Lecture outline: Chapter 13

Transcription

1 Lecture outline: Chapter 13 Properties of solutions Why solutions form at the molecular l levell Units of solution concentration Colligative properties: effects of solutes on BP, MP, and vapor pressure e Osmotic pressure 1

2 Review of solution terms Solution: a homogeneous mixture of 2 or more substances Solvent: the component of the solution present in greatest quantity Solute: a substance dissolved in the solvent Solubility: a term that refers to how much of a solute can dissolve in a given solvent (grams solute dissolved/100 ml solution) Miscible: ibl two substances that t can form a solution at all solute-solvent proportions 2

3 The solution process soluble in water: glucose, NaCl, NaO, acetic acid, MgSO 4 ( soluble means a significant amount can dissolve) Insoluble in water: CaCO 3, AgCl, C 6 14, I 2 ( insoluble means a very little amount dissolves) miscible with water: ethanol, glycerol, formaldehyde ( miscible means soluble at all proportions) 3

4 The solution process Why do some substances dissolve in water while others don t? Consider strengths of: 1. Solvent-solvent intermolecular forces (BREAK) 2. Solute-solute intermolecular forces (BREAK) 3. Solvent-solute intermolecular forces (solvation, hydration) (FORM) Remember, it takes energy to break a bond or intermolecular attraction, while the formation of a new bond or intermolecular attraction releases energy 4

5 Solvent (e.g. water) Solute (e.g. glucose) 5

6 Sodium chloride dissolves in water to form hydrated sodium and choride ions + - 6

7 ydrated chloride and sodium ions: ion- dipole attractions ti Cl - Na + 7

8 The dissolving process can be exothermic, endothermic, or isothermic NaCl in water: ΔT ~ 0 NaO in water: ΔT > 0 (N 4 )(C 3 COO) in water: ΔT <0 8

9 Another term important in dictating whether a reaction occurs Entropy Thermodynamics, chemistry 2: chemical reactions that occur with an increase in the disorder (randomness) of the system are favored 9

10 Dynamic equilibrium: when two opposing processes occur at the same rate, so that there is no net change in the state of the system Example 1: vapor pressure in a sealed container of water rate escape = rate return Example 2: a saturated solution containing undissolved solute rate solute dissolving = rate solute crystallizing 10

11 Factors affecting solubility Nature of solute-solvent interactions Pressure Temperature 11

12 Factors affecting solubility Nature of solute-solvent interactions Pressure Temperature like dissolves like polar + polar nonpolar + nonpolar 12

13 What molecules dissolve in water? soluble salts (ion-dipole interactions) polar molecules -bonding polar but no -bonding 13

14 Some polar molecules in water 14

15 Solubility of alcohols in water C O C C O C C C O C C C C O 15

16 Small compounds with hydroxyl groups tend to dissolve in water Glycerol (nontoxic) C O C O C O Propylene glycol (nontoxic) C O C O C Ethylene glycol (highly toxic) C O C O 16

17 Glucose, C 6 12 O 6 C O C C O O C C O C O C O C C O O O C C C O C O O C O 17

18 nonpolar + nonpolar 18

19 nonpolar + nonpolar Cl Cl Cl C Cl C C C C C 19

20 Mix solutions of CCl 4 and 2 O together: density, 2 O ~ 1 g/ml density, CCl 4 ~ 1.6 g/ml 20

21 Mix solutions of CCl 4 and 2 O together: density, 2 O ~ 1 g/ml density, CCl 4 ~ 1.6 g/ml 21

22 Mix solutions of CCl 4 and 2 O together: density, 2 O ~ 1 g/ml density, CCl 4 ~ 1.6 g/ml 22

23 If you added CuSO 4 to the separatory funnel, then shook it, where would the CuSO 4 end up? 23

24 If you added I 2 to the separatory funnel, then shook it, where would the I 2 end up? 24

25 Gases in water N 2 CO O 2 Ar Kr 25

26 Molecule Mr Solubility data for some gases in water at 298 K structure Predominant intermolecular attraction with water Solubility (g solute/kg 2 O) London C C London N 2 28 N N London O 2 32 O O London CO 2 44 O C O London S S Dipole-dipole 3.3 SO 2 O S O Dipole-dipole and hydrogen bonding 94 N 3 N ydrogen bonding

27 Factors affecting solubility Nature of solute-solvent interactions Pressure Temperature For gases: C g = kp g enry s law constant for the gas-solvent sol pair at a defined temperature Solubility of gas in solution phase Partial pressure of gas over solution 27

28 Dynamic equilibrium: when two opposing processes occur at the same rate, so that there is no net change in the state t of the system Example 1: vapor pressure in a sealed container of water Example 2: a saturated t solution containing undissolved solute Example 3: the solubility of a gas in water 28

29 Air: ~78% N 2, 21% O 2 by volume Rate escape = rate return,o 2,O 2 Rate escape = rate return,n 2,N 2 29

30 What is the concentration of dissolved oxygen in a fresh-water stream in equilibrium with air at 25 C and 1 atm. P? k 166 O2 = 1.66 x 10-6 M/mm g X O2 = 0.21 P 1 = X 1 P T C g = kp g 30

31 Examples of enry s law Bottle of soda pop Scuba diving 31

32 Examples of enry s law Scuba diving 32

33 Factors affecting solubility Nature of solute-solvent interactions Pressure Temperature 33

34 In general, but not always, the solubility of solid solutes increases with increasing temperature The solubility of gases decreases with increasing temperature 34

