Redox Reactions Class 11 Notes Chemistry Chapter 7

 Page 1


  Oxidation  Reduction
1. Addition of oxygen 1. Removal of oxygen
2. Removal an Hydrogen 2. Addition of Hydrogen
3. Addition of an electronegative 3. Removal of an electronegative
element. element.
4. Removal of an electropositive 4. Addition of an electropositive
element element.
5. Loss of electron(s) 5. Gain of electron(s)
6. Increase in oxidation number. 6. Decrease in oxidation number.
? Reducing Agent : Donor of electrons.
? Oxidising Agent : Acceptor of electrons.
? Redox Reaction : Reactions in which oxidation and reduction takes place
simultaneously.
? Oxidation Number : It is charge that an atom appears to have in a given
species when the bonding electron are counted towards more electro-
negative atom.
? Calculation of Oxidation Number :
(a)  Oxidation number of all the elements in their elemental form (in
standard state) is taken as zero. Oxidation number of element in a molecule
Cl
2
, F
2
, O
2
, P
4
, O
3
, Fe, H
2
, N
2
, C (graphite) is zero.
(b) Common Oxidation number of elements of first group is +1. Common
Oxidation number of elements of second group + 2.
(c)   For ions composed of only one atom, the oxidation number is equal
to the charge on the ion.
Oxidation and Reduction :
Page 2


  Oxidation  Reduction
1. Addition of oxygen 1. Removal of oxygen
2. Removal an Hydrogen 2. Addition of Hydrogen
3. Addition of an electronegative 3. Removal of an electronegative
element. element.
4. Removal of an electropositive 4. Addition of an electropositive
element element.
5. Loss of electron(s) 5. Gain of electron(s)
6. Increase in oxidation number. 6. Decrease in oxidation number.
? Reducing Agent : Donor of electrons.
? Oxidising Agent : Acceptor of electrons.
? Redox Reaction : Reactions in which oxidation and reduction takes place
simultaneously.
? Oxidation Number : It is charge that an atom appears to have in a given
species when the bonding electron are counted towards more electro-
negative atom.
? Calculation of Oxidation Number :
(a)  Oxidation number of all the elements in their elemental form (in
standard state) is taken as zero. Oxidation number of element in a molecule
Cl
2
, F
2
, O
2
, P
4
, O
3
, Fe, H
2
, N
2
, C (graphite) is zero.
(b) Common Oxidation number of elements of first group is +1. Common
Oxidation number of elements of second group + 2.
(c)   For ions composed of only one atom, the oxidation number is equal
to the charge on the ion.
Oxidation and Reduction :
(d) The oxidation number of oxygen in most compounds is – 2. While in  
peroxides (e.g., H
2
O
2
, Na
2
O
2
), each oxygen atom is assigned an  
oxidation number of – 1, in super oxides (e.g., KO
2
, RbO
2
) each  
oxygen atom is assigned an oxidation number of – (½).
(e) In oxygen difluoride (OF
2
) and dioxygen difluoride (O
2
F
2
), the oxygen  
is assigned an oxidation number of + 2 and + 1, respectively.
(f) The oxidation number of hydrogen is + 1 but in metal hydride its  
oxidation no. is – 1.
(g) In all its compounds, fluorine has an oxidation number of – 1.
(h) The algebraic sum of the oxidation number of all the atoms in a  
compound must be zero.
(i) In polyatomic ion, the algebraic sum of all the oxidation numbers of 
atoms of the ion must equal the charge on the ion.
? Types of Redox Reactions:
(i) Combination Reaction :      0 0      + 2    – 3
3 Mg (s) + N
2
 (g) ? Mg
3
N
2
 (s) 
  (ii) Decomposition Reaction :  +1 +5 – 2           + 1 – 1 0
                2KClO
3
(s) ?    2 KC1(s)    +  3O
2
 (g) 
  (iii) Metal Displacement : + 2 + 6 – 2  0  + 2 + 6 – 2  0
   CuSO
4
 (aq) + Zn(s) ? ZnSO
4
 (aq) + Cu (s) 
  (iv) Non-metal displacement : 0      + 1 – 2          + 2 – 2 + 1  0
Ca (s) + 2 H
2
O (1) ? Ca (OH)
2
 + H
2
 (g)
  (v) Disproportionation reactions : It is a reaction in which same 
element is reduced and oxidized simultaneously.
0 –1             + 1
  C1
2
 (g) + 2 OH
–
 (aq) ? Cl
–
 (aq) + ClO
–
 (aq) + H
2
O (1)
? Stock Notation : Representing oxidation number of metal in Roman
numerals within parenthesis after the symbol or name of metal in the 
molecular formula or name of a compound. For e.g., Stock Notation of 
Ferric oxide is Fe
2
(III)O
3
 or Iron (III) oxide.
? Fractional Oxidation Number : When two or more atoms of an element
are present in different oxidation states, then calculated oxidation number 
may comes out as fractional due to average of all the different oxidation 
states.
?
?
Page 3


