9.6 LeChatelier’s Principle

9.6 Chemical Equilibrium

Chemical Equilibrium

Chemical Equilibrium occurs in chemical reactions that are reversible.

  • Chemical Equilibrium is when the rates of the forward and reverse reactions are equal in a chemical reaction (chemical change).
  • At equilibrium the concentration of reactants and products of the reaction remains constant.

LeChatelier’s Principle

  • States that “If a system at equilibrium is subjected to a stress, the equilibrium will shift in the direction that relives the stress.”

Factors that will ‘stress’ a reversible reaction are:

  1. Change in Concentration
  2. Change in Temperature
  3. Change in Pressure
  4. Common ion Effect
  5. Catalyst

1. CONCENTRATION

  • A + B (reactants – Left  Side) ↔ C + D  (products – Right Side)
  • An increase in the concentration of A causes the reaction to go to the right (shifts equilibrium to the right) in order to reduce stress caused by too much A.
  • A and B are use up faster, and more C and D are formed.
  • At equilibrium, there’s a higher concentration (more) of C and D and a lower concentration of B.
  • Any increase in stress to the left side will shift the equilibrium to the right. Similarly any increase in stress to the right side will shift the equilibrium to the left side.

2. TEMPERATURE

  • A + B ↔ C + D + Heat  (or energy)
  • The addition of heat, according to LeChatelier’s Principle, shifts equilibrium so that heat is absorbed (to relive the stress). This favors an endothermic reaction.
  • This shifts the equilibrium to the left, more A and B is produced, while C and D are decreased.
  • The decrease of heat, will shift the equilibrium to the right, since there will be less stress on the right, since heat is being removed.

3. Pressure

  • Changing the pressure only affects gases.
  • If the pressure is increased, the reaction is driven in the direction which relives the pressure (stress), in the direction of less gas molecules (smaller number of gas molecules).
    • In the Haber process for making ammonia NH3(g):
    • N2 (g) + 3H2 (g) ↔ 2NH3(g)
    • There are 4 molecules of gas on the Left Side of the equation (reactants), and two molecules of ammonia gas on the right side (products).
    • Therefore when there is more pressure, the equilibrium shifts to the right to relive the stress, decreasing the number of nitrogen and hydrogen molecules on the Left Side, and increasing the number of ammonia molecules on the Right Side.
    • If there is equal number of molecules of gases on the Left Side and on the Right Side, pressure will have NO EFFECT.

4. COMMON ION EFFECT

  • AgCl(s) ↔ Ag+ (aq) + Cl– (aq)
  • If you add NaCl, the additional Cl ions will stress the Right Side causing the equilibrium to shift to the Left Side and create more AgCl(s).
  • The silver ions will decrease, while the AgCl(s) will increase.

5. CATALYST

  • Catalyst increase the rate of the forward and reverse direction equally.