Trend of oxidising power of halogens

Don't get confused with Halide ions

Trend in reducing power of Hydrogen halide

**HCl doesn’t have enough reducing power to reduce conc. H₂SO₄ Acid.

** HBr & HI have enough reducing power to reduce concentrated H2SO4 further .

** HI is a stronger reducing agent than HBr since HI reduce the sulphur in H₂SO₄             from +6 to -2 whereas HBr reduces sulphur from +6 to +4 .

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

Reaction of Concentrated sulfuric acid with metal bromide :

Again, Sulphuric acid reacts with Metal bromide to form HBr :

e.g.         KBr (s)      +    H₂SO₄ (l)  → HBr  (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.

This HBr then further reduces H₂SO₄ to SO₂

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

HBr (g) +   H₂SO₄ (l)  → Br₂ (g)  +   SO₂ (g)  + 2H₂O (l) 

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

Here two reactions are occurring.

Observations :

1)  Steamy / Misty fumes due to HBr gas.

2)  Red brown vapour due to Br₂ gas.

** Bromine i.e. formed here is in the gaseous state since the reaction is exothermic.

Reaction of Metal Chloride With Concentrated Sulphuric Acid:

Any metal chlorides react with concentrated sulphuric acid to give Hydrochloric acid and Sodium hydrogen sulphate.

e.g.    NaCl (s)      +      H2SO4 (l)            →          HCl (g)      +       NaHSO4 (s)

                                    (concentrated)

HCl that is produced cannot reduce H₂SO₄ Acid any further:

HCl (g)         +            H₂SO₄ (l)             →     No reaction

Observation : Misty/ Steamy Fumes Due to the formation of HCl gas because HCl gas is extremely soluble in water vapour in the air.  

Chemical Test For Iodine

Chemical  Test For Iodine 1:

Bleaching action: Iodine doesn’t bleach damp litmus paper.

Chemical  Test For Iodine 2:

Displacement reaction: Iodine cannot displace bromine or chlorine from a solution of bromide or 
chloride ions respectively.

Chemical  Test For Iodine 3:

With Starch-iodide paper : Iodine turns starch- iodide paper deep blue / blue-black.

With Starch-iodide solution: Iodine makes a deep blue / blue-black colour with starch solution.

Chemical Test For Bromine 3 :

Action with starch-iodide paper :

Bromine gas also turns damp starch iodide paper deep blue / blue black coloured complex.

Just like chlorine, Bromine first reacts with KI (aq) in the starch iodide paper :

Br₂ (l)     +     2KI (aq)       →   2KBr (aq)       +                                         I₂ (aq)

                                                                           Here I₃^- Ion formed which is actually soluble in water   not I^- Ion

Then iodine that is formed will react with starch in the starch-iodine Paper  :

I₂ (aq)      +       Starch  →       blue black / deep blue coloured complex