9.5 Equilibrium

9.5 Equilibrium

Equilibrium – Physical Equilibrium

Equilibrium  is when the forward and reverse reaction occur at the same rate.

The rates are equal but the quantities (amounts) of reactants and products are not necessarily equal.

  • Equilibrium can only occur:
  1. In a system in which changes that are taking place are reversible.
  2.  In a closed system. A closed system in which nothing is allowed in or out.
  • When equilibrium is reached in a system
  1. Rate of the forward process is equal to the rate of the reverse process.
  2. Concentration (amount) of substances remains constant.
  • Types of equilibrium.

1. Physical Equilibrium

A. Phase Equilibrium

B. Solution Equilibrium

2. Chemical Equilibrium 

PHYSICAL EQUILIBRIUM– occurs in certain reversible physical changes.

A. Phase Equilibrium  occurs in a closed system in which phase changes are occurring, at a specific temperature.

e.g 1. Ice/Water equilibrium occurs at 0°C or 273 K (melting and freezing point of water)

H2O(s) ↔ H2O (l)

e.g 2. Water/Steam (vapor) equilibrium at 100°C or 373 K (boiling point of water)

H2O(l) ↔ H2O (g)

B. Solution equilibrium occurs in a closed system in which a substance is dissolving in a liquid.

e.g. 3. Solid in Liquid Equilibrium: In a SATURATED SOLUTION.

NaCl(s) ↔Na+ (aq) + Cl (aq)

  • Equilibrium exists between dissolved particles and undissolved particles.
  • Rate of dissolving of the solid is equal to the rate of crystallization of the particles.
  • Amounts of solid and ions remain constant in the solution.

e.g. 4. Gas in Liquid equilibrium: In a gaseous solution

  • Equilibrium exists between dissolved gases in the liquid and undissolved gas above the liquid.
  • Rate of dissolving of the gas is equal to the rate of the gas being undissolved.
  • Amounts of the undissolved gas (above the liquid) and dissolved gas (in liquid) remain constant.

CO2 (g) ↔ CO2 (aq)