Reaction of Concentrated sulfuric acid with metal iodide:

Again, Sulphuric acid reacts with Metal iodide to form HI :

e.g.         KI (s)      +    H₂SO₄ (l)  → HI  (g)   +    KHSO₄   (s)

This is the primary reaction that would occur in every reactions of Concentrated sulfuric acid with Metal Halide.

If you have remembered from early reaction with metal chloride, HCl can’t reduces H₂SO₄ further whereas HBr can. HBr can reduces H₂SO₄ once. HI can ALSO reduces H₂SO₄ 3 times.

In First reaction , HI  further reduces H₂SO₄ to SO₂ .

         (-1)                            (+6)                               ( 0 )                        (+4)

2HI (g) +   H₂SO₄ (l)  →  I₂ (g)  +   SO₂ (g)  + 2H₂O (l) 

Here  HI reduces Sulphur in sulphuric acid from +6 to +4 in SO₂

In Second reaction , HI  further reduces H₂SO₄ to S.

        (-1)                             (+6)                                   ( 0 )                             ( 0 )

6HI (g) +   H₂SO₄ (l)  →  3I₂ (g)  +     S (s)     +   4H₂O (l)

Here  HI reduces Sulphur in sulphuric acid from +6 to 0 in S .

In Third reaction, HI  further reduces H₂SO₄ to H₂S.

          (-1)                           (+6)                                   ( 0 )                                      ( -2 )

8HI (g) +   H₂SO₄ (l)  →  4I₂ (g)  +     H₂S (s)     +   4H₂O (l)

Here  HI reduces Sulphur in sulphuric acid from +6 to -2 in H₂S.

Here three reactions are occurring.

Observations :

1)  Steamy / Misty fumes due to HI gas.

2)  Purple vapour due to I₂ gas. Iodine i.e. formed is in gaseous state since the reaction is exothermic.

3)  Black solid due to formation of solid iodine.

4) Yellow solid due to the formation of sulphur.

5) Rotten Egg smell / Smell of bad eggs due to formation of H₂S