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Solutions - PowerPoint PPT Presentation

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Solutions. A homogeneous mixture in which all of the particles have the sizes of atoms. Driving forces for solution formation Spontaneous tendency for increasing disorder (entropy!) Intermolecular forces. Sugar or alcohol in water.

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Solutions l.jpg

A homogeneous mixture in which all of the particles have the sizes of atoms.

  • Driving forces for solution formation

  • Spontaneous tendency for increasing disorder (entropy!)

  • Intermolecular forces

Sugar or alcohol in water

Water and ethyl alcohol are completely "miscible". Both water and ethanol are polar molecules with hydrogen bonding. The similarity of the two molecules results in solutions where the water and alcohol molecules are interchangeable. Note that hexanol is only partially soluble in water (miscible with hexane though!).

Glucose dissolves in water because polar water molecules attach to the glucose molecules by dipole-dipole (H-bond) forces. When the attractive forces of the water molecules for the glucose exceeds the attractive forces between the glucose and its neighbouring glucose molecules the water can rip the sugar molecule out of the crystal. The glucose is "solvated" when it surrounded solvent molecules. The solvent has "dissolved" the molecule.

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Engine coolants

  • We can now explain why car radiator coolants dissolve in water. The coolants typically contain either ethylene glycol or propylene glycol, which, like ethanol and water, contain hydrogen-bonding O-H bonds.

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Solublility & Miscibility (like dissolves like)

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How do stain removers work?

  • Like dissolves like

    dry cleaning, surfactants and oxidizing agents

    (and enzymes!)

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Energy Changes and Solution Formation

Heat of solution: The enthalpy change between system and surroundings when 1 mole of a solute dissolves in solvent at constant pressure

  • - In general, solutions form when Hsoln is negative.

  • When Hsoln is too positive, a solution will not form.

  • Heat of solution is zero for an ideal solution

  • Examples:

    • MgSO4 added to water has Hsoln = -91.2 kJ/mol.

    • NH4NO3 added to water has Hsoln = + 26.4 kJ/mol.

    • MgSO4 is used in hot packs and NH4NO3 is used in cold packs.

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Pressure and Solubility

  • “The Bends” - deep sea divers

    • N2 in compressed air has low solubility in blood. At great depths, partial pressure of N2 increases and solubility increases. After returning to surface, solubility decreases substantially and N2 come out of blood causing small bubbles in the capillaries which may rupture the blood vessels - fatal - risk reduced if He is used since it has lower Henry’s constant.

  • Soft drinks and champagne

    • produced by dissolving CO2 in a liquid under pressure

    • opening bottle reduces the partial pressure of CO2 above solution, solubility reduced, CO2 leaves solution (effervescence)

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Colligative Properties of Solutions

Physical properties that depend only upon the populations of particles in a mixture

Effect of solutes on the vapour pressure of solutions

Psoln = xsolventP*solvent Raoult’s Law

Molecular interpretation of Raoult’s Law

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Volatile solutes and Raoult’s Law

Each component contributes its own partial pressure to the solution vapour pressure (Dalton’s Law)

Real mixtures

Deviations because of intermolecular attractions



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Boiling point elevation Freezing point depression

Entropy effect (see later): when a solute is added to a pure liquid, the entropy (disorder) is increased relative to the vapour phase. Therefore there is a weaker tendency to form a vapour (boiling point elevation). A similar molecular interpretation explains freezing point depression. The effect shifts the s-l and l-g vp curve of a phase diagram down.

  • Applications:

  • Cooking

  • De-icing and anti-freeze

  • Molar mass determination

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Osmotic pressure

Osmotic membrane: semi-permeable membrane that allows passage of only solvent molecules

Dialysis membrane: membrane that allows passage of solvent and small solutes.

Van’t Hoff equation

  • Applications:

  • Reverse osmosis

  • Crenation and hemolysis of red blood cells

  • Molar Mass determination

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Colligative properties of solutions of electrolytes

1.00 m NaCl: F.P= -3.37C (not –1.86C as expected)!

Colligative properties depend on the concentration of particles

Remember: NaCl Na+ + Cl-

We have 2.00m of particles and should get F.P: -(2x1.86C) = -3.72C

Effect of interionic attractions account for discrepancy between actual and calculated F.P. for ionic species.

Van’t Hoff Factor compares degrees of dissociation of electrolytes