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Chemistry 232

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  1. Chemistry 232 Properties of Solutions

  2. Concentration Terms • Dilute - not a lot of solute. • Concentrated - a large amount of solute. • Concentration can be expressed quantitatively is many ways: • Molarity • Molality • Percentage • Mole fraction

  3. Molarity and Molality • The molarity is the number of moles of solute in 1 litre of solution. • M = moles of solute / V sol’n (litres) • The molality is the number of moles of solute in 1 kg of solvent. • M = moles of solute / kg solvent • Conversion between the two requires the solutions density.

  4. Partial Molar Thermodynamic Properties Define a partial molar thermodynamic property as Euler’s Theorem

  5. The Chemical Potential We define the chemical potential of a substance as

  6. The Wider Significance of  Shows how all the extensive thermodynamic properties depend on system composition

  7. Thermodynamics of Mixing Spontaneous mixing of two or more substances to form solutions Gibbs energy of the solution must be less than G(pure components)

  8. The Gibbs Energy of Mixing

  9. The Enthalpy and Entropy

  10. The Ideal Solution TmixS/n kJ/mol TmixH/n 0 TmixG/n XA

  11. The Volume and Internal Energy of Mixing

  12. Ideal Solution Def’n For an ideal solution

  13. Raoult’s Law Consider the following system

  14. Raoult’s Law #2 The chemical potential expressions

  15. Raoult’s Law: Depression of Vapour pressure VP of solution relates to VP of pure solvent PA = XAP*A Solutions that obey Raoult’s law are called ideal solutions.

  16. Raoult’s Law Example The total vapour pressure and partial vapour pressures of an ideal binary mixture Dependence of the vp on mole fractions of the components.

  17. An Ideal Solution Benzene and toluene behave almost ideally Follow Raoult’s Law over the entire composition range.

  18. Henry’s Law Henry’s law relates the vapour pressure of the solute above an ideally dilute solution to composition.

  19. The Ideal Dilute Solution Ideal Dilute Solution Solvent obeys Raoult’s Law Solute obeys Henry’s Law

  20. Henry’s Law #2 The chemical potential expressions • JO(H)is the Henry’s law standard state. • It is the chemical potential of J in the vapour when PJ = kJ.

  21. Henry’s Law #3 The Standard State Chemical potential for Henry’s Law When the system is in equilibrium The chemical potential expressions reduce to Henry’s Law

  22. Henry’s Law in terms of molalities The Standard State Chemical potential for Henry’s Law When the system is in equilibrium The chemical potential expressions reduce to Henry’s Law in terms of molalities

  23. Chemical Potentials in terms of the Molality The chemical potential expressions oJ,m = chemical potential of the solute in an ideal 1 molal solution

  24. The Gibbs-Duhem Equation The Gibbs-Duhem gives us an interrelationship amongst all partial molar quantities in a mixture

  25. Colligative Properties

  26. Colligative Properties • All colligative properties • Depend on the number and not the nature of the solute molecules • Due to reduction in chemical potential in solution vs. that of the pure solvent • Freezing point depression • Boiling Point Elevation • Osmotic Pressure

  27. Boiling Point Elevation Examine the chemical potential expressions involved

  28. Boiling Point Elevation #2 The boiling point elevation

  29. Freezing Point Depression Examine the chemical potential expressions involved

  30. Freezing Point Depression #2 Define the freezing point depression

  31. Osmosis

  32. Osmosis • The movement of water through a semi-permeable membrane from dilute side to concentrated side • the movement is such that the two sides might end up with the same concentration • Osmotic pressure: the pressure required to prevent this movement

  33. Osmosis – The Thermodynamic Formulation  - the osmotic pressure Equilibrium is established across membrane under isothermal conditions

  34. The Final Equation The osmotic pressure is related to the solutions molarity as follows

  35. Terminology Isotonic: having the same osmotic pressure Hypertonic: having a higher osmotic pressure Hypotonic: having a lower osmotic pressure

  36. Terminology #2 Hemolysis: the process that ruptures a cell placed in a solution that is hypotonic to the cell’s fluid Crenation: the opposite effect

  37. The Partial Molar Volume In a multicomponent system

  38. Volume Vs. Composition The partial molar volume of a substance slope of the variation of the total sample volume plotted against composition. PMV’s vary with solution composition

  39. The PMV-Composition Plot The partial molar volumes of water and ethanol at 25C. Note the position of the maxima and minima!!

  40. Experimental Determination of PMV’s Obtain the densities of systems as a function of composition Inverse of density – specific volume of solution

  41. Example with Methanol. Plot volumes vs. mole fraction of component A or B Draw a tangent line to the plot of volume vs. mole fraction. Where the tangent line intersects the axis – partial molar volume of the components at that composition

  42. The Solution Volume vs. Composition

  43. The Mean Molar Volume Define the mean mixing molar volume as V*J – the molar volume of the pure liquid Vm = V/nT

  44. The Mean Molar Volume Plot VB-VB* VA-VA*

  45. Infinite Dilution Partial Molar Properties The value of a partial molar thermodynamic property in the limit of zero volume is its infinite dilution value E.g., for the volumes

  46. The Definition of the Activity For any real system, the chemical potential for the solute (or solvent) is given by

  47. Activities of Pure Solids/Liquids The chemical potential is essentially invariant with pressure for condensed phases

  48. Pure Solids and Pure Liquids or aJ = 1 For a pure solid or a pure liquid at standard to moderately high pressures

  49. Activities in Gaseous Systems The chemical potential of a real gas is written in terms of its fugacity

  50. Define the Activity Coefficient The activity coefficient (J) relates the activity to the concentration terms of interest. In gaseous systems, we relate the fugacity (or activity) to the ideal pressure of the gas via