Chemistry PREPARATION, PROPERTIES, STRUCTURES AND USES OF HYDROGEN SULPHIDE, SULPHUR DIOXIDE AND SULPHUR TRIOXIDE

Hydrogen Sulphide (`H_2S`) :

(i) Also known as sulphuratted hydrogen.

(ii) Lab method : Kipps apparatus

`FeS + text(Conc.) 2HCl -> FeCl_2 + H_2S`

In this reaction conc. `HNO_3` cannot be employed because `HNO_3` is a oxidising agent and converts `H_2S` into colloidal sulphur.

(iii) `Sb_2S_2 + 6HCl -> 2SbCl_3 + 3H_2S`.
Antimony sulphide

Here `H_2S` is rotten egg smelling gas. If conc. `HNO_3` or `H_2SO_4` is added the rotten smell of `H_2S` disappears.

(iv) `text(Properties :)`

(i) `H_2S + Pb (CH_3COO)_2 -> PbS` (Black) `+ 2CH_3COOH`

This would be confirmatory test for `H_2S`.

(ii) It burns in air with a blue flame.

`2H_2S + O_2 -> 2H_2O + S`

Jf the air supply is in excess.

`2H_2S + 3O_2 -> 2H_2O + 2SO_3`

(iii) It reduces moist chlorine to `HCl`

`H_2S + Cl_2 text(moist) -> 2HCl + S`

(iv) It is a mild acid

`H_2S ⇋ S^(-2) +2H^(+)`

`H_2S ⇋ SH^(-) +H^(+)`

(v) Ferric salts reduced to ferrous salts

`H_2S + 2FeCl_3 -> 2FeCl_2 + 2HCl + S`

(vi) Acidified `KMnO_4` gets decoloured by `H_2S`.

`2KMnO_4 + 5H_2S + 3H_2SO_4 -> K_2SO_4 + 2MnSO_4 + 8H_2O + 5S`

(vii) Acidified `K_2Cr_2O_7` becomes green in colour.

`3H_2S + K_2Cr_2O_7 + 4H_2SO_4 -> K_2SO_4 + Cr_2(SO_4)_3 + 8H_2O + 3S`
green colour

Sulphur di-oxide (`SO_2`) :

See fig.

`ast` `pi` bond for nuclei `d` `pi` - `p` `pi` bond.

`ast` V-shape or trigonal planar.

(ii) `text(Bleaching Property :)` `SO_2` acts as a bleaching agent in following two ways.

(a) In the presence of `H_2O`, it is oxidised with the liberation of nascent hydrogen which reduces the colouring matter to colourless.

`SO_2 + 2H_2O -> H_2SO_4 + 2[H]`

Colouring matter `+ 2[H] ⇋` Colourless compound.

(b) `undersettext(Coloured)(X - OH) + SO_2 -> undersettext(Colourless)(X - HSO_3)`

(iii) `text(Preparations :)`

(a) `text(Lab Method :)` By heating copper turnings with conc. `H_2SO_4`.

`Cu + 2H_2SO_4 text((conc.)) -> CuSO_4 + SO_2 + 2H_2O`

(b) By roasting `ZnS` or iron pyrites.

`2ZnS + 3O_2 -> 2ZnO + 2SO_2`

`4FeS_2 + HO_2 -> 2FeO_3 + 8SO_2`.

(c) Reaction of anhydride with coke :

`2CaSO_4 + C-> 2CaO +CO_2 + 2SO_3`.

(iv) `text(Physical Properties :)`

(a) It is pungent smelling suffocating gas

(b) It is soluble in `H_2O`

Sulphur Trioxide (`SO_3`) :

See fig.1.

(i) `text(Preparation :)`

(a) Action of heat on Ferric sulphate

`Fe_2(SO_4)_3 -> Fe_2O_3 + 3SO_3`

(b) Commercial Method : By the oxidation of dry `SO_2` with dry oxygen.

`2SO_2 + O_2 -> 2SO_3`.

`SO_2` and `O_2` are first dried by passing through conc. `H_2SO_4` and then passed over heated (`400^oC`) platinsed asbestos contained in a hard glass tube.

(ii) `text(Physical Properties :)` It exists in atleast three forms a `alpha-SO_3, beta- SO_3` and `gamma- SO_3`. The structure of solid `SO_3` is complex. It possesses cyclic trimer (`alpha`-form) structure.

(a) `alpha` `text(form :)` It is branched and sheet like structure.

(b) `beta` `text(and)` `gamma` `text(forms :)` It possesses an infinite helical chain made up of linked `SO_4` tetrahedron. See fig.2.

(iii) `text(Chemical Reactions :)`

(a) It dissolves in conc. `H_2SO_4` to form pyrosulphuric acid (oleum)

`SO_2 + H_2SO_4 -> H_2S_2O_7`

(b) `SO_3 + 2HBr -> H_2O + Br_2 + SO_2`. In this `SO_3` is used as an oxidising agent.