Chemistry · Chemistry of the elements · Grade 8

The oxygen group and sulfur

How is yellow sulfur obtained from natural gas in Muborak, and why is H₂S dangerous?

The situation

Part of the natural gas that arrives at the Mubarek gas processing plant in Kashkadarya is “sour” — it contains poisonous hydrogen sulfide, which smells of rotten eggs. The plant purifies this gas and obtains hundreds of thousands of tonnes of yellow sulfur from it every year. Sulfur is a “relative” of oxygen — an element of group VI A. When heated it behaves strangely: first it melts, then it darkens and becomes thick as honey. Why? Why is hydrogen sulfide dangerous, and how can sulfur be obtained from it?

Topic

The oxygen group and sulfur. Mubarek gas processing plant: removing hydrogen sulfide from sour natural gas to obtain yellow sulfur

Key ideas

Allotropy
Hydrogen sulfide
Claus process

Formulas

2 H_{2} S + SO_{2} \overset{}{\to} 3 S + 2 H_{2} O

The Claus reaction: one third of the hydrogen sulfide is burned, and the SO₂ formed reacts with the remaining H₂S to give sulfur.

2 H_{2} S + 3 O_{2} \overset{}{\to} 2 SO_{2} + 2 H_{2} O

H₂S burns in excess oxygen; when oxygen is short: $2 H_{2} S + O_{2} \overset{}{\to} 2 S + 2 H_{2} O$ .

FeS + 2 HCl \overset{}{\to} FeCl_{2} + H_{2} S ↑

Preparing hydrogen sulfide in the laboratory (in a fume cupboard!).

4 Ag + 2 H_{2} S + O_{2} \overset{}{\to} 2 Ag_{2} S + 2 H_{2} O

Tarnishing of silver — traces of H₂S in the air form silver sulfide.

ω (S) = \frac{n A_{r} (S)}{M_{r}}

Mass fraction of sulfur in a compound: in FeS₂, $n$ = 2.

Theory

The oxygen group (§29)

Group VI A: oxygen, sulfur, selenium, tellurium and radioactive polonium. There are 6 electrons in the outer shell ( $n s^{2} n p^{4}$ ): two are missing to complete an octet, so in compounds with hydrogen and metals the oxidation state is −2. Sulfur, selenium and tellurium also show +4 and +6 in oxygen compounds (SO₂, SO₃), whereas oxygen is positive only with fluorine (+2 in OF₂). Going down the group, the atomic radius increases (O 66 → Te 138 pm), electronegativity decreases (3.44 → 2.10) and non-metallic properties weaken: oxygen and sulfur are typical non-metals, selenium and tellurium are semiconductors, and polonium is a metal. In the series of hydrides $H_{2} O$ , $H_{2} S$ , $H_{2} Se$ , $H_{2} Te$ stability decreases and acidity increases.

Sulfur: allotropy and natural compounds (lab work 7)

The sulfur molecule is an 8-atom crown-shaped ring, S₈. Rhombic sulfur (ρ = 2.07 g/cm³) is stable up to 95.3 °C; if the melt is cooled slowly, needle-like monoclinic crystals (ρ = 1.96) form at ≈ 119 °C, and at room temperature they turn back into rhombic sulfur within a few days. On heating, sulfur turns into a pale yellow, mobile liquid at ≈ 113–119 °C; above ≈ 159 °C the rings break and long chains form — the liquid darkens and becomes very thick (at ≈ 187 °C its viscosity is ≈ 93 Pa·s — 10 000 times that at 150 °C). If such a liquid is poured into cold water, plastic sulfur is obtained, which stretches like rubber. The boiling point is 444.6 °C. In nature: native sulfur, sulfides — pyrite FeS₂, chalcopyrite CuFeS₂, sphalerite ZnS, galena PbS, cinnabar HgS; sulfates — gypsum CaSO₄·2H₂O. Uses: sulfuric acid, vulcanisation of rubber, matches, plant-protection products.

