Solutions

1. Solutions Chapter 15

2. What Are Solutions? Section 15.1

3. What Are Solutions? • Solution: homogeneous mixture of 2 or more substances • Solid, liquid, or gas • Solvent: dissolving medium • Solute: substance that dissolves • When in solution, you cannot distinguish solvent and solute

4. What is a Solution? • Soluble – a substance that can dissolve in a given solvent • Miscible: two liquids that can dissolve in each other • Example: water and antifreeze • Insoluble – substance cannot dissolve • Immiscible: two liquids that cannot dissolve in each other • Example: oil & water

5. Why Do Some Substances Dissolve and not Others? • To dissolve, solute particles must dissociate from each other and mix with solvent particles • Attractive forces between solute and solvent must be greater than attractive forces within the solute • Process of surrounding solute particles with solvent particles is called SOLVATION • In water, it is also called HYDRATION

6. Aqueous Solutions of Ionic Compounds • Remember: • Water molecules are polar (+ and – ends) • Water molecules are in constant motion • When you put salt in water, water molecules collide with surface of crystal • Charged ends of water attract ions of salt • Dipole interaction (water/salt) is stronger than ions in crystal, so it pulls them away

7. Solvation

8. Aqueous Solutions of Molecular Compounds • Water is also a good solvent for many molecular compounds (Example: sugar) • Sugar has many O-H bonds (polar) • When water is added, the O-H bond becomes a site for hydrogen bonding with water • Water’s hydrogen bonds pulls the sugar molecules apart • Oil is not a good solute because it has many C-H bonds (not polar) and few or no O-H (polar) bonds

9. Factors that Affect Solvation Rate • Increase Solvation Rate (Dissolve Faster) by: • Agitation (stirring) • Increase surface area (make particles smaller) • Temperature (make it hotter) • All these increase the number of collision between water and the solute

10. Heat of Solution • During Solvation it takes energy to make the solute particles come apart. • Solvent particles must also move apart • This energy requirements is called “Heat of Solution”

11. Solubility • Has Anyone ever made rock candy? • How much water does it take to dissolve the sugar at room temperature? • What happens when we raised the temperature? • Only a limited amount of solute can dissolve in a given amount of solvent • Every solute is unique for the solvent • This is ‘Solubility’ – the amount of solute that can dissolve in a given amount of solvent at a specified temperature and pressure

12. Solubility Continued • Solubility can also be understood at the particle level: • As particles collide, some particles are deposited back to the solute • Some particles are removed from the solute. • When the rate of deposit equals the rate of solvation, then the solution is SATURATED • Saturated Solution – no more solute can be dissolved in the solvent at this temperature and pressure • Unsaturated Solution – there is still room for more solute to be dissoved

13. Factors that Affect Solubility • Most substances are MORE soluble at high temperature than at low • If you dissolve a substance until saturated at high temperature and then reduce the temperature, the solution becomes “supersaturated” • Supersaturated solutions are unstable • A small change makes the solute reappear • Rock candy worked that way. How?

14. Factors that Affect Solubility S2 S1P1 P2 S1 P1 = S2 P2 = • Pressure affects solubility of gaseous solutes • Carbonated beverages • Henry’s Law • At a given temperature solubility (S) of a gas in a liquid is directly proportional to pressure (P) OR

15. Solution Concentration Section 15.2

16. Expressing Concentration • Concentration is a measure of how much solute is dissolved in a specific amount of solvent. • Concentration can be qualitative or quantitative • Qualitative: strong, weak, etc. • Quantitative: percent by mass, percent by volume, molarity, molality

17. Using Percent to Express Concentration Mass of solute Mass of solution X 100 Percent by mass = Volume of solute Volume of solution X 100 Percent by volume =

18. Molarity Moles of solute Liters of solution Molarity (M) = • Molarity is the most common method of expressing concentration in Chemistry • Molarity is moles of solute in 1 liter of solution. • You make it by taking 1 mole of a solute and filling up with solvent to the 1 liter level.

