Page 167 - Chemistry ICSE Class IX

Study of the First Element : Hydrogen 155
(iii) Removal of hydrogen: Hydrogen sulphide is oxidised by bromine
water to sulphur.
H S + Br o 2HBr + S
2
2
Here, hydrogen has been removed from hydrogen sulphide.
Hydrochloric acid is oxidised to chlorine by manganese dioxide on
heating.
'
4HCl + MnO o MnCl + Cl + 2H O
2
2
2
2
(iv) Removal of an electropositive radical: Potassium iodide is
oxidised by hydrogen peroxide to iodine.
2KI + H O o 2KOH + I 2
2
2
Potassium iodide has lost potassium ion, an electropositive radical.
Potassium ferrocyanide is oxidised by chlorine to potassium ferricyanide.
2K [Fe(CN) ] + Cl o 2K [Fe(CN) ] + 2KCl
2
4
6
3
6
Oxidising Agent
What is an oxidising agent
The substance which can bring about oxidation of other substances is called
an oxidising agent. Thus, an oxidising agent is the substance that provides
oxygen or removes hydrogen from any other substance. Some important
oxidising agents are oxygen, ozone, hydrogen peroxide, chlorine, bromine,
nitric acid, conc. sulphuric acid, potassium permanganate, potassium
dichromate, etc. A few illustrative examples are:
(i) S + O o SO 2 (O is oxidising agent)
2
2
(ii) Cl + H O o 2HCl + [O]
2
2
H S + [O] o H O + S
2
2
Cl + H S o 2HCl + S (Cl is oxidising agent)
2
2
2
(iii) 2HNO o H O + 2NO + [O]} × 3
2
2
3
S + 3[O] + H O o H SO 4
2
2
S + 6HNO o H SO + 2H O + 6NO 2 (HNO is oxidising agent)
3
3
2
2
4
(iv) 2KMnO + 3H SO o K SO + 2MnSO + 3H O + 5[O]
4
2
4
4
2
4
2
2FeSO + H SO + [O] o Fe (SO ) + H O]} × 5
2
4
4 3
4
2
2
2KMnO + 10FeSO + 8H SO o K SO + 2MnSO + 5Fe (SO ) + 8H O (KMnO is
4
2
4 3
2
4
2
2
4
4
4
4
oxidising agent)
Reduction
What is reduction
A process which involves, (i) the addition of hydrogen or any other
electropositive radical or (ii) the removal of oxygen or any other
electronegative radical is termed reduction. Thus, reduction is reverse of
oxidation. This is illustrated below.
ƒ Addition of hydrogen:
H S + Cl o 2HCl + S
2
2
Here, hydrogen has been added to chlorine which is said to have been
reduced.