Energetics of Solution Formation

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Energetics of Solution Formation

• When a solute is dissolved in a solvent,
  sometimes the overall dissolving process is
  endothermic and sometimes it is exothermic.
• The overall ?H of solution is the sum of the ?H
  values for three separate processes
• Solvent molecules must be moved apart.
  Overcoming their mutual attractions is an
  endothermic process.
• Solute molecules or ions must be moved apart,
  also an endothermic process.
• Solvent molecules are attracted to and surround
  the solute particles. This is an exothermic
  process.

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Figure 11.1The Steps in the Dissolving Process
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• The result
• If the total of the endothermic processes is less
  than the value of the exothermic term, the
  overall solution process will be exothermic. A
  large exothermic term results when there is a
  very strong attraction between the solute and
  solvent, e.g., sulfuric acid and water.
• If the total of the endothermic processes is
  greater than the value of the exothermic term,
  the overall solution process will be endothermic.
  This situation arises when there is weak
  attraction between solute and solvent, and/or
  when there are very strong attractive forces
  within the pure solute and pure solvent, e.g.,
  sucrose in water.

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Figure 11.2 The Heat of a Solution
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• What causes the attraction between solute and
  solvent particles to be strong or weak?
• Like dissolves like, the principle of
  solubility.
• Strong attraction results if the two species have
  similar types of intermolecular forces
• Polar solute in polar solvent
• ex. Acid, base, or ionic solute in water
• Nonpolar solute in nonpolar solvent
• ex. Hydrocarbons in other hydrocarbons

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Effect of Temperature on Solubility

• 1. If the ?H of solution is negative
  (exothermic), the solubility will decrease as the
  temperature increases.
• solute solvent ? solution heat
• If the ?H of solution is positive (endothermic),
  the solubility will increase as the temperature
  increases.
• solute solvent heat ? solution

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Figure 11.6The Solubilities of Several Solids as
a Function of Temperature
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Figure 11.7 The Solubilities of Several Gases in
Water
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Effect of Pressure on Solubility of Gases

• Henrys Law The solubility of a gas in a liquid
  is directly proportional to the partial pressure
  of that gas above the liquid.
• In mathematical terms Pi kH ci
• where Pi partial pressure of ith gas above
  liquid
• ci molarity of dissolved gas i
• kH Henrys law constant, specific to the
  particular gas and solvent

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Figure 11.5 A Gaseous Solute
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• Ex. What is the solubility of CO2 in water at
  25C under a partial pressure of 900 torr of CO2?
• The kH for CO2 in water is 32 L atm/mol at 25C.
• Ans. c P(CO2) / kH
• (900 torr)(1 atm/760 torr) / 32 L atm/mol
• 0.037 mol/L
• Question Why do bubbles form when the cap is
  removed from a bottle of soda?
• Another application deep-sea diving and the
  bends