Halide Ions

What is the trend in reducing power down group 7?

• How easy it is for a halide ion to lose an electron depends on the attraction between the nucleus and the outer electrons.

• As you go down the group, the attraction gets weaker because ions get bigger, so electrons are further away from the positive nucleus

• There is also increased shielding

• Therefore reducing power increases down the group

How does NaF or NaCl react with sulfuric acid?

• NaCl/ NaF reacts for form either HCl or HF

• You will see misty fumes as the gas comes into contact with moisture in the air.

• HF and HCl aren’t strong enough reducing agents to reduce sulfuric acid so the reaction stops

• This is an acid base reaction, not a redox reaction

How does NaBr react with sufuric acid?

• 1st reaction: NaBr + H2SO4 → NaHSO4 + HBr

• This gives misty fumes of hydrogen bromide gas. But the HBr is stronger reducing agent than HCl and reacts with the H2SO4 in a redox reaction

• 2nd reaction: 2HBr + H2SO4 O2 and orange fumes of Br2

How does NaI react with sulfuric acid?

• 1st reaction: NaI + H2SO4 → NaHSO4 + HI

• HI then reduces H2SO4

• 2nd reaction: 2HI + H2SO4 → I2 + SO2 + 2H2O

• But HI keeps going and reduces the SO2 to H2S

• 3rd reaction: 6HI + SO2 → H2S + 3I + 2H2O

• The reaction produces fumes of H2S and solid iodine

What is the test for halides?

• Add dilute nitric acid to remove ions which might interfere with the test.

• Add a few drops of silver nitrate solution

• A precipitate is formed (of the silver halide)- Chloride is white, bromide is cream, iodide is yellow

• You can also test your results by adding ammonia solution. Each silver halide has different solubility in ammonia

• Chlorides dissolve in dilute ammonia

• Bromide dissolves in conc. ammonia

• Iodide is insoluble in conc. ammonia