Reactivity of Halogens

Halogens React with Metals

hlorine + Iron

3Cl₂ + 2Fe → 2FeCl₃

Observation

The iron wool burns vogorously with bright flame, forming a brown solid after reaction.

Discussion

1. Chlorine react with iron to form brown iron(III) chloride.
2. The reaction of potassium with concentrated hydrochloric acid produces chlorine gas.

2KMnO₄ + 16HCl → 2KCl + 2MnCl₂ + 5Cl₂ + 8H₂O

Bromine + Iron

3Br₂ + 2Fe → 2FeBr₃

Observation

The iron wool glows brightly but less vigorously. A brown solid is formed.

Discussion

Bromine react with iron to form brown iron(III) bromide.

Iodine + Iron

3I₂ + 2Fe → 2FeI₃

Observation

The iron wool glows slowly with dim light. A brown solid is formed after reaction.

Discussion

Iodine react with iron to form brown iron(III) iodide.

Note:

1. Halogen is a reactive non-metal. It form salt when react with metal.
2. The reactivity decreases down the group.
3. Chlorine gas is a poisonous. Excess chlorine gas is absorbed by the soda lime (sodium hydroxide) so that it does not escape to the surrounding.
4. Iron wool rather than iron piece is used to increase the rate of reaction.

Halogens React with Water

Chlorine + Water

Cl₂ + 2H₂O → HCl + HOCl

Observation:

1. Chlorine gas dissolves in water to form a pale yellow solution.
2. The solution turn blue litmus paper red before it is bleached.

Discussion:

1. The solution formed is acidic and contains bleaching agent.
2. Hypochlorous(I) acid is a strong bleaching agent. It decolourises the colour of litmus paper.

Bromine + Water

Br₂ + 2H₂O → HBr + HOBr

Observation:

1. Bromine liquid dissolves slowly in water to form a yellowish-brown solution.
2. The solution turn blue litmus paper red before it is bleached slowly.

Discussion:

Hypobromous(I) acid is a weak bleaching agent.

Iodine + Water

I₂ + 2H₂O → HI + HOI

Observation:

1. Only a little iodine dissolves in water to form a yellowish solution.
2. The solution turn blue litmus paper red but does not bleach it.

Discussion:

Hypoiodous(I) acid has very weak bleaching characteristic.

Notes:

1. Chlorine, bromine and iodine are soluble in water to form an acidic solution.
2. The solubility decreases down the group.
3. Aqueous chlorine and bromine are bleaching agent.
4. Aqueous iodine does not act as bleaching agent.

Halogens React with Sodium Hydroxide

Chlorine + Sodium Hydroxide

Cl₂ + 2NaOH → 2NaCl + 2NaOCl + H₂O

Observation:

The greenish chlorine gas dissolves quickly in sodium hydroxide, forming a colourless solution.

Discussion:

Chlorine gas react with sodium hydroxide to form salt of sodium chloride, sodium chlorate(I) and water.

Bromine + Sodium Hydroxide

Br₂ + 2NaOH → 2NaBr + 2NaOBr + H₂O

Observation:

The reddish-brown liquid bromine dissolves in sodium hydroxide, forming a colourless solution.

Discussion:

1. Bromine liquid react with sodium hydroxide to form salt of sodium bromide, sodium bromate(I) and water.
2. The reaction is less reactive compare with chlorine.

Iodine + Sodium Hydroxide

I₂ + 2NaOH → 2NaI + 2NaOI + H₂O

Observation:

The black iodine crystals dissolve slowly in sodium hydroxide, forming a colourless solution.

Discussion:

1. The reddish-brown liquid bromine dissolves in sodium hydroxide, forming a colourless solution.
2. Solid iodine react slowly with sodium hydroxide to form salt of sodium iodide, sodium iodate(I) and water.
3. The reaction is least reactive among the three.

Safety Precaution

1. Fluorine, chlorine and bromine gases are poisonous.
2. Therefore all the experiments involving these gases should be carried out in a fume chamber.
3. The experiments involve fluorine are nor done in school.
4. This is because fluorine is so reactive that it will react with most of the substance it comes into contact with.
5. It is very difficult to conduct experiments involving fluorine.