會考化學，電化序，20分，勁人請來

concentrated sulphuric acid is a oxiding agent.
it can oxidize iodide ion into iodine. and it can
oxidize bromide ion into bromine.
but it cannot oxidize chloride ion into chlorine.
it is because electronegativity decrease down the group .
since chlorine atom is very electronegative, the chloride formed
will be very stable and is not easily changed back to chlorine.