Decomposition of hydrogen iodide (HI) into iodine (I2) and hydrogen (H2)

Overall reaction: 
\( 2 \text{NH}_3 (g) \rightarrow \text{N}_2 (g) + 3 \text{H}_2 (g) \)

Mechanism

Step 01: Surface-Catalyzed Reaction
\( 2 \text{HI} \text{ (adsorbed on the surface)} \rightarrow \text{I}_2 \text{ (adsorbed on the surface)} + 2 \text{H} \text{ (adsorbed on the surface)}  \) (slow)

Step 02: Desorption
 \(  \text{I}_2 \text{ (desorbs)} + 2 \text{H}_2 \text{ (desorbs)} \) (fast)

Rate Law:
\( \text{Rate} = k  \)

Order of Reaction:
\( 0  \)