  Oxidation  Reduction
1. Addition of oxygen 1. Removal of oxygen
2. Removal an Hydrogen 2. Addition of Hydrogen
3. Addition of an electronegative 3. Removal of an electronegative
element. element.
4. Removal of an electropositive 4. Addition of an electropositive
element element.
5. Loss of electron(s) 5. Gain of electron(s)
6. Increase in oxidation number. 6. Decrease in oxidation number.
? Reducing Agent : Donor of electrons.
? Oxidising Agent : Acceptor of electrons.
? Redox Reaction : Reactions in which oxidation and reduction takes place
simultaneously.
? Oxidation Number : It is charge that an atom appears to have in a given
species when the bonding electron are counted towards more electro-
negative atom.
? Calculation of Oxidation Number :
(a)  Oxidation number of all the elements in their elemental form (in
standard state) is taken as zero. Oxidation number of element in a molecule
Cl
2
, F
2
, O
2
, P
4
, O
3
, Fe, H
2
, N
2
, C (graphite) is zero.
(b) Common Oxidation number of elements of first group is +1. Common
Oxidation number of elements of second group + 2.
(c)   For ions composed of only one atom, the oxidation number is equal
to the charge on the ion.
Oxidation and Reduction :
(d) The oxidation number of oxygen in most compounds is – 2. While in  
peroxides (e.g., H
2
O
2
, Na
2
O
2
), each oxygen atom is assigned an  
oxidation number of – 1, in super oxides (e.g., KO
2
, RbO
2
) each  
oxygen atom is assigned an oxidation number of – (½).
(e) In oxygen difluoride (OF
2
) and dioxygen difluoride (O
2
F
2
), the oxygen  
is assigned an oxidation number of + 2 and + 1, respectively.
(f) The oxidation number of hydrogen is + 1 but in metal hydride its  
oxidation no. is – 1.
(g) In all its compounds, fluorine has an oxidation number of – 1.
(h) The algebraic sum of the oxidation number of all the atoms in a  
compound must be zero.
(i) In polyatomic ion, the algebraic sum of all the oxidation numbers of 
atoms of the ion must equal the charge on the ion.
? Types of Redox Reactions:
(i) Combination Reaction :      0 0      + 2    – 3
3 Mg (s) + N
2
 (g) ? Mg
3
N
2
 (s) 
  (ii) Decomposition Reaction :  +1 +5 – 2           + 1 – 1 0
                2KClO
3
(s) ?    2 KC1(s)    +  3O
2
 (g) 
  (iii) Metal Displacement : + 2 + 6 – 2  0  + 2 + 6 – 2  0
   CuSO
4
 (aq) + Zn(s) ? ZnSO
4
 (aq) + Cu (s) 
  (iv) Non-metal displacement : 0      + 1 – 2          + 2 – 2 + 1  0
Ca (s) + 2 H
2
O (1) ? Ca (OH)
2
 + H
2
 (g)
  (v) Disproportionation reactions : It is a reaction in which same 
element is reduced and oxidized simultaneously.
0 –1             + 1
  C1
2
 (g) + 2 OH
–
 (aq) ? Cl
–
 (aq) + ClO
–
 (aq) + H
2
O (1)
? Stock Notation : Representing oxidation number of metal in Roman
numerals within parenthesis after the symbol or name of metal in the 
molecular formula or name of a compound. For e.g., Stock Notation of 
Ferric oxide is Fe
2
(III)O
3
 or Iron (III) oxide.
? Fractional Oxidation Number : When two or more atoms of an element
are present in different oxidation states, then calculated oxidation number 
may comes out as fractional due to average of all the different oxidation 
states.
?
?
In reality no element can have a fractional oxidation state.
? Balancing of Redox Reactions :
(A) Oxidation number method
(B) Half reaction method
? Electrode Potential (E) : Potential difference between electrode and
electrolytic solution due to charge separation.
? Standard Electrode Potential (E
?
) : Electrode Potential measured at 298
K and 1M concentration of metal ions (or 1 bar pressure of gas).
? Electrochemical Cell : A device in which chemical energy of a spontaneous 
redox reaction is converted into electrical energy.
Cell diagram: Zn | Zn
2+
 || Cu
2+ 
| cu
LHS oxidation, Zn ? Zn
2+
 + 2e
–
RHS reduction Cu
2+
 + 2e
– 
? Cu
Overall reaction    Zn(s) + Cu
2+
 (aq) ? Zn
2+
 (aq) + Cu(s)
? Representation of an Electrochemical cell :
———— Flow of electrons ——— ?
?———— Flow of current ———
Zn(s) | Zn
2+ 
(aq) || Cu
2+
(aq) | Cu(s)
Left Electrode Salt Bridge Right Electrode
  LOAN Oxidation Reduction
Anode Cathode
Negative Positive
?
Functions of Salt Bridge : (i) To complete inner circuit. (ii) To maintain
electrical neutrality around electrudes.
Page 4