35 Units of solution concentration Molarity mols solute mol = = M liters solution l Molality mols solute lt = m kg solvent Mol fraction Mass % Volume % Parts per million 35

36 The density of water changes as a function of temperature t Density of water at different temperatures densit ty (g/ml) temperature ( C) 36

37 Solution prepared at 25 C Density of water at different temperatures (g/ml) density l temperature ( C) 37

38 Solution cooled to 25 C Density of water at different temperatures (g/ml) density l temperature ( C) 38

39 Molarity Molality Mol fraction Mass % Volume % Units of solution concentration mols solute liters solution mols solute lt kg solvent x = 1 n n 1 mass % A = v/v; m/v t mol = l = m mass total mass total = M of A mass of A mass Parts per million 6 ppm A = x 10 x

40 Units of solution concentration (cont.) Parts per million ppm A mass of A = x 10 total mass 6 g A g total x 10 6 g A g total x 1000 mg 1g x kg g 40

41 Units of solution concentration (cont.) When the units of a solution are expressed in units of mg/kg, they are already expressed in units of ppm, so you don t have to multiply py by 10 6 anymore! ppm A = g g A total x 10 6 ppm A = kg mg A solution 41

42 Units of solution concentration (cont.) In dilute aqueous solutions, density, 2 O ~ 1g/ml = 1 kg/l ppm A = kg mg A solution = liter mg A solution = ml μg A solution 42

43 What is the mass % (w/w) composition of a solution prepared by mixing 46.1 grams ethanol with 162 grams water?? mass % A = mass total of A mass x

44 What is the volume % (v/v) composition of a solution prepared p by mixing 46.1 grams ethanol with 162 grams water?? d 2O = 1.0 g/ml d ethanol = 0.79 g/ml 44

45 Sodium hydrogen sulfite, NaSO 3, is used to adjust the p of swimming pools. What is the concentration of Na + ion in ppm in a 110,000 gallon (4.5 x 10 5 liter) swimming pool to which 560 grams NaSO 3 was added? ppm A = kg mg A solution = liter mg A solution = ml μg A solution 45

46 Colligative properties: the effects of solute concentration on the properties of a solution Vapor pressure lowering Boiling point elevation (raising) Freezing point depression (lowering) Osmosis and osmotic pressure 46

47 Vapor pressure lowering Add solute Equilibrium in pure water rate capture > rate escape Equilibrium reestablished 47

48 The extent of vapor pressure lowering is proportional to the concentration of solute particles 48

49 Vapor pressure lowering Mol fraction of solvent Raoult s law:p A = X A P A Vapor pressure of solution Vapor pressure of pure solvent 49

50 What is the vapor pressure at 25 C above a solution that contains 46 grams of sucrose in 355 ml of water? The vapor pressure of pure water at 25 C is238torr 23.8 torr. Raoult s law: P A = X A P A 50

51 Colligative properties: the effects of solute concentration on the properties of a solution Vapor pressure lowering Boiling point elevation (raising) Freezing point depression (lowering) Osmosis and osmotic pressure 51

52 Boiling point elevation (raising) ΔT b = K b m Molal conc. of solute particles Change in boiling point from pure solvent Molal-boiling point elevation constant 52

53 Boiling point elevation For 2 O, K b = 0.52 C/m Molal conc. of solute particles ΔT b =Km b Change in boiling point from pure solvent Molal-boiling point elevation constant 53

54 Colligative properties: the effects of solute concentration on the properties of a solution Vapor pressure lowering Boiling point elevation (raising) Freezing point depression (lowering) Osmosis and osmotic pressure 54

55 Freezing point depression (lowering): molecular explanation 55

56 Equilibrium between ice and liquid water at 0 C Rate melting = rate freezing 56

57 Addition of a solute to ice and liquid water at 0 C Rate melting > rate freezing 57

58 The ice dissolves if temperature stays 0 C Rate melting > rate freezing 58

59 Lowering the temperature below 0 C slows rate of melting allowing equilibrium to be reestablished Melting is endothermic, freezing is exothermic liquid freezing melting Δ out Δ in solid Rate melting = rate freezing 59

60 Freezing point depression (lowering) For 2 O, K f = 1.86 C/m ΔT f = K f m Molal conc. of solute particles Change in freezing point from pure solvent Molal-boiling point elevation constant 60

61 Colligative properties: the effects of solute concentration on the properties of a solution Vapor pressure lowering Boiling point elevation (raising) Freezing point depression (lowering) Osmosis and osmotic pressure 61

62 Osmosis: The net movement of solvent across a semipermeable membrane from the side with lower solute concentration to the side with higher solute concentration The key to osmosis is the semi-permeable membrane: water solute 62

63 Separation of water on the sides of two glass tubes by the semi-permeable membrane water solute Water levels are equal on both sides 63

64 Addition of solute molecules to the left side slows rate of water movement from left to right, but not right to left Water levels become uequal water solute 64

65 Equilibrium is established when sufficient pressure on the left side causes water to move between sides at the same rate Δh water solute Water levels at equilibrium dicated by osmotic pressure 65

66 Osmotic pressure: The pressure that must be applied on the side with higher solute concentration in order to prevent net movement of solvent molecules 66

67 The membranes of cells are semipermeable membranes Figure Credit: Ladyofats, (Mariana Ruiz Villarreal ) 67

68 Some more solution terms ydrophilic ydrophobic Amphipathic Soap/detergent t Micelle Colloidal suspension macromolecule 68