Hydrogen sulfide (§30)

Hydrogen sulfide H₂S is a colourless, very poisonous gas with a smell of rotten eggs, slightly denser than air. In nature it is found in volcanic gases, some mineral springs and “sour” natural gas, and it also forms when proteins rot. In the laboratory: $FeS + 2 HCl \overset{}{\to} FeCl_{2} + H_{2} S$ . It burns: in plenty of oxygen — $2 H_{2} S + 3 O_{2} \overset{}{\to} 2 SO_{2} + 2 H_{2} O$ , with little oxygen — sulfur is released: $2 H_{2} S + O_{2} \overset{}{\to} 2 S + 2 H_{2} O$ (a yellow stain on a cold porcelain dish). H₂S is a strong reducing agent; its aqueous solution is a weak hydrosulfuric acid, and its salts are sulfides. Qualitative test: with lead salts it gives black PbS (lead acetate paper turns black); silver objects tarnish from traces of H₂S in the air ( $Ag_{2} S$ ). Danger: the smell can be detected even at 0.0005 ppm, but at 100–150 ppm the olfactory nerve is paralysed and a person no longer notices the danger; half of those who breathe air with ≈ 800 ppm for 5 minutes die.

Oxides of sulfur (§31)

Sulfur burns in air with a bluish flame to form sulfur(IV) oxide: $S + O_{2} \overset{}{\to} SO_{2}$ . SO₂ is a colourless, pungent, poisonous gas and an acidic oxide: $SO_{2} + H_{2} O \overset{}{⇌} H_{2} SO_{3}$ (sulfurous acid — weak and unstable; its salts are sulfites). SO₂ bleaches many dyes (it is used to bleach wool, silk and straw) and kills microbes — it is the preservative E220: dried apricots are often treated with SO₂ so that they keep their orange colour. SO₂ in industrial waste causes acid rain, so it is captured and converted into acid or sulfur. In the presence of a catalyst SO₂ is oxidised to sulfur(VI) oxide: $2 SO_{2} + O_{2} \overset{}{⇌} 2 SO_{3}$ ; SO₃ (boiling point 45 °C) combines violently with water to give sulfuric acid — in detail in the “Sulfuric acid and the contact process” experiment.

Mubarek: sulfur from sour gas (the Claus process)

Sour natural gas is first washed with amine solutions: H₂S and CO₂ pass into the solution, and the purified gas goes into the pipeline. When the solution is heated, the “acid gas” released is sent to the Claus unit: in the furnace one third of the H₂S is burned ( $2 H_{2} S + 3 O_{2} \overset{}{\to} 2 SO_{2} + 2 H_{2} O$ ), then in the furnace and over a catalyst (Al₂O₃) at 200–350 °C $2 H_{2} S + SO_{2} \overset{}{\to} 3 S + 2 H_{2} O$ . After each stage the sulfur vapour is cooled and flows as a liquid into a pit, and is then made into lumps or granules. The amount of air must be exact: unless H₂S : SO₂ = 2 : 1, the excess gas goes to waste. With two or three catalytic reactors ≈ 95–98 % of the sulfur is recovered. The Mubarek plant (Kashkadarya) produces hundreds of thousands of tonnes of sulfur a year (on average ≈ 340 thousand t, according to 2021 data). The world's main consumer of sulfur is the production of sulfuric acid. The problems of practical 2: mass fraction in a compound, gas volume and mass from a reaction.

Safety and the model

Safety: work with H₂S and SO₂ only in a fume cupboard, and do not sniff the gas directly; liquid sulfur sticks to the skin and burns deeply — do not point the test tube at yourself or others, heat slowly (the vapour can ignite); wear safety goggles. Polonium is radioactive — it is not used in schools. Model: the heating rate is schematic (3 °C/s), the viscosity is based on measured values, and above 200 °C it is approximate; H₂S in the room is a two-zone model (area by the bench 2 m³, room 180 m³, the cupboard draws away 99.9 % of the gas); the efficiency of the Claus stages is constant (0.65; 0.70; 0.60; 0.50), COS and CS₂ are not taken into account; the plant model is not to scale.

Did you know?

The Mubarek gas processing plant produces on average ≈ 340 thousand tonnes of sulfur a year from sour gas (2021 data); the sulfur is shipped as lumps and granules.
Pyrite is called “fool's gold” because it glitters like gold. But it is hard and brittle and leaves a black-green streak on a porcelain plate, whereas gold is soft and leaves a golden streak.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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Water: composition and properties — Grade 7
Mineral resources — Grade 7
Lime and classes of compounds — Grade 8
Non-metals and halogens — Grade 8
Chlorine and hydrochloric acid — Grade 8
Sulfuric acid and the contact process — Grade 8
Nitrogen, ammonia and nitric acid — Grade 8
Phosphorus and mineral fertilisers — Grade 8
Carbon and its compounds — Grade 9
Silicon and silicates — Grade 9