GCSE 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 produce on reacting an acid with a  sulfide

H^+_{(aq)} + S^2^-_{(aq)} \rightarrow H_2S_{(aq)}

It is a very weak acid, and therefore it can be collected over water, since it will not dissolve in water. Since it is insoluble in water it can therefore be prepared by mixing two soluble salts to give the H+ cation, such as HCl, and the S2- anion, such as Na2S.

2HCl + Na_2S \rightarrow 2NaCl + H_2S

Sulfur dioxide SO2

SO2 is prepared by the reaction of acids on sulfites.

2HCl + Na_2SO_3 \rightarrow SO_2 + H_2O
2H^+ + SO_3^2^- \rightarrow SO_2 + H_2O

SO2 is a colourless gas but it easily forms moist since it is highly soluble in water. This would form H2SO3.

SO_2 + H_2O \rightarrow H_2SO_3

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

SO2 + NaOH \rightarrow Na_2SO_3 + H_2O

SO2 is a good reducing agent and in fact its test is that it turns orange potassium dichromate to green.

Sulfur trioxide SO3

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

SO_3 + H_2O \rightarrow H_2SO_4

SO3 reacts with bases to form the sulfate and water.

SO_3 + NaOH \rightarrow Na_2SO_4 + H_2O

SO3 is an oxidising agent.

Sulfuric (VI) acid

Contact process

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

2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2

2SO_2 + O_2 \rightarrow 2SO_3 (This is in equilibrium, 99% conversion) V2O5 catalyst, 2 atm, 450oC

SO_3 + H_2SO_4 \rightarrow H_2S_2O_7 (Oleum)

H_2S_2O_7 + H_2O \rightarrow 2H_2SO_4

Properties

Acid

Sulfuric acid is a strong acid, immediately giving a proton and hydrogensulafte 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.

2H_2SO_4 + Cu \rightarrow CuSO_4 + SO_2 +2 H_2O

Dehydrating agent

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

C_6H_1_2O_6 \rightarrow 6C+ 6H_2O

Drying agent

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

Displacement reactions to prepare acids

NaCl + H_2SO_4 \rightarrow NaHSO_4 + HCl

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