Which of the following reactions would produce a halogen?

Question

1993 CE CHEM # 9
Which of the following reactions would produce a halogen?
(1) sodium chloride + conc sulphuric acid
(2) sodium bromide + conc sulphuric acid
(3) sodium iodide + conc sulphuric acid

A (1) only
B (3) only
C (1) and (2) only
D (2) and (3) only

Why the answer is D?

Answer 1

This question has been out of HKCE syllabus since 1995, but is included in HKAL syllabus.

(1)
Sodium chloride reacts with concentrated sulphuric acid to give hydrogen chloride.
NaCl + H2SO4 → NaHSO4 + HCl
The reducing power of HCl is low. HCl does not react with concentrated sulphuric acid. Therefore, no chlorine is formed.

(2)
Sodium bromide reacts with concentrated sulphuric acid to give hydrogen bromide.
NaBr + H2SO4 → NaHSO4 + HBr
HBr is a stronger reducing than HCl. Concentrated sulphuric acid can oxidize HBr to give bromine.
2HBr + H2SO4 → 2H2O + SO2 + Br2

(3)
Sodium iodide reacts with concentrated sulphuric acid to give hydrogen iodide.
NaI + H2SO4 → NaHSO4 + HI
HI is a strong reducing agent. Concentrated sulphuric acid can oxidize HI to give iodine.
8HI + H2SO4 → 4H2O + H2S + I2

Conclusion : Halogens are formed in (2) and (3).

Answer 2

Because the bromide and iodide are strong reducing agent.Br- can reduce the conc. H2SO4 to SO2 and be oxidizied to give Br2. I- can reduce the conc. H2SO4 to H2S and be oxidizied to give I2.

Chloride are not strong reducing agent. The NaCl react with conc. H2SO4 to give HCl gas but not chlorine