Question

Why quantum yield of formation of hcl is very high than formation of hbr?

Answer 1

A striking feature of the photochemical reaction between hydrogen and chlorine is the extremely large quantum yield that is obtained 

Answer 2

Answer:

In the photosynthesis of HCl, the first step of the secondary process which immediately follows the primary process is exothermic and hence takes place easily and consequently chain reaction start and the quantum yield is very high.

while, in the photosynthesis of HBr, the first reaction of secondary process is endothermic and has very little landers to take place because of which there is no chain reaction and the quantum yield is very lows.