Reversible reactions and equilibrium

In a reversible reaction the products can react to form the reactants again, shown by the symbol ⇌. Heating blue hydrated copper sulfate makes white anhydrous copper sulfate and water; adding water reverses it. If one direction is exothermic, the other is endothermic by the same amount of energy.

In a closed system a reversible reaction reaches dynamic equilibrium: the forward and backward reactions continue at the same rate, so the amounts stay constant. Le Chatelier's principle says that if conditions change, the equilibrium shifts to oppose the change. In the Haber process, N₂ + 3H₂ ⇌ 2NH₃ (forward exothermic), higher pressure favours ammonia because there are fewer gas molecules on the right; lower temperature favours ammonia but is too slow, so a compromise of about 450 °C, 200 atmospheres and an iron catalyst is used.