Reversible and Irreversible Reactions

the two types of chemical reaction.

A reaction is reversible if its direction depends on the concentrations of the participating compounds. For example, the heterogeneously catalyzed reaction

(1) N₂ + 3H₂ = 2NH₃

proceeds to the right at low concentrations of ammonia in the gaseous mixture and high concentrations of nitrogen and hydrogen. On the other hand, ammonia decomposes if present in high concentrations, and the reaction proceeds to the left. When a reversible reaction is complete, that is, at chemical equilibrium, the system contains both the starting materials and reaction products.

A reaction is irreversible if it proceeds in only one direction and results in the complete conversion of starting materials into products. An example is the decomposition of explosives. A single reaction can be largely reversible or practically irreversible, depending on temperature and pressure.

A one-step reversible reaction consists of two substituent reactions that proceed simultaneously and differ from each other only in the direction of chemical conversion. The directly observable direction of the overall reaction is determined by the direction of the faster of the two substituent reversible reactions. For example, the one-step reaction

(2) N₂O₄ ⇆ 2NO₂

is composed of the substituent reactions N₂O₄ → 2NO₂ and 2NO₂ → N₂O₄.

In order that a multistep reaction, such as (1), may be reversible, all its component steps must also be reversible.