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.

sulfur 1

Compounds Sulfur

Hydrogen sulfide

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

H+ + S2- –> 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:

S2- –> S

MnO4–> Mn2+

Cr2O72- –> Cr3+

Cl2 –> Cl

SO2 –> S

Sulfur dioxide SO2

sulfur 2

SO2 is prepared by the reaction of acids on sulfites.

2H+ + SO32- –> 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 SO32-  are good reducing agents being oxidised to SO3 and SO42- respectively.

SO2 –> SO3

SO32- –> SO42-

MnO4 –> Mn2+

Cr2O72- –> Cr3+

Cl2 –> Cl

Fe3+ –> Fe2+

Sulfur trioxide SO3

sulfur 3

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

sulfur 4

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, 450oC

SO3 + H2SO4 –> H2S2O7 (Oleum)

H2S2O7 + H2O –> 2H2SO4



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.

SO42- –> 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)

Sulfur 5

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


Sulfur 6

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:

SO32- + S –> S2O32- (boil)

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

S2O32- + 2H+ –> S + SO2 + H2O

Thiosulfate is oxidised differently by different halogens:

S2O32- –> SO42-

Cl2 à Cl


S2O32- –> SO42-

I2 à I