute / kg solvent molal solutionIf the concentration of a solution is given in terms of molality, it is referred to as a molal solution. Q. Calculate the molality of a solution consisting of 25 g of KCl in 250.0 mL of pure water at 20 o C? First calculate the mass in kilograms of solvent using the density of solvent: 250.0 mL of H 2 O (1 g/ 1 mL) = 250.0 g of H 2 O (1 kg / 1000 g) = 0.2500 kg of H 2 O Next calculate the moles of solute using the molar mass: 25 g KCl (1 mol / 54.5 g) = 0.46 moles of solute Lastly calculate the molality: 1.8 m (molal) solution m = n / kg = 0.46 mol / 0.2500 kg = 1.8 m (molal) solution

15. HEAT EFFECT ON THE GAS DILUTION IN WATER

16. PRESSURE AFFECTS GAS SOLUBILITY

17. HENRY’S LAW m = kP m – mass of soluble gas; k – Henry’s constant; P – partial gas pressure. СО 2 pressure in bottle is 4 atm.

18. Decreases of pressure of saturated vapor under solution

19. Colligative: particles are particles Colligative comes from colligate – to tie together Colligative properties depend on amount of solute but do not depend on its chemical identity Solute particles exert their effect merely by being rather than doing The effect is the same for all solutes

20. COLLIGATIVE PROPERTIES FOR NONVOLATILE SOLUTES: Vapour pressure is always lower Boiling point is always higher Freezing point is always lower Osmotic pressure drives solvent from lower concentration to higher concentration

21. NON-VOLATILE SOLUTES AND RAOULT’S LAW Vapor pressure of solvent in solution containing non-volatile solute is always lower than vapor pressure of pure solvent at same T

22. –At equilibrium rate of vaporization = rate of condensation –Solute particles occupy volume reducing rate of evaporation the number of solvent molecules at the surface –The rate of evaporation decreases and so the vapor pressure above the solution must decrease to recover the equilibrium

23. Molecular view of Raoult’s law: Boiling point elevation In solution vapor pressure is reduced compared to pure solvent Liquid boils when vapor pressure = atmospheric pressure Must increase T to make vapor pressure = atmospheric

24. Colligative Properties – BP Elevation The addition of a nonvolatile solute causes solutions to have higher boiling points than the pure solvent. –Vapor pressure decreases with addition of non-volatile solute.  Higher temperature is needed in order for vapor pressure to equal 1 atm.

25. MOLECULAR VIEW OF RAOULT’S LAW: FREEZING POINT DEPRESSION –Ice turns into liquid –Lower temperature to regain balance –Depression of freezing point Depends on the solute only being in the liquid phase –Fewer water molecules at surface: rate of freezing drops

26. Colligative Properties - Freezing Pt Depression  Freezing point of the solution is lower than that of the pure solvent. The addition of a nonvolatile solute causes solutions to have lower freezing points than the pure solvent. Solid-liquid equilibrium line rises ~ vertically from the triple point, which is lower than that of pure solvent.

27. RAOULT’S LAW Vapor pressure above solution is vapor pressure of solvent times mole fraction of solvent in solution Vapour pressure lowering follows:

28. MAGNITUDE OF ELEVATION Depends on the number of particles present Concentration is measured in molality (independent of T) K b is the molal boiling point elevation constant

29. Boiling point elevation (ebullioscopy) The boiling point of a pure solvent is increased by the addition of a non-volatile solute, and the elevation can be measured by ebullioscopy.ebullioscopy Here i is the van't Hoff factor as above, K b is the ebullioscopic constant of the solvent (equal to 0.512°C kg/mol for water), and m is themolality of the solutionvan't Hoff factorebullioscopic constantmolality

30. MAGNITUDE OF DEPRESSION Analagous to boiling point, the freezing point depression is proportional to the molal concentration of solute particles For solutes which are not completely dissociated, the van’t Hoff factor is applied to modify m:

31. Freezing point depression (cryoscopy) The freezing point of a pure solvent is lowered by the addition of a solute which is insoluble in the solid solvent, and the measurement of this difference is called cryoscopy. Here K f is the cryoscopic constant, equal to 1.86°C kg/mol for the freezing point of water. Again i is the van't Hoff factor and m the molality.cryoscopic constant

32. OSMOSIS: MOLECULAR DISCRIMINATION A semi-permeable membrane discriminates on the basis of molecular type –Solvent molecules pass through –Large molecules or ions are blocked Solvent molecules will pass from a place of lower solute concentration to higher concentration to achieve equilibrium

33. OSMOTIC PRESSURE Solvent passes into more conc solution increasing its volume The passage of the solvent can be prevented by application of a pressure The pressure to prevent transport is the osmotic pressure

34. CALCULATING OSMOTIC PRESSURE The ideal gas law states But n/V = M and so Where M is the molar concentration of particles and Π is the osmotic pressure

35. Determining molar mass A solution contains 20.0 mg insulin in 5.00 ml develops an osmotic pressure of 12.5 mm Hg at 300 K

36. VOLATILE SOLUTE: TWO LIQUIDS Total pressure is the sum of the pressures of the two components

37. OSMOMETER p =  gh

38. Colligative Properties - Osmosis Osmosis plays an important role in living systems: –Membranes of red blood cells are semipermeable. Placing a red blood cell in a hypertonic solution (solute concentration outside the cell is greater than inside the cell) causes water to flow out of the cell in a process called CRENATION.

39. Colligative Properties Placing a red blood cell in a hypotonic solution (solute concentration outside the cell is less than that inside the cell) causes water to flow into the cell. –The cell ruptures in a process called HEMOLYSIS.