Hydrogen Halides (HCl, HBr, HI)

The diatomic inorganic compounds with the formula HX are called hydrogen halides, where X is one of the halogens: fluorine, chlorine, bromine, iodine, or astatine. Hydrogen halides are gases that dissolve in water to give acids which are commonly known as hydrohalic acids.

Lab preparation of hydrogen chloride gas

HCl gas is prepared in the lab by heating common salt with concentrated sulphuric acid.

Preparation of aqueous HCl

HCl gas is highly soluble in water. Its aqueous solution is called hydrochloric acid. In order to prepare aqueous HCl, an anti suction device should be made by attaching an inverted funnel to the delivery tube such that the rim of the funnel just touches the surface of the water in the beaker.

HCl gas is dried by conc. H₂SO₄.

Preparation of HBr and HI

HBr and HI are more powerful reducing agents than HCl. Hence, they reduce conc. H₂SO₄ and evolves SO₂ gas. So, conc. H₂SO₄ is not used for the preparation of HBr and HI.

NaBr + H₂SO₄ → NaHSO₄ + HBr ] x 2
H₂SO₄ → H₂O + SO₂ + [O]
2HBr + [O] → Br₂ + H₂O

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2NaBr + 3H₂SO₄ → 2NaHSO₄ + SO₂ + Br₂ + 2H₂O

Similarly,

KI + H₂SO₄ → KHSO₄ + HI
H₂SO₄ → H₂O + SO₂ + [O]
2HI + [O] → I₂ + H₂O

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2KI + 3H₂SO₄ → 2KHSO₄ + SO₂ + I₂ + 2H₂O

However, HBr and HI can be prepared by the reaction of bromide and iodide salt with non-oxidizing acid.

NaBr + H₃PO₄ → NaH₂PO₄ + HBr
NaI + H₃PO₄ → NaH₂PO₄ + HI

Lab preparation of Hydrogen bromide:

HBr gas is prepared in the lab by the action of bromine on moist red phosphorous.

P₄ + 6Br₂ → 4PBr₃
PBr₃ + 3H₂O → H₃PO₃ + 3HBr ] x 4

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P₄ + 6Br₂ + 12H₂O → 4H₃PO₃ + 12HBr

Moist HBr gas is dried over anhydrous CaCl₂ or P₂O₅. Conc. H₂SO₄ or CaO can’t be used for drying.

Lab preparation of Hydrogen iodide:

HI gas is prepared in the lab by the action of water on a mixture of red phosphorous and iodine.

P₄ + 6I₂ → 4PI₃
PI₃ + 3H₂O → H₃PO₃ + 3HI ] x 4

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P₄ + 6I₂ + 12H₂O → 4H₃PO₃ + 12HI

Moist HI gas is dried over anhydrous CaCl₂ or P₂O₅. Conc. H₂SO₄ or CaO can’t be used for drying.

Physical properties:

1. All hydrogen halides are colorless gas with a pungent odour.
2. All hydrogen halides are highly soluble in water.
3. The boiling point of hydrogen halides is in the order: HCl<HBr<HI<HF. HF has a high boiling point due to the intermolecular hydrogen bond.
   H – F……H – F……H – F

Chemical properties:

1. Acidic nature: Hydrogen halides are soluble in water and their aqueous solutions are called hydrohalic acid. HF is hydrofluoric acid, HCl is hydrochloric acid, HBr is hydrobromic acid and HI is hydroiodic acid.
   HCl + H₂O → H₃O⁺ + Cl^–
   
   Acidic strength of halogen acid:
   HF<HCl<HBr<HI

i. Action with base:

HCl + NaOH → NaCl + H₂O
2HCl + Na₂O → 2NaCl + H₂O
2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂
HCl + NaHCO₃ → NaCl + H₂O + CO₂

ii. Action with metal:

2Na + 2HCl → 2NaCl + H₂
Zn + 2HCl → ZnCl₂ + H₂

iii. They turn blue litmus to red, phenolphthalein from pink to colourless and methyl orange from yellow to orange.

2. Reducing nature:

Hydrogen halides are reducing nature. HI>HBr>HCl>HF
HCl reduces MnO₂, KMnO₄ and K₂Cr₂O₇.

– With HCl
MnO₂ + HCl → MnCl₂ + H₂O + [O]
[O] + 2HCl → Cl₂ + H₂O

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MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O

– With KMnO₄:
2KMnO₄ + 6HCl → 2KCl + 2MnCl₂ + 3H₂O + 5[O]
[O] + 2HCl → H₂O + Cl₂ ] x 5

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

– With K₂Cr₂O₇:
K₂Cr₂O₇ + 8HCl → 2KCl + 2CrCl₃ + 4H₂O + 3[O]
[O] + 2HCl → H₂O + Cl₂ ] x 3

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K₂Cr₂O₇ + 14HCl → 2KCl + 2CrCl₃ + 7H₂O + 3Cl₂

HBr and HI react similarly as HCl. HBr and HI can even reduce conc. H₂SO₄ and HNO₃ also.

– With conc. H₂SO₄
H₂SO₄ → H₂O + SO₂ + [O]
[O] + 2HBr →H₂O + Br₂

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H₂SO₄ + 2HBr → 2H₂O + SO₂ + Br₂

– With conc. HNO₃:
2HNO₃ → H₂O + 2NO₂ + [O]
[O] + 2HBr → H₂O + Br₂

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2HNO₃ + 2HBr → 2H₂O + 2NO₂ + Br₂

HI is a more powerful reducing agent. Hence, it reduces copper sulphate too.

2CuSO₄ + 4HI → Cu2I₂ + I₂ + H₂SO₄

Uses of hydrogen halides:

• HF is used for glass etching, metal cleaning, etc.
• HCl is used to manufacture glue, purify common salt, bleaching textiles, etc.
• HBr is used to prepare AgBr used in photography, bromination of organic compounds, etc.
• HI is used to prepare iodine, reducing agent, etc.

Some Important Questions

1. Why HBr can’t be prepared using NaBr and conc. H₂SO₄? Give balanced reaction for preparation of HCl.
2. Give the balanced chemical reaction for laboratory preparation of HBr gas.
3. How can you show that HBr is a better reducing agent than HCl?
4. Order the hydrogen halide on a. boiling point b. acidic strength c. reducing agent d. solubility.