Solubility

Last updated Jun 15, 2023 Edit Source

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• Solubility is the mass of a substance that will dissolve in 100g of water at 25$\degree$C
• Saturated Solution → No more solute will dissolve at set temperature
• Unsaturated Solution → Less solute than needed to make a saturated solution
• Supersaturated solution → Unstable solution that causes crystals of solute to form if disturbed (forms from cooling saturated solution)

• # Solubility Curves

  • Solubility curves highlight the change in solubility for a substance when the temperature changes
  • Example Solubility Graph: 300
    • Points below the curve represent an unsaturated solution
    • Points on the curve represent a saturated solution
    • This is a solubility graph for different compound in water

• For bonds to be broken → endothermic → requires energy
• Attractions form between the solvent and solute → exothermic → releases energy
• If energy released > energy required → compound will dissolve
  • Requires attraction between solute and solvent to generate energy
    • Polar dissolves polar as they can form dipole-dipole interaction
    • Non-polar dissolves non-polar due to dispersion forces
      • The dispersion forces must be stronger than the forces of the bonds if the solute
    • Non-polar cannot dissolve/be dissolved by polar solvents/solutes

• Water and sodium chloride ion-dipole interaction diagram:
  • Water is a highly effective solvent for dissolving compounds due to its highly polar nature
  • Solvent molecules form attraction between oppositely charged ions in ionic lattice
    • This is weak, but there are so many molecules that it can break about the network
      • Imagine far more water molecules than seen on the diagram
  • This is a polar dissolves polar interaction

• You need to be able to convert between different units for concentration!
• Units for Concentration:
  • Moles per litre → mol L$^{-1}$
    • $\frac{number\\ of \\ moles}{litre}$
  • Grams per litre → g L$^{-1}$
    • $\frac{grams}{litre}$
  • Parts per million → ppm
    • $\frac{mg\\ solute}{kg\\ solution}$
    • Milligrams per kilogram
  • Mass per 100g of water → % m/m
    • $\frac{solute\\ mass\\ (g)}{solvent\\ mass\\ (g)}\\ ×\\ {100%}$
      • g/100g
  • Volume per 100ml of water → % v/v
    • $\frac{solute\\ volume\\ (ml)}{solvent\\ volume\\ (ml)}\\ ×\\ {100%}$
      • ml/100ml
  • Mass per 100ml water → % m/v
    • $\frac{solute\\ mass\\ (g)}{solvent\\ volume\\ (ml)}\\ ×\\ {100%}$
      • g/100ml

• $\rho$ - density
  • Density of water → 1 gram mL$^{-1}$
    • 1 mL of water = 1 gram of water
  • mass (g) = density (g mL$^{-1}$) × volume (mL)

• In precipitation reactions, there are often ions that are not truly part of the reaction as they do not undergo a chemical reaction
• This is because when compounds are dissolved, ions are delocalised and are free to move in the solution
  • NaCl${(aq)}$ + KNO$3$${(aq)}$ actually looks like → Na$^+$${(aq)}$ + Cl$^-$${(aq)}$ + K$^+$${(aq)}$ + NO$3^-$${(aq)}$
    • No reaction is taking place
• A reaction only occurs when two solutions are mixed if a non-aqueous product is formed
• When writing a precipitation (ionic) reaction, omit the ions that are not involved in the reaction
  • e.g. NaCl${(aq)}$ + AgNO$3$${(aq)}$ → AgCl${(s)}$ + NaNO$3$${(aq)}$ should be written instead as:
  • Ag$^+$${(aq)}$ + Cl$^-$${(aq)}$ → AgCl$_{(s)}$ where the sodium and nitrate ions are omitted due to them not actually reacting
    • These ions are known as spectator ions