Reversible Reactions and Equilibria

What equilibrium looks like

• Place a reversible reaction in a sealed container with no escape route for any chemical — a closed system
• At first only the forward reaction occurs (no products to react in the reverse direction yet); its rate is highest
• As the products build up, the reverse reaction starts and speeds up
• The forward rate falls (reactants becoming scarcer) and the reverse rate rises (products becoming more abundant)
• Eventually the two rates become equal: the system is at dynamic equilibrium

Two defining features

• The rate of the forward reaction equals the rate of the reverse reaction
• The concentrations of reactants and products stay constant from this point onward (provided temperature and pressure do not change)

Why "dynamic", not "static"

• At equilibrium the reactions have not stopped — both still happen at full speed
• Each forward conversion of a reactant into a product is matched by a reverse conversion of a product back into a reactant
• The chemicals interchange constantly; only the overall amounts stay fixed

Closed-system requirement

• Equilibrium can only be reached when nothing can leak out (or leak in)
• Open systems lose products as they form (a gas escaping into the air, for instance), so the reverse reaction never has enough material to keep up and equilibrium is never reached