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.

lab preparation of hydrogen chloride(HCl)
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.

preparation of aqueous HCl
Fig: Preparation of aqueous HCl

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

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


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

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

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

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:

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.

  1. 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

MnO2 + 4HCl → MnCl2 + Cl2 + 2H2O

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

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

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

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

H2SO4 + 2HBr → 2H2O + SO2 + Br2

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

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.

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