Understanding H₂S: Hydrogen Sulphide

Hydrogen sulphide naturally occurs in the atmosphere from the gases produced by volcanoes, petroleum (which is crude), natural gas and even from the decay of organic matter. It contains two hydrogen atoms along with one sulphur.

Let us understand what happens in the H₂S compound. Read on to learn its preparation, structure and even other aspects.

What is Hydrogen Sulphide?

H₂S is a covalent hydride of sulphur (Group 16), VSEPR: bent (sp³ on S, 2 lone pairs), H–S–H ≈ 92°. We can also see that the formula of
H₂S is very similar to that of water, H₂O. But despite the similarity in the structure, because of sulphur’s low electronegativity and larger atomic size, they differ in physical and chemical properties.

Hydrogen Sulphide

Breakdown of Hydrogen Sulphide Properties

For the molecular structure and chemical behaviour it exhibits, hydrogen sulphide is widely studied.

Here is a complete understanding of its properties.

Physical Properties

The physical properties of H₂S :

Appearance: It is a colourless gas
It exhibits a rotten eggy odour
Melting point: –82 °C
Boiling point: –60 °C
If we talk about its density, it is heavier than air
Sparingly soluble in water but enough to form a weakly acidic solution (hydrosulphuric acid).

Chemical Properties

The chemical properties of H₂S :

Weak dibasic acid → since it forms HS⁻ and S²⁻ ions, it ends up acting like a weak dibasic acid
Strong reducing behaviours (especially in moist air) → Reducing agent (E.g., in deposition of sulphur, 2 H₂S + O₂ → 2 H₂O + 2 S)
This compound burns in oxygen → Combustion (E.g., exhibits complete combustion in: 2 H₂S + 3 O₂ → 2 SO₂ + 2 H₂O, it also exhibits limited combustion: 2 H₂S + O₂ → 2 S + 2 H₂O)
When we look through the eyes of qualitative analysis, its reactivity with metal ions causes precipitation of metal sulphides.

Preparation of Hydrogen Sulphide

H₂S can be prepared by various methods. Below are some -

1. Laboratory Preparation

From sulphides and acids reaction
- FeS + 2 HCl → FeCl₂ + H₂S ↑
By passing steam over heated sulphides
- CaS + H₂O → CaO + H₂S
Laboratory preparation
- In this, we treat iron(II) sulphide with dilute HCl in a Kipp’s apparatus, which produces H₂S.

2. Industrial Preparation

It can either be produced by aluminium or phosphorus sulphide hydrolysis, or else it comes up as a byproduct while there is processing of petroleum and natural gas in industries.

Details At A Glance

Property	Details
Formula	H₂S
Molar mass	34 g/mol
State	Colourless gas
Odour	Rotten eggs
Bond angle	≈ 92° (bent).
Nature	Weak acid, reducing agent
Solubility	Sparingly soluble; aqueous solution weakly acidic
Density	Heavier than air

Toxicity and Hazards in Hydrogen Sulphide

Flammable → forms explosive mixtures with air.
If humans are exposed to this gas, it can cause unconsciousness, dizziness, nausea or even death.
Lower concentrations → irritation of eyes, nose, and throat.
Higher concentrations → respiratory paralysis and even death
In order to prevent these mishaps, it is necessary that proper ventilation and the use of gas detectors are done, especially in industrial production.

Summing Up

We have seen that Hydrogen Sulphide, found naturally in volcanic gases, hot springs, and crude oil, is a toxic, foul-smelling gas that has important applications in qualitative inorganic analysis, metallurgy, and industry.

Frequently Asked Questions

Q1. Why does H₂S smell like rotten eggs?

Because hydrogen sulphide itself has a strong, unpleasant odour similar to that of decaying organic matter, due to characteristic volatile sulphur content present in it.

Q2. Is hydrogen sulphide acidic or basic?

In aqueous solution, it acts as weak dibasic acid.

Q3. Is H₂S dangerous for humans?

Yes, it is highly toxic and can be fatal in high concentrations when exposed to humans.

Q4. Is H₂S polar or non-polar?

Polar, but much less polar than water.

Q5. How is H₂S used in qualitative analysis?

It can help identify cations in different groups of inorganic analysis by precipitating characteristic metal sulphides.