When concentrated sulphuric acid is added to an inorganic mixture containing chloride, we get colourless pungent smelling gas HCl, but if the mixture contains bromide then we get red vapour of bromine. Why ?

When concentrated sulfuric acid interacts with an inorganic mixture containing halides, the reactions differ significantly based on whether the halide is a chloride or a bromide. This distinction arises from the unique properties of chlorine and bromine, particularly their reactivity and the nature of the products formed during the reaction.

The Reaction with Chlorides

When concentrated sulfuric acid is added to a chloride, such as sodium chloride (NaCl), the following reaction occurs:

• NaCl + H2SO4 → NaHSO4 + HCl↑

In this reaction, hydrogen chloride (HCl) gas is produced. HCl is a colorless gas with a pungent odor, which is why you observe a colorless gas being released. The reaction is straightforward because chlorine is less reactive than bromine and does not undergo further oxidation under these conditions.

The Reaction with Bromides

In contrast, when concentrated sulfuric acid is added to a bromide, such as sodium bromide (NaBr), the reaction is more complex:

• NaBr + H2SO4 → NaHSO4 + HBr↑
• 2HBr + H2SO4 → Br2 + SO2 + 2H2O

Initially, hydrogen bromide (HBr) gas is formed, similar to HCl. However, HBr is more reactive than HCl and can be further oxidized by concentrated sulfuric acid. This oxidation leads to the formation of bromine (Br2), which is a reddish-brown gas. The presence of this red vapor is a clear indication of the bromide's reactivity and the subsequent reactions that take place.

Key Differences in Behavior

The fundamental reason for the different outcomes lies in the relative positions of chlorine and bromine in the periodic table. Bromine is lower than chlorine, making it a stronger reducing agent. This means that bromide ions can be oxidized more readily than chloride ions, leading to the production of elemental bromine in the presence of a strong oxidizing agent like concentrated sulfuric acid.

Summary of Observations

To summarize:

• Chloride + concentrated sulfuric acid → HCl (colorless gas)
• Bromide + concentrated sulfuric acid → HBr (initially colorless gas) → Br2 (reddish-brown vapor)

This difference in behavior highlights the importance of understanding the chemical properties of different halides and how they interact with strong acids. Each halide's unique characteristics lead to distinct products, which can be observed visually during the reaction.