19. Molarity - Example 1 mol glucose 180 g/mol glucose = 0.028 mol glucose 5.10g glucose X 1 L 1000 ml = 0.1005 L 100.5 ml X An IV solution contains 5.10 g of glucose (C6H12O6) in 100.5ml of water. What is the molarity of this solution? Known: Mass of solute = 5.10 g glucose Molar mass of glucose = 180.0 g/mol Volume of solvent = 100.5 ml SOLVE for Mole/Liter: Convert ml to liters: 0.028 mol 0.1005 L Moles Liter = 0.28 M Molarity = =

20. Preparing Molar Solutions • How do you prepare a 1.5M solution of sucrose (C12H22O11) ? • 1.5 moles sucrose x 342g/mol = 513 g sucrose • 1.5 Molar would be 513g in 1 L of water • Measure out 513 g sucrose • Put it in a 1L graduated cylinder • Add distilled water to make 1L total solution • To make 100ml, use 1/10th of each • 51.3g sucrose in 100ml of water

21. Making Dilute Solutions • Concentrated HydroChloric Acid is 12M. • How would I make ½ the concentration, or 6 M ? • Use M1V1 = M2V2 • 12moles x 1L = 6moles x ?L = 12moles x 1L/6 moles = 2 L • So put in twice the solvent and you have ½ the concentration.

22. Molality and Mole Fraction • Molality is Moles of Solute per 1 kg of Solvent • Abbreviated m and is read as molal. • This is because volume increases with temperature and this changes the molarity.

23. Calculating Molality 1 mol NaCl 58.5 g NaCl = 0.077 mol NaCl 4.5g NaCl X 1 kg H2O 1000 g H20 = 0.1000 kg H2O 100.0 g water X 0.077 mol NaCl 0.1000 kg H2O Moles of solute Kg solvent = 0.77 mol/kg = m = • If a student adds 4.5 g of sodium chloride to 100.0g of water, what is the molality? Known: Mass of water is 100.0 g Mass of sodium Chloride is 4.5g Unknown: m or mol/kg

24. Mole Fraction ___nA__ nA + nB Mole Fraction (X) = 1 Mol HCl 36.5 g HCl 37.5 g HCl x = 1.03 mol HCl 1 mole H2O 18.0 g H2O = 3.47 mol H2O 62.5 g H2O x _____1.03 mol HCl_______ 1.03 mol HCl + 3.47 mol H2O ___nHCL___ nHCl + nH2O XHCl = = = 0.229 • Mole fraction = moles solute divided by total moles of solute + solvent Example: What is the mole fraction of hydrochloric acid if for every 100 g of solution, 37.5 g is HCl

25. Colligative Properties of Solutions Chapter 15.3

26. Electrolytes and Colligative Properties • When solutions are made, the physical properties of the solutions are affected by the number of particle dissolved • Colligative: depending on the collection

27. Colligative Properties • Electrolytes vs non-electrolytes • Ionic compounds ARE electrolytes because they form ions in solution that conduct electricity • Molecular compounds ARE NOT electrolytes because they do not conduct electricity • Vapor Pressure Lowering • Adding a non-volatile solute lowers the vapor pressure of the solution (vs. solvent) • More solute  more vapor pressure lowering

28. Colligative Properties • Boiling Point Elevation • Because vapor pressure is lowered, it takes more energy to make it boil • Boiling point temperature is raised • Boiling point elevation is directly proportional to solution molality • What is the benefit of adding salt to boiling water for pasta? • Freezing Point Depression • Freezing point temperature is lowered • Solute particles interfere with attractive forces of solvent • Freezing point of a solution is always lower than the freezing point of a pure solvent • FP Depression is directly proportional to molality

29. Colligative Properties • Osmosis and Osmotic Pressure • Diffusion: mixing of gasses or liquids through random motions • Osmosis is diffusion of solvent through a semi permeable membrane from high solvent concentration to lower solvent concentration • Living cells use this to get materials in/out of cells

30. Colligative Properties • Example: Salt/water • During Osmosis, Water molecules move both directions through membrane • But only water can move through the membrane • So pure water builds up on one side of the membrane • Water/salt builds up on the other. • Higher concentration of water on one side creates: • Osmotic pressure • A pressure or push to equalize the water/salt concentrations • Pressure depends on concentration of solute

31. Colligative Properties - Antifreeze

32. B. Types Freezing Point Depression View Flash animation.

33. B. Types Boiling Point Elevation Solute particles reduce vapor pressure of solvent, requiring more energy for vapor pressure to reach atmospheric pressure (boiling), thus raising the boiling point temperature.

