OXIDATION NUMBER at a glance

 Oxidation number (O.N) of an element is the charge on an atom of the element whether it is by itself or bonded to another atom. It indicates the number of electrons the atom has gained or lose at that moment. That is, it is the charge an element will have if electrons were transferred to or from it. It is usually zero (0) for an element in the uncombined state. It is also referred to as the oxidation state of the element

          NOTE: - The sign or charge for O.N is written before the number (–2) but it is written after the number for an ionic charge i.e O^2–

Rules for calculating oxidation number

The following rules are applied when assigning an oxidation number or calculating the oxidation number of an element thus

1.       The O.N of oxygen is always equal to -2 except in peroxides (–1)

2.   O.N of hydrogen is always equal to plus one (+1) except again in metallic hydrides (–1).

3.        The O.N for an element in the elemental (or ground) state O.N = O (zero) e.g.

          Na = O, Cl₂ = O,  O₂ = O etc.

4.   For an for a simple ion is equal to the charge on it for example 

   Na+ = +1,   Cl^– = –1,     O^2–  =  -2

5. Oxidation number of a radical is equal to the charge on it e.g  CO₃^2- = –2,  NO₃^- =-1,  SO₄^2- =-2

3   The  O.N of a compound is equal O (because it is the sum of the e.g.

   H₂O = O, 

 i.e (O.N of H × no of H atoms) + (O.N of O) =

      (+1 x 2) + ( -2 x1) 

          2 -2 = 0

NaOH = (+1 x1) + (-2 x 1) + (1 x 1) 

                         +1 - 2+ 1

                           +2-2=0

      

Rules for specific groups in the PT.

I        For group 1A elements (comprising Li,Na,K, e.t.c) their O.N = +1

II       For group 2A elements (comprising Be, Mg, Ca e.t.c) their O.N = +2 in all compounds

III     For group 3 elements (B, Al, e.t.c) their O.N = +3 especially in their binary compounds.

IV     For group 5 = -3

V     For group 6 = -2 except Oxygen (O) in peroxides).

 VI   For group 7 = –1 respectively especially in their binary compounds 

 

Determination of the oxidation number of an element

1   Find the O.N of the underlined elements in the following

  a). ZnCl₂        b). SO₃      c). NO₃^-         d). Ca²⁺

          Solution

  To determine the O.N of the underlined elements, we must follow the general rules for calculating O.N of an element.

   a). ZnCl₂: The O.N of a compound is zero, i.e. ZnCl₂  = O. 

Since Cl is a group 7(A) element and ZnCl₂ is a binary compound then the O.N of Cl is –1, therefore, the O.N of Zn is

    (O.N of Zn) + (O.N of Cl ´ 2) = 0
                   x + (–1 ´ 2) = 0
                   x – 2 = 0
                   x = +2

 

          b)      SO₃

                    Solution

(O.N of S) + (O.N of 0 ´ 3) = 0
            x + (–2 ´ 3) = 0
             x – 6 = 0
           x = +6

      Trioxosulphate(IV) ion

   c)        NO₃^-

                   Solution

        The O.N of a radical is equal to the charge on it, hence

                    NO₃^- = –1
                   that is,
     

 (O.N of N) + (O.N of 0x3) = –1

                   x + (–2x3) = –1
                   x – 6 = –1
                   x = +6 – 1
                    = +5

   Trioxonitrate (V) ion

    d) Ca²⁺  The O.N of an ion is the charge on it, i.e., Ca²⁺ = +2  Calcium  = ion

 Uses of oxidation number

Oxidation number is used for the

1.   It is used in the IUPAC (International Union of Pure and Applied chemistry) system of naming compounds e.g. H₂SO₄: Tetraoxosulphate(VI) acid

2. It is used to know the oxidation state or number of an element in a compound 

 

