or

Why can't HBr and HI be prepared by treating conc.H2SO4 with bromide and iodide?

HBr and and HI can't be prepared by treating conc. H2 SO4 with Bromide and Iodide  as HBr and HI are strong reducing agents and they reduce H2SO4 into SO2 and themselves into Br2 and I2 respectively. 

i.e.

2NaBr+H2SO4-----------heat--------------> Na2SO4+2HBr          

2HBr +H2SO4----------------------------> SO2+2H2O+Br2         


2NaI+H2SO4-----------heat-------------->  Na2SO4+2HI

2HI+H2SO4----------------------------> SO2+2H2O+I2