Question

Consider the following reaction mechanism:

1) Initiation
Br2 → 2 Br•

2) Propagation (here a cycle of two steps)
Br• + H₂ → HBr + H•
H• + Br2 → HBr + Br•

3) Termination 2 Br• → Br2

This reaction is catalyzed by platinum.

Wat causes the bromine to split in the initiation step? Does the platinum influences this or is it just due the weak bonding between Br-Br?

Answer 1

Platinum catalyzed this reaction. It shows that it should catalyze the slow step in the mechanism. Here propogation seems to be important or rate determining step. Platinum is a transition metal, it adsorbs Hydrogen molecules in metal surface. Due to large surface area of metal, hydrogen also covers much surface area for reaction. Now easily Bromine radical attack on adsorbed Hydrogen molecule facilitating the propagation step. This increase the rate of reaction or catalyse the reaction.

Platinum does not play any role in splitting of Bromine into radicals. The molecule turns to radical due to it's weak bonding energy.