34. B. Types • Applications • salting icy roads • making ice cream • antifreeze • cars (-64°C to 136°C) • fish & insects

35. C. Calculations t: change in temperature (°C) k: constant based on the solvent (°C·kg/mol) m: molality (m) n: # of particles t = k · m · n

36. C. Calculations • # of Particles • Nonelectrolytes (covalent) • remain intact when dissolved • 1 particle • Electrolytes (ionic) • dissociate into ions when dissolved • 2 or more particles

37. C. Calculations • At what temperature will a solution that is composed of 0.73 moles of glucose in 225 g of phenol boil? GIVEN: b.p. = ? tb = ? kb = 3.60°C·kg/mol WORK: m = 0.73mol ÷ 0.225kg tb = (3.60°C·kg/mol)(3.2m)(1) tb = 12°C b.p. = 181.8°C + 12°C b.p. = 194°C m = 3.2m n = 1 tb = kb · m · n

38. C. Calculations • Find the freezing point of a saturated solution of NaCl containing 28 g NaCl in 100. mL water. GIVEN: f.p. = ? tf = ? kf = 1.86°C·kg/mol WORK: m = 0.48mol ÷ 0.100kg tf = (1.86°C·kg/mol)(4.8m)(2) tf = 18°C f.p. = 0.00°C - 18°C f.p. = -18°C m = 4.8m n = 2 tf = kf · m · n

39. Heterogeneous Mixtures Chapter 15.4

40. Types of Heterogeneous Mixtures • Heterogeneous Mixtures • Look like a solution, but are really mixtures • Mixtures of substances that exist in 2 different phases • 2 Types: • Suspensions • Colloids • Solutions: • Particles of solute are atomic sized compared to solvent

41. Suspensions • Particle Size • Suspended particles are large compared to solvent • Larger than 1000 nm for solvated particles • CAN be filtered • When stirred, solid-like state begins to flow like a liquid • Called Thixotropic • Examples • housepaint

42. Colloids • Particle Size • Particles of solute are intermediate sized (between atomic and large suspension sized) compared to solvent • Between 1 nm and 1000 nm diameter • Cannot be Separated by filtration or settling • Example: milk, butter, cheese,

43. Colloids • Types of Colloids • Solid Sol: Solid in solid (gemstones) • Sol: Solid in Liquid (Blood, gelatin) • Solid emulsion: Liquid in solid (butter, cheese) • Emulsion: liquid/liquid (milk, mayonaise) • Solid foam: gas/solid (marshmallows, soap that floats) • Foam: gas/liquid (whipped cream, beaten egg whites) • Aerosol: solid/gas (smoke, dust in air) • Aerosol: liquid/gas (clouds, spray deodorant

44. Brownian Motion • Liquid colloid under a microscope shows random, jerky motions of dispersed particles • Brownian motion • From collisions of particles of dispersion medium with dispersed particles • Collisions prevent particles from settling out • Due to polar or charged atomic particles • Link for display • http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=24

45. Tyndall Effect • Dilute Colloids sometimes appear as clear solutions (concentrated colloids do not) • Because particles are too small to be seen with naked eye • But: dispersed colloid particles are large enough to scatter light • Tyndall effect • Solutions do not scatter light (particles are too small)

46. Tyndall Effect Colloid Solution

47. Chapter Summary • Chapter 15 Test on Friday (5/16) • Chapter 15 Vocabulary Due Friday (5/16) • Chapter 15 HO’s (remaining) Due Friday (5/16)

48. Chapter Test Covers: • Characteristics of Solutions • Solvation – ionic vs molecular • Factors Affecting Solubility • Solution Concentrations: Calculate • Percent by mass • Percent by volume • Molarity • Molality • Colligative Properties of Solutions • Suspensions and Colloids