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SOLUTIONS

SOLUTIONS. Solutions. Solution – a homogeneous mixture of pure substances (occur in all phases, but we will focus on aqueous solutions) The SOLVENT is the medium in which the SOLUTES are dissolved. (The solvent is usually the most abundant substance.) Example: Solution: Salt Water

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SOLUTIONS

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  1. SOLUTIONS

  2. Solutions • Solution – a homogeneous mixture of pure substances (occur in all phases, but we will focus on aqueous solutions) • The SOLVENT is the medium in which the SOLUTES are dissolved. (The solvent is usually the most abundant substance.) • Example: • Solution: Salt Water • Solute: Salt • Solvent: Water

  3. Concentration of Solution • Concentration refers to the amount of solute dissolved in a solution. Ways of measuring concentration…

  4. MOLARITY (M)

  5. MOLALITY (m) Arrr…Don’t be confusing this with molarity.

  6. MASS PERCENT (%) Recall that ppt, ppm and ppb are similar measurements, but the multiplier becomes 103, 106 or 109 respectively.

  7. MOLE FRACTION ()

  8. NORMALITY (N) See page 487

  9. Practice Problems (to turn in):Concentration Units • Complete the table below for aqueous solutions of urea, CO(NH2)2: Check your answers with online key

  10. Energy of Making Solutions • Heat of solution ( DHsoln ) is the energy change for making a solution. • Most easily understood if broken into steps. • Break apart solvent • Break apart solute • Mixing solvent and solute

  11. 1. Break apart Solvent • Have to overcome attractive forces. DH1 >0 2. Break apart Solute. • Have to overcome attractive forces. DH2 >0

  12. 3. Mixing solvent and solute • DH3 depends on what you are mixing. • Molecules can attract each other - DH3 is large and negative. • Molecules can’t attract - DH3 issmall and negative. • This explains the rule “like dissolves like”

  13. Solute and Solvent DH3 DH2 Solvent DH1 DH3 Solution Solution • Size of DH3 determines whether a solution will form Energy Reactants

  14. Types of Solvent and solutes • If DHsoln is small and positive, a solution will still form because of entropy. • There are many more ways for them to become mixed than there is for them to stay separate.

  15. Solution Formation – Factors Favoring Spontaneity • Processes in which the energy content of the system decreases (exothermic) tend to occur spontaneously. H < 0 • Processes in which the disorder (entropy) of the system increases tend to occur spontaneously. S < + G = H - TS

  16. “LIKE DISSOLVES LIKE”

  17. It seems the extent to which one substance dissolves in another depends on the nature of both the solute and the solvent. (“like dissolves like”). It also depends on temperature and at least for gases, on pressure.

  18. As a rule, network covalent solids, such as graphite and quartz (SiO2), do not dissolve in any solvent. Nor do metals, technically speaking “dissolve.” Ionic and molecular compounds, on the other hand will dissolve in certain solvents.

  19. Structure and Solubility • Water soluble molecules must have dipole moments -polar bonds. • To be soluble in non polar solvents the molecules must be non polar.

  20. O- P O- CH2 CH2 CH2 CH3 CH2 O- CH2 CH2 CH2 Soap

  21. O- P O- CH2 CH2 CH2 CH3 CH2 O- CH2 CH2 CH2 Soap • Hydrophobic non-polar end

  22. O- P O- CH2 CH2 CH2 CH3 CH2 O- CH2 CH2 CH2 Soap • Hydrophilic polar end

  23. O- P O- CH2 CH2 CH2 CH3 CH2 O- CH2 CH2 CH2 _

  24. A drop of grease in water • Grease is non-polar • Water is polar • Soap lets you dissolve the non-polar in the polar.

  25. Hydrophobic ends dissolve in grease

  26. Hydrophilic ends dissolve in water

  27. Water molecules can surround and dissolve grease. • Helps get grease out of your way.

  28. Pressure effects • Changing the pressure doesn’t affect the amount of solid or liquid that dissolves • They are incompressible. • Pressure does affect solubility of gases.

  29. Dissolving Gases • Pressure affects the amount of gas that can dissolve in a liquid. • The dissolved gas is at equilibrium with the gas above the liquid.

  30. The gas is at equilibrium with the dissolved gas in this solution. • The equilibrium is dynamic.

  31. If you increase the pressure the gas molecules dissolve faster. • The equilibrium is disturbed.

  32. The system reaches a new equilibrium with more gas dissolved. • Henry’s Law: P= kC Pressure=(constant)(gas concentration)

  33. Temperature Effects • Increased temperature increases the rate at which a solid dissolves. • We can’t predict whether it will increase the amount of solid that dissolves. • We must read it from a graph of experimental data.

  34. Solubilities of Various Solids in Water Solubility (g solute/100 g H2O) 40 60 80 20 100 Temp (C)

  35. Gases are predictable • As temperature increases, solubility decreases. • Gas molecules can move fast enough to escape. • Thermal pollution.

  36. Vapor Pressure of Solutions • A nonvolatile solvent lowers the vapor pressure of the solution. • The molecules of the solventmust overcome the force of both the other solvent molecules and the solute molecules.

  37. Raoult’s Law: Psoln = csolvent Psolvent Sol’n vap. press = mole fraction of solvent x vapor pressure of pure solvent • Applies only to an ideal solution where the solute doesn’t contribute to the vapor pressure.

  38. To determine whether a sol’n is IDEAL… • Liquid-liquid solutions where both are volatile. • Modify Raoult’s Law to Ptotal= PA + PB = cAPA0 + cBPB0 • Ptotal= vapor pressure of mixture • PA0= vapor pressure of pure A • If this equation works then the solution is ideal. • Solvent and solute are alike.

  39. Deviations • If Solvent has a strong affinity for solute (H bonding)… • Lowers solvents ability to escape. • Lower vapor pressure than expected. • Negative deviation from Raoult’s law. DHsoln is large and negative exothermic. • Endothermic DHsoln indicates positive deviation.

  40. Colligative Properties of Solutions Colligative properties = physical properties of solutions that depend on the # of particles dissolved, not the kind of particle.

  41. Colligative Properties • Lowering vapor pressure • Raising boiling point • Lowering freezing point • Generating an osmotic pressure

  42. Boiling Point Elevation • a solution that contains a nonvolatile solute has a higher boiling pt than the pure solvent; the boiling pt elevation is proportional to the # of moles of solute dissolved in a given mass of solvent.

  43. Ok… but WHY???

  44. Consider evaporation….only the solvent molecules evaporate. But you see some of the solute molecules block the surface so that fewer solvent molecules can reach the surface and evaporate.

  45. This lowers vapor pressure because fewer molecules evaporate.

  46. Lowering Vapor Pressure The thing to remember is that this is a physical effect. The solute molecules get in the way, and prevent some of the solution molecules from reaching the surface where they can evaporate.

  47. Lowering Vapor Pressure It doesn’t matter much what the solute molecules are, just how many of them there are. Thus, this is a colligative property.

  48. Measuring Vapor Pressure

  49. Pure Water Aqueous Solution

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