Question

In the preparation of HBr or HI ,NaX(X=Br, I) is treated with H₃PO₄ and not by concentrated H₂SO₄ since,

(A) H₂SO₄ makes the reaction reversible

(B) H₂SO₄ oxidises HX to X₂ (Br₂ , I₂)

(C) Na₂SO₄ is water soluble and Na₃PO₄ is water insoluble

(D) Na₃PO₄ is water insoluble and Na₂SO₄ is water soluble

Answer 1

Correct Answer is:(B) H₂SO₄ oxidises HX to X₂ (Br₂ , I₂)

HBr (or HI) cannot be prepared by heating bromide (iodide) with concentrated H₂SO₄ because HBr and HI are strong reducing agents and reduce H₂SO₄ to SO₂ and get themselves oxidised to bromine and iodine respectively.

KX + H₂SO₄ → KHSO₄ + HX

H₂SO₄ + 2HX → SO₂ + X₂ + 2H₂O (X = Br or I)

Hence, HBr and HI are prepared by heating bromides and iodides respectively with concentrated H₃PO₄ .

3KBr(KI) + H₃PO₄ → K₃PO₄ + 3HBr (HI)