Reversible reaction

A reversible reaction is a reaction that can proceed in both the forward and backward directions under the same conditions, written with a double half-arrow, ⇌, instead of a single arrow. Malay: tindak balas berbalik · Chinese: 可逆反应.

In a reversible reaction the products can react together to re-form the reactants at the same time as the reactants are forming the products. Because neither direction ever fully finishes, some of every substance is present together. In a closed container the reaction can reach equilibrium, the state in which the forward and backward reactions go at the same rate and the amounts of everything stay constant. The two industrial reactions in this chapter are both reversible, which is exactly why their yields are less than complete and why conditions must be chosen with care.

Example. In the Contact process, sulfur dioxide and oxygen form sulfur trioxide in a reversible reaction: 2SO₂ + O₂ ⇌ 2SO₃. In the Haber process, nitrogen and hydrogen form ammonia reversibly: N₂ + 3H₂ ⇌ 2NH₃. In each case the double arrow warns you that not all the reactants convert to product.

Confusion to avoid. Do not confuse a reversible reaction with equilibrium itself. Reversible describes the reaction; equilibrium is the particular state that reversible reaction can settle into. Also, the double arrow ⇌ is not the same as an equals sign or a single reaction arrow.

A useful check in answers is to remember that a reversible reaction never reaches completion in a closed system, so its yield can always be improved, but never made total, by adjusting temperature and pressure. This is precisely why manufacturers must reason about the position of equilibrium rather than simply running the reaction to the end.

Recognising a reaction as reversible is the first step in explaining why industry chooses compromise conditions, a recurring theme in SPM Chemistry.