SULPHATES

Action of heat on sulphates

1. Hydrated copper (II) sulphate crystals.

On gentle heating, a lourless liquid which turns anhydrous copper (II) sulphate blue is given off (water of crystallization).The blue crystals change to a dirty white solid (anhydrous copper (II) sulphate).

Gentle heating

CuSO₄.  5H₂O _[S]                                 CuSO_4[S] + 5H₂O _[L]

blue crystals                                                    dirty white solid

[hydrated]                                                                   [anhydrous]

On strong heating, a dirty white solid decompose to give white fumes (of sulphur dioxide) and block residue (of copper (II) oxide).

CuSO_{4 [s]}                                 Cu O_[s] + SO_{3 [g]}

Black               white fumes

Residue

Qn:

Copper (II) sulphate crystals were heated gently then strongly until no further change occurred. Stale what was observed and write equations for the reaction.

Iron (II) sulphate water crystals

On the gentle heating a colourlless liquid was given off which turned anhydrous copper (II) sulphate blue. The pale green crystals changed to a dirty white solid (anhydrous Iron (II) sulphate.)

Fe SO₄. 7H₂O                         FeSO₄ + 7H₂O

Pale green crystal                    dirty white solid

[hydrated]                               [anhydrous]

On strong heating, the dirty white solid turned grey, and finally a reddish brown solid was formed as a residue (iron (III) oxide) white fumes were given off (sulphur trioxide) and a colourless gas which turned blue litmus pink and bleached it (sulphur dioxide)

Fe SO_4[s]                                  Fe₂O_{3 [s]} + SO_{3 [g] +} SO_{2 [g]}

Solubility of the sulphates in water

All common sulphates are souluble except lead (II) sulphate and barium sulphate,

• Calcium sulpahte is only souluble in water.

 Testing for the sulphate ion SO42-

To test the solution containing the sulpahte ion, add barium nitrate solution followed by dilute niric acid.

Formation of white precipitat insoluble in the acid conforms presence of the sulphate ion.

Ba ²⁺ _[aq] + SO₄^2-                                  BSO_4[s]

White precipitate

• Dilute niric acid is added to eliminate possibility of the carbonate ion or sulphate ion, both of which white precipitate, soluble in acid with evolution of a gas (the carbonate ion gives off carbondioxide while the barium sulphite ions gives sulphurdioxide gas).

Barium chloride solution can also be used followed by dilute hydrochloric cid.

Test (II)

Lead (II) nitrate solution can be used followed by diute nitric acid

• Formation of the white precipitate insoluble in acid confirms the presence of the sulphate ion.
• Dilute nitric acid is added to eliminate the possibility of carbon ion or sulphate ion.

Hydrogen sulphide, H₂S

Laboratory preparation

The gas is formed / prepared by reacting concentrated hydrochloric acid with an etal sulphide (e.g iorn (II) sulphide).

Fe S_[s]   + 2HCI                            H₂SO _[g] + Fe CL_{[s] [aq]}

The gas is collected over warm water. (Hydrogen sulphide is fairly soluble in cold water). If required dry, it can be dried using fused calcium chloride and is collected by down ward delivery.

Note:

1. Hydrogen sulphide should be prepared in the fume cupboard or in open air because it is highly poisonous.
2. Iron (II) sulphide contains irom metal as an impurity, iron containing iron (II) reacts with the acid to form hydrogen gas

Fe _[s] + 2HCI aq                                         H_{2 [g]} + Fe CL_[aq]

Therefore the hydrogen sulphide prepared contains hydrogen as an impurity

Test for hydrogen sulphide

• It turns filter paper wetted with lead (II) nitrate solution or lead (II) ethanote solution brown / black. The colour change is due to formation of insoluble lead (II) sulphide

E.g

P b [No₃]₂ + H₂S_[g]                              PbS_[s] + 2HNO_{3 [aq]}

Ionic equation

P b²⁺ _[aq] + S^2-                                        PbS_[s]

Properties of hydrogen sulphide

 Physical properties

1. Colourless gas
2. Has a repulsive smell to that of rotten eggs. Rotten eggs and decaying cabbages contain hydrogen sulphide.

• Fairly soluble in cold water forming an acid solution (which urns blue litmus pink.)
• Denser than air hence collected by down ward delivery

Chemical properties

1. As a reducing agent – it reduces oxidizing agents and it is itself oxidized to sulphur which forms as a yellow precipitate.

Reaction with sulphurdioxide

In the presence of water (moisture) hydrogen sulphide reduces dioxide to sulphur.

2H₂S_[g] + SO_{2 [g]}                                  3S + 2H₂O

Yellow

ppt

Hydrogen sulphide is a stronger reducing agent compared to sulphurdioxide.

2F³⁺e + S^2-                                           2Fe³⁺ S

Yellow                                                            pale green

Brown

Observation

• Yellow or brown solution turns pale green
• Yellow precipitate of sulphur

Hydrogen sulphide reduces concentrated nitric acid to reddish brown nitrogen dioxide gas. The mixture becomes hot (exothermic).

            H₂S_[g] + 2HNO₃                                  2NO₂ + 2H₂O _[L] + S_[s]

            R.A                 O.A

 Combustion of H₂S

1. Limited air (limited oxygen)

H2S burns in limited air with a quiet blue flame forming water vapor and sulphur.

2H₂S_[g] + O₂                            2H₂O_[l] + 2S_[s]

In excess air (excess oxygen)

The yellow deposit is not formed. Products are water vapour and sulphur dioxide gas

2 H₂S_[g] +  O_{2 [g]}                                 H₂O_[g] + SO_{2 [g]}

2H₂S_[g] + 3O_{2 [g]}                                   2H₂O_[g] + 2SO_{2 [g]}

1. Reaction with solution of metals salts

It reacts with some solutions of metals slats to form insoluble metal sulphide e.g. with a solution containing copper (II) ions, black precipitate of copper (II) ions were formed.

Cu²⁺ _[aq] + S^2- _[aq]                                               CuS_[s]

With solution contaiing lead (II) ions – a black or brown precipitate of lead (II) sulphide is formed.

Pb²⁺ _[aq] + S^2- _aq]                                                PbS_[s]

Black /brown

 Hydrogen sulphide as a pollutant

One of the major sources is sewage and marshe.

The gas is very poisonous and has a epulsive smell of rotten eggs.

Lead based paints react with hydrogen sulphide forming lead (II) sulphide which is black or brown this explains why lead based paints change colour.