2.3.2 Chemical Properties of Group 7

The halogens (Group 7 elements) are known for their strong oxidising and reducing abilities, which vary down the group. Their reactions with other elements and compounds, particularly with sulfuric acid, demonstrate their distinct chemical properties. Understanding these reactions is critical for analysing their behaviour in different redox processes.

Reactions of Halides with Concentrated Sulfuric Acid

When sodium halides (NaCl, NaBr, NaI) react with concentrated sulfuric acid H_2SO_4, different outcomes are observed depending on the halide involved. These reactions demonstrate both acid-base interactions and redox reactions, depending on the halide.

Fluoride and Chloride Ions ($F^-$) and ($Cl^-$)

Reaction

NaF or NaCl reacts with H_2SO_4 to produce hydrogen fluoride (HF) or hydrogen chloride (HCl).

Observations: Misty white fumes of HF or HCl are evolved.

Equation

$$NaF + H_2SO_4 → NaHSO_4 + HF$$ $$NaCl + H_2SO_4 → NaHSO_4 + HCl$$

Type of Reaction

Acid-base reaction, not redox, as no change in oxidation state occurs.

Bromide Ions (Br^-)

First reaction

Initially, bromide ions undergo an acid-base reaction similar to chloride ions.

$$NaBr + H_2SO_4 → NaHSO_4 + HBr$$

Observations: Misty white fumes of HBr.

Second reaction

Bromide ions are strong enough to reduce sulfuric acid, leading to a redox reaction where sulfur is reduced and bromine is oxidised.

$$2HBr + H_2SO_4 → Br_2 + SO_2 + 2H_2O$$

Observations: Red-brown vapour of bromine (Br_2) and misty fumes of HBr are produced, along with sulfur dioxide gas (SO_2).

Type of Reaction

Redox reaction: bromine is oxidised from -1 (HBr) to 0 (Br_2) while sulfur is reduced from +6 (H_2SO_4) to +4 (SO_2).

Iodide Ions (I^-)

First reaction

Iodide ions also first undergo an acid-base reaction, producing hydrogen iodide.

$$NaI + H_2SO_4 → NaHSO_4 + HI$$

Observations: Misty white fumes of HI.

Subsequent Reactions

Iodide ions are much stronger reducing agents, and as a result, several redox reactions occur in succession. Iodine reduces sulfur further than bromide, producing a range of reduction products such as sulfur dioxide (SO_2), sulfur (S), and hydrogen sulfide (H_2S).

$$2HI + H_2SO_4 → I_2 + SO_2 + 2H_2O$$ $$6HI + H_2SO_4 → 3I_2 + S + 4H_2O$$ $$8HI + H_2SO_4 → 4I_2 + H_2S + 4H_2O$$

Observations: Purple iodine vapour, yellow sulfur, and a rotten egg smell (H_2S) are produced. Black solid iodine also forms.

Type of Reaction

Redox reaction: iodine is oxidised from -1 (HI) to 0 (I_2), while sulfur is reduced from +6 (H_2SO_4) to +4 (SO_2), 0 (S), and -2 (H_2S).