[leena]

Cl₂ is prepared by the action of a Cl^- with MnO₂ and conc' H₂SO₄ while it is not necessary to add MnO2 in the preparation of Br₂ and I₂ with conc' H2SO4 .Give reasons.

Ok,so I think that both MnO₂ and conc' H₂SO₄ act as oxidising agents.and I think Cl is a stronger oxidising agent than Br and I , so Cl should undergo reduction easily,therefore the oxidation of Cl^- to Cl₂ would be not so favourable,so... we may need an extra oxidising agent ,like MnO2,for the reaction to takeplace.
Does this make any sense?
Thanks

[Borek]

Doesn't sound wrong to me. Second opinion won't hurt.

[dufengtao]

I think you are right, althought in some book the MnO2 is considered as a catalyzer.

[leena]

Thank you Borek and dufengtao,for confirming.