Sulfur

GCSE Sulfur

Sulfur

Allotropes

Sulfur is found as S₈ rings and it can exist as 2 different allotropes. These allotropes have the same reactivity since they are made up of exactly the same molecule.

Compounds Sulfur

Hydrogen sulfide

H₂S is a poisonous gas which is produced on reacting an acid with a  sulfide

H⁺ + S^2- –> H₂S

It is a very weak acid, and therefore it can be collected over water since it will not dissolve.

H₂S is a very strong reducing agent, and it can reduce the number of ions:

S^2- –> S

MnO₄^– –> Mn²⁺

Cr₂O₇^2- –> Cr³⁺

Cl₂ –> Cl^–

SO₂ –> S

Sulfur dioxide SO₂

SO₂ is prepared by the reaction of acids on sulfites.

2H⁺ + SO₃^2- –> SO₂ + H₂O

SO₂ is a colourless gas but it easily forms moist since it is highly soluble in water.

It is an acidic oxide and therefore reacts with bases for form water and a sulfite salt.

SO₂ + NaOH –> Na₂SO₃ + H₂O

SO₂ and SO₃^2-  are good reducing agents being oxidised to SO₃ and SO₄^2- respectively.

SO₂ –> SO₃

SO₃^2- –> SO₄^2-

MnO₄^– –> Mn²⁺

Cr₂O₇^2- –> Cr³⁺

Cl₂ –> Cl^–

Fe³⁺ –> Fe²⁺

Sulfur trioxide SO₃

SO₃ is the same as sulfuric (VI) acid since the latter is SO₃ dissolved in water. Therefore all reaction will be described for sulfuric acid.

SO₃ can be produced by the oxidation of SO₂ in air.

SO₃ fumes in air since it dissolves in moisture to produce H₂SO₄.

SO₃ reacts with bases to form the sulfate and water.

SO₃ is an oxidising agent and it behaves exactly the same os sulfuric (VI) acid.

Sulfuric (VI) acid

Contact process

The process by which sulfuric (VI) acid is prepared from S.

2ZnS + 3O₂ –> 2ZnO + 2SO₂

2SO₂ + O₂ –> 2SO₃ (This is in equilibrium, 99% conversion) V₂O₅ catalyst, 2 atm, 450^oC

SO₃ + H₂SO₄ –> H₂S₂O₇ (Oleum)

H₂S₂O₇ + H₂O –> 2H₂SO₄

Properties

Acid

Sulfuric acid is a strong acid, immediately giving a proton and hydrogensulfate ion. Sulfuric acid can react with an alkali, base, metal and carbonate.

Oxidising agent

H₂SO₄ is a very good oxidising agent being reduced to SO₂.

SO₄^2- –> SO₂

Cu –> Cu²⁺

C –> CO₂

Br^– –> Br₂

Dehydrating agent

Concentrated sulfuric acid can also dehydrate a molecule by taking a water molecule from the reactant.

HCOOH –> H₂O + CO₂

C₆H₁₂O₆  –> 6C+ 6H₂O

Drying agent

Sulfuric acid can be used to remove water molecules from a mixture. Any excess water can be removed from a reaction mixture by addition of sulfuric acid.

Displacement reactions to prepare acids

NaCl + H₂SO₄ –> NaHSO₄ + HCl

NaBr + H₂SO₄ –> NaHSO₄ + HBrà Br₂ if H₂SO₄ is concentrated

Nal + H₂SO₄ –> NaHSO₄ + Hl –> I₂ if H₂SO₄ is concentrated

NaNO₃+ H₂SO₄ –> NaHSO₄ + HNO₃

 

Sulfates (VI)

Soluble sulfates can be prepared by the reaction of the acid with a base, alkali, metal or carbonate.

Insoluble sulfates can be prepared via precipitation reactions

All sulfates are thermally stable except for CuSO₄ and FeSO₄

CuSO₄ –> CuO + SO₃

FeSO₄ –> Fe₂O₃ + SO₂ + SO₃

Thiosulfates

In thiosulfate, there are 2 S atoms with different oxidation states. This is due to the fact that the central Sulfur atom extends its valence shell while the other sulfur does not.

This is prepared by the reaction of a sulfite with sulfur:

SO₃^2- + S –> S₂O₃^2- (boil)

Thiosulfates decompose in the presence of acids to form S and SO₂

S₂O₃^2- + 2H⁺ –> S + SO₂ + H₂O

Thiosulfate is oxidised differently by different halogens:

S₂O₃^2- –> SO₄^2-

Cl₂ –> Cl^–

 

S₂O₃^2- –> S₄O₆^2-

I₂ –> I^–