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Chemistry 232. Properties of Solutions. 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. Molarity and Molality.

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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