Lecture on Solubility and Solution Equilibria

Equilibrium Process in Dissolution

• Dissolution of a solute in a solvent is an equilibrium process, similar to evaporation.
• Initially, the rate of dissolving is high as there are no dissolved ions to recrystallize.
• As ion concentration increases, recrystallization starts, leading to dynamic equilibrium.
• Dynamic Equilibrium: Rates of dissolution and recrystallization are equal.

Sodium Chloride Dissolution

• Sodium chloride dissociates into aqueous sodium and chloride ions in water.
• The process is reversible; ions can recombine to form solid sodium chloride.

Types of Solutions

• Saturated Solution: Rate of recrystallization equals the rate of dissolution.
• Unsaturated Solution: Less than equilibrium amount of solute; additional solute can dissolve.
• Supersaturated Solution: More solute dissolved than at equilibrium; unstable.
  • Disturbance causes crystallization.

Supersaturated Solutions and Demonstrations

• Supersaturated solutions appear like water but crystallize upon disturbance.
• Hot Ice (Sodium Acetate): Crystallizes upon disturbance, exothermic.
• Demonstrated by videos showing crystallization upon disturbance.

Solubility and Temperature

• Solubility often increases with temperature for solids.
• Exceptions: Some compounds like cerium sulfate decrease in solubility with temperature.
• Recrystallization Process: Purifies solids by forming larger, purer crystals upon slow cooling.

Solubility of Gases

• Temperature Effect: Solubility decreases with increasing temperature.
  • Example: Cold soda keeps fizz longer than warm soda.
• Pressure Effect: Solubility increases with pressure (Henry's Law).
  • Releasing pressure from a soda causes gas to escape.

Environmental Impacts

• Temperature affects dissolved oxygen in water, impacting aquatic life.
• Thermal Pollution: Warm water discharge reduces dissolved oxygen levels, harming aquatic life.

Henry's Law

• Solubility of a gas is proportional to its partial pressure.
• Formula: ( S = k_H \times P )
  • ( S ) = Solubility
  • ( k_H ) = Henry's Law Constant
  • ( P ) = Partial Pressure

Application and Calculations

• Example calculation of oxygen solubility in water using Henry's Law.
  • Given: ( P_{O_2} = 0.21 ) atm, ( k_H = 1.3 \times 10^{-3} ) M/atm.
  • Solubility ( S = 2.7 \times 10^{-4} ) moles/L._

Note: Understanding these concepts is crucial for applications in chemistry and environmental science, particularly in areas involving solution chemistry and gas solubility.