  Oxidation  Reduction
1. Addition of oxygen 1. Removal of oxygen
2. Removal an Hydrogen 2. Addition of Hydrogen
3. Addition of an electronegative 3. Removal of an electronegative
element. element.
4. Removal of an electropositive 4. Addition of an electropositive
element element.
5. Loss of electron(s) 5. Gain of electron(s)
6. Increase in oxidation number. 6. Decrease in oxidation number.
? Reducing Agent : Donor of electrons.
? Oxidising Agent : Acceptor of electrons.
? Redox Reaction : Reactions in which oxidation and reduction takes place
simultaneously.
? Oxidation Number : It is charge that an atom appears to have in a given
species when the bonding electron are counted towards more electro-
negative atom.
? Calculation of Oxidation Number :
(a)  Oxidation number of all the elements in their elemental form (in
standard state) is taken as zero. Oxidation number of element in a molecule
Cl
2
, F
2
, O
2
, P
4
, O
3
, Fe, H
2
, N
2
, C (graphite) is zero.
(b) Common Oxidation number of elements of first group is +1. Common
Oxidation number of elements of second group + 2.
(c)   For ions composed of only one atom, the oxidation number is equal
to the charge on the ion.
Oxidation and Reduction :
(d) The oxidation number of oxygen in most compounds is – 2. While in  
peroxides (e.g., H
2
O
2
, Na
2
O
2
), each oxygen atom is assigned an  
oxidation number of – 1, in super oxides (e.g., KO
2
, RbO
2
) each  
oxygen atom is assigned an oxidation number of – (½).
(e) In oxygen difluoride (OF
2
) and dioxygen difluoride (O
2
F
2
), the oxygen  
is assigned an oxidation number of + 2 and + 1, respectively.
(f) The oxidation number of hydrogen is + 1 but in metal hydride its  
oxidation no. is – 1.
(g) In all its compounds, fluorine has an oxidation number of – 1.
(h) The algebraic sum of the oxidation number of all the atoms in a  
compound must be zero.
(i) In polyatomic ion, the algebraic sum of all the oxidation numbers of 
atoms of the ion must equal the charge on the ion.
? Types of Redox Reactions:
(i) Combination Reaction :      0 0      + 2    – 3
3 Mg (s) + N
2
 (g) ? Mg
3
N
2
 (s) 
  (ii) Decomposition Reaction :  +1 +5 – 2           + 1 – 1 0
                2KClO
3
(s) ?    2 KC1(s)    +  3O
2
 (g) 
  (iii) Metal Displacement : + 2 + 6 – 2  0  + 2 + 6 – 2  0
   CuSO
4
 (aq) + Zn(s) ? ZnSO
4
 (aq) + Cu (s) 
  (iv) Non-metal displacement : 0      + 1 – 2          + 2 – 2 + 1  0
Ca (s) + 2 H
2
O (1) ? Ca (OH)
2
 + H
2
 (g)
  (v) Disproportionation reactions : It is a reaction in which same 
element is reduced and oxidized simultaneously.
0 –1             + 1
  C1
2
 (g) + 2 OH
–
 (aq) ? Cl
–
 (aq) + ClO
–
 (aq) + H
2
O (1)
? Stock Notation : Representing oxidation number of metal in Roman
numerals within parenthesis after the symbol or name of metal in the 
molecular formula or name of a compound. For e.g., Stock Notation of 
Ferric oxide is Fe
2
(III)O
3
 or Iron (III) oxide.
? Fractional Oxidation Number : When two or more atoms of an element
are present in different oxidation states, then calculated oxidation number 
may comes out as fractional due to average of all the different oxidation 
states.
?
?
In reality no element can have a fractional oxidation state.
? Balancing of Redox Reactions :
(A) Oxidation number method
(B) Half reaction method
? Electrode Potential (E) : Potential difference between electrode and
electrolytic solution due to charge separation.
? Standard Electrode Potential (E
?
) : Electrode Potential measured at 298
K and 1M concentration of metal ions (or 1 bar pressure of gas).
? Electrochemical Cell : A device in which chemical energy of a spontaneous 
redox reaction is converted into electrical energy.
Cell diagram: Zn | Zn
2+
 || Cu
2+ 
| cu
LHS oxidation, Zn ? Zn
2+
 + 2e
–
RHS reduction Cu
2+
 + 2e
– 
? Cu
Overall reaction    Zn(s) + Cu
2+
 (aq) ? Zn
2+
 (aq) + Cu(s)
? Representation of an Electrochemical cell :
———— Flow of electrons ——— ?
?———— Flow of current ———
Zn(s) | Zn
2+ 
(aq) || Cu
2+
(aq) | Cu(s)
Left Electrode Salt Bridge Right Electrode
  LOAN Oxidation Reduction
Anode Cathode
Negative Positive
?
Functions of Salt Bridge : (i) To complete inner circuit. (ii) To maintain
electrical neutrality around electrudes.
Valency: V alency is the combining capacity of en element. V alency of an element 
cannot be zero.
Limitation of Concept of Oxidation Number
Acc. to the concept of oxidation number, oxidation means increase oxidation 
number, during, oxidation there is decrese in elecron density while increase in 
electron density around the atom under going reduction.