OBJECTIVE QUSETION

1. The oxidation number of an element in its free (uncombined) state is
   A. +1
   B. 0
   C. –1
   D. variable

2. The oxidation number of hydrogen in most of its compounds is
   A. –1
   B. 0
   C. +1
   D. +2

3. Hydrogen has an oxidation number of –1 in
   A. HCl
   B. H₂O
   C. NaH
   D. NH₃

4. The oxidation number of oxygen in most compounds is
   A. –1
   B. –2
   C. 0
   D. +2

5. Oxygen has an oxidation number of –1 in
   A. CO₂
   B. H₂O
   C. H₂O₂
   D. Na₂O

6. The oxidation number of chlorine in NaCl is
   A. +1
   B. –1
   C. 0
   D. +7

7. The sum of oxidation numbers of all atoms in a neutral compound is always
   A. zero
   B. one
   C. two
   D. equal to the number of atoms

8. The sum of oxidation numbers of atoms in a polyatomic ion is equal to
   A. zero
   B. the charge on the ion
   C. the atomic number
   D. the mass number

9. What is the oxidation number of sulphur in H₂SO₄?
   A. +2
   B. +4
   C. +6
   D. –2

10. The oxidation number of nitrogen in NH₃ is
    A. –3
    B. –1
    C. +3
    D. +5

11. What is the oxidation number of manganese in KMnO₄?
    A. +2
    B. +4
    C. +6
    D. +7

12. The oxidation number of iron in Fe²⁺ is
    A. +1
    B. +2
    C. +3
    D. 0

13. In the compound CaCO₃, the oxidation number of carbon is
    A. +2
    B. +4
    C. –2
    D. –4

14. Which of the following elements always has an oxidation number of +1 in compounds?
    A. Calcium
    B. Sodium
    C. Magnesium
    D. Aluminium

15. The oxidation number of aluminium in Al₂O₃ is
    A. +1
    B. +2
    C. +3
    D. +6

16. What is the oxidation number of chromium in K₂Cr₂O₇?
    A. +3
    B. +4
    C. +6
    D. +7

17. An increase in oxidation number indicates
    A. reduction
    B. oxidation
    C. neutralization
    D. hydrolysis

18. A decrease in oxidation number indicates
    A. oxidation
    B. reduction
    C. combustion
    D. displacement

19. The oxidation number of chlorine in Cl₂ is
    A. –1
    B. 0
    C. +1
    D. +5

20. Which of the following statements about oxidation number is correct?
    A. It is always positive
    B. It represents the actual charge on an atom
    C. It is a theoretical charge assigned to an atom
    D. It is equal to the atomic number

     21.       Which species undergoes reduction in the reaction represented by the equation below?

  H₂S_(aq)+2FeCl_3(aq)→S_(s) + 2HCl +3FeCl₂

   (a) Fe³⁺    

   (b). H₂S            

   (c). Cl^–     

   (d)  S

22. Oxidation is a reaction which involves the following except 

   (a). Loss of electrons

   (b). Increase in oxidation number 

   (c). Gain of oxygen 

   (d). addition of hydrogen

23.  The O.N of the following underlined elements are Na₂SO₄               

          (a)  +4,  

          (b)  -2,  

          (c)  +6

          (d)  –5

  24.     Al (H₂O)₆]³⁺         

           (a) +3

           (b) –3

           (c) +6

           (d) –6

     25.     K₂Cr₂O₇             

           (a) +5

           (b) +4

           (c) –6

           (d) +6

      26.     Mn                                     

           (a) +6

           (b) +7

           (c) +5

           (d) +3

THEORY QUESTION

2. Find the oxidation numbers of the following underlined elements.

  (a) K₂Cr₂O₇             (b) KMnO₄         (c) HNO₃

  (d). S^2-                   (e). Cl^-              (f). Cr₂

2.a State two applications of oxidation numbers 

b.  What is the oxidation state of manganese in each of the following species?

 i.  MnO₂   ii MnO₄^-    iii.   MnCl₂