Chapter 13 - Properties of Solutions

• Solutions - Homogeneous mixtures of two or more pure substances.
• Solute - Part being dissolved. Usually smaller in quantity.
• Solvent - Thing that dissolves.
• Solvation - Dissolving process.
• Hydration - The dissolving process with water as the solvent.
• Substances depend on these to form solutions.
  • Intermolecular forces
  • Natural tendency to mix.

Natural Tendency to Mix

• Mixing of gases is spontaneous.
• Mixing causes more randomness in the position of molecules increasing the entropy.

Attractions involved when forming a solution

• Solute-solute interactions - Must be overcome to disperse these particles when making a solution.
• Solvent-solvent interactions - Must be overcome to make room for the solute.
• Solvent-solute interactions - Occur as the particles mix.

Energetics of Solution Formation

• ΔH solvent - Energy required to vaporize.
• 
• Solubility - How much solute can be dissolved in a given amount of solvent at a given temperature.
• 
• Saturated solutions - Can’t add more solute. More than enough.
• Unsaturated solutions - Can add more. Not enough.
• Supersaturated solutions - Temporary situation where the solution is cooled slowly and for it to react, a crystal from the solute is added.

Factors that affect solubility

• Solute-solvent interactions
• Temperature
  • As it goes up, more solute is dissolved
• Pressure (for gas solutes)
  • As it goes up, more solute can be added.

Pressure Effects

• Solubility of solids and liquids isn’t affected by pressure.

• Gas solubility is affected by pressure

• Henry’s Law - The solubility of a gas is proportional to the partial pressure of the gas above the solution. It’s only true at a constant temperature.

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Temperature Effects on Solubility

• For most solids, as temperature increases, solubility increases.
• For all gases, as temperature increases, solubility decreases.

Solution Concentration Conversion Between Units

• Mass percentage (% m/m)
• 
• Parts per million (ppm)
• 
• Parts per billion (ppb)
• 
• Mole fraction (X)
• 
• Molarity (M)
• 
• Molality (m)
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Colligative Properties

• They depend only on the quantity, not on the identity of the solute.
  • Vapor pressure reduction
  • Boiling point elevation
  • Freezing point depression
  • Osmotic pressure

Vapor pressure reduction

• Raoult’s Law
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Boiling point elevation

• 
• i - Van-Hoff factor, the number of ions you get when dissolving the compound.
• Kb is a constant different for all compounds.

Freezing point depression

• 
• Kf is a constant different for all compounds.

Osmotic Pressure

• Osmosis - The net movement of solvent molecules from a solution of low to a high concentration of solute across a semipermeable membrane.
  • Semipermeable membrane - Smaller particles pass through it, but it blocks larger particles.
• Osmotic pressure - The applied pressure to stop bigger particles.
• 
• If two solutions separated by a semipermeable membrane have the same osmotic pressure, no osmosis occurs.

Colloids

• Colloids - Suspension of particles larger than individual ions or molecules, but too small to be settled out by gravity.
  • Ex. → Blood
• They have a hydrophobic end (tail) and a hydrophilic end (head).

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