Reversible Reactions

KEY POINTS:

• Despite what you may have been told, many chemical reactions can proceed in either direction. For example the decomposition of ammonium chloride to ammonia and hydrogen chloride gas:
  • Forwards:
    NH₄Cl _(s)     →     NH_{3 (g)}     +     HCl _(g)
  • Backwards:
    NH_{3 (g)}     +     HCl _(g)     →     NH₄Cl _(s)
• If this is set up in a sealed container we can reach a point where both the forwards and the backwards reaction are occurring at exactly the same rate! This is referred to as dynamic equilibrium.
• We represent dynamic equilibrium using reversible reaction arrows:
  NH₄Cl _(s)     ⇄     NH_{3 (g)}     +     HCl _(g)
• The dynamic part indicates that reactions are still on going. As the ammonium chloride is decomposing as fast as the ammonia and hydrogen gas is coming together there is no net change in the amount of each substance present.
• The position of equilibrium i.e. how far to the left or right the equilibrium lies, is dependent on variables such as temperature and pressure. Le Chatelier’s principle helps to understand how the equilibrium shifts as conditions change.
• As chemists we want high yields of product – but in a sufficiently short amount of time. So compromises between yield and rate must be made.