9701 · 11.3
Some reactions of the halide ions — FAQ
Frequently asked questions for 9701 Some reactions of the halide ions. Direct answers first, then deeper explanation — then practise with marking.
Why is concentrated sulfuric acid not used to prepare HBr and HI in the lab?
Because concentrated sulfuric acid is a strong enough oxidising agent to oxidise the HBr and HI formed into bromine (Br₂) and iodine (I₂) respectively. This results in an impure sample of the hydrogen halide. A weaker, non-oxidising acid like concentrated phosphoric(V) acid is used instead.
Why does the reducing power of halide ions increase down the group?
It's due to atomic structure. As you go down the group, the ions get larger (increasing ionic radius) and have more inner shells of electrons (increased shielding). This means the outermost electron is further from the nucleus and feels a weaker electrostatic pull. Consequently, this electron is lost more easily, making the ion a stronger reducing agent.
Can fluorine displace chloride ions from a solution of NaCl?
Yes, in principle, fluorine is the most powerful oxidising agent and would readily displace chloride ions. However, this reaction is not performed in a typical school laboratory because fluorine is extremely reactive and reacts violently with water, the solvent, producing oxygen and HF.