Understanding Oxidation and Reduction: Modern Concept and Redox Reactions, Schemes and Mind Maps of Chemistry

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Redox Reaction

Chemical reactions in which reduction and oxidation takes place are known as redox reactions. In order to define oxidation and reduction, there were many concepts which are as following:

Classical concept

OXIDATION REDUCTION

Addition of oxygen or Removal of hydrogen

Addition of hydrogen or Removal of oxygen

Oxidation of Mg Reduction of Cl^2

e.g.

Oxidation of HCl Reduction of H 2 O, Oxidation of H 2 O

Note: Above definitions failed to define oxidation and reduction in all reactions. Hence, its concept was modified later as following:

OXIDATION REDUCTION

Addition of electronegative element Or Removal of electropositive element

Addition of electropositive element or Removal of electronegative element

Introduction

Oxidation of Zn and reduction of Cu2+.

Oxidation of H 2 and reduction of Ag+.

Note: Modern concept can be used to identify oxidation and reduction only in ionic reactions.

Redox Reaction 4.

Oxidation Number The average charge per atom possessed by an element in a specie is called “oxidation number” of that element in that species (atom, molecule, ion). In a specie, the charge is developed on atoms due to diɬerence in electronegativity between bonded atoms or due to transfer of electrons.

e.g. H 2

  ^ 

  Oxidation number of hydrogen in H 2 = zero

e.g. i) HCl       (^)    (^)     

More electronegative

ii) HCN

   

    

    Order of electronegativity H < C < N

Note: Oxidation number may be positive, negative, integer, fractional or even zero.

Redox Reaction 6.

e.g.

i)

 

O = C = O and

Oxidation number of carbon in CO 2 is +4 and in CH 4 is −4.

ii)

 

Oxidation number of hydrogen in H 2 , HCl and NaOH is 0, +1 and +1 respectively.

7. Redox Reaction

Determination of Oxidation Number

For a specie, the sum of oxidation number of each atom present is equal to the charge possessed by that specie. Let AXByCz x(p) + y(q) + z(r) = 0 (AxByCz) x(p) + y(q) + z(r) =

e.g. i) H 2 O 2(+1) + 1 ( 2) = 0 ii) CF 4 1(+4) + 4( 1) = 0 iii) CO 2 1(+4) + 2 ( 2) = 0

Oxidation Number of Some Common Elements For elements present in their free state (Natural form at room temperature and pressure), the oxidation number is zero.

e.g.

  (^)    

All IA group elements show (+1) oxidation number in their compounds.

i.e.

    ^    

All IIA group elements show (+2) oxidation numbers in their compounds

i.e.

    ^    

9. Redox Reaction

Where, ‘n’ is number of valence electrons.

For halogens (except F)

  

   

 ^ ^ ^ ^  

Oxidation number of Chlorine

Case I Chlorine in chlorides  1(x) = 1 x = 1

Case II Chlorine molecule

2(x) = 0 x = 0

Case III Chlorine with more electronegative atom

ClO- 1(x) + 1(–2) = – x = +

For chalcogens except oxygen Oxidation number range = (–2) to (+6)

e.g.

For Nitrogen family Oxidation number =(-3) to (+5)

e.g.

 ^ ^ ^ ^ ^ 

d block elements show multiple but fixed oxidation number in their compounds. e.g. Fe = (+2), (+3) Ti = (+2), (+4) Cu = (+2), (+1) Mn = (+2), (+4), (+5), (+6), (+7) Cr = (+6), (+3) Zn = (+2) Ag = (+1) etc.

Redox Reaction 10.

Note: Maximum possible oxidation number of an element in species is  which is for Osmium and Xenon in their particular compounds.

e.g. CrO 5 Oxidation of Cr 1(x) + 5( 2) = 0 x= (+10) Not possible i.e.

Note: In calculations , if O.N of p block elements comes beyond the range then it is incorrect.

e.g. +1 x 2 H 2 SO 5

Redox Reaction 12.

x = 1 O.N. of S in FeS 2 is ( 1). viii) PbS      Oxidation number of Pb in PbS is +2. ix) CS 2        Oxidation number of C in CS 2 is +4. x) CrO 5 x+( 2) + 4 ( 1) = 0 x 2 4 = 0 O.N. of Cr in CrO 5 is +

Note: 4 oxygen atoms in peroxide linkages.

xi) (N 2 H 5 ) 2 SO 4 2(2x + 5(1)) + ( 2) = 0 4x + 8 = 0 4x = 8 x = -8/4 = 2 Oxidation number of N in (N 2 H 5 ) 2 SO 4 is ( 2). xii) N 2 O 5 2(x) + 5( 2) = 0 2x 10 = 0 2x = 10 x = 10/ x = + Oxidation number of N in N 2 O 5 is + 5. xiii) HCN Oxidation number of C: (1) + x + ( 3) = 0   Oxidation number of N: 1 + (+2) + x = 0  

13. Redox Reaction

Oxidation number of C in HCN is (+2). Oxidation number of N in HCN is ( 3). xiv) Ba[H 2 PO 2 ] 2 Oxidation number of Ba is + Oxidation number of P is: (+2)+ 2 [2(+1) + x+2( 2)] = 0 2 + 2 (2+x 4) = 0 2 + 4 + 2x 8 = 0 2x 2 = 0 2x = 2   O.N of P in Ba[H 2 PO 2 ] 2 is (+1) xv) OsO 4 x+4( 2) = 0 x = 8 Oxidation number of Os in OsO 4 is (+8). xvi) H 2 S 2 O 3 2(+1) + 2(x) + 3( 2) = 0 2 + 2x 6 = 0 2x 4 = 0 2x = 4 x = 2 Oxiation number of S in H 2 S 2 O 3 = (+2) xvii) CH 3 SO 3 H Oxidation number of C is ( 2) Oxidation number of S is (+4) xix) Ba 2 XeO 6 Oxidation number of Xe is +8. 2(+2) + x + 6( 2) = 0 x = + xx) Ba(SCN) 2 Oxidation number of S is ( 2). Oxidation number of C is (+4). Oxidation number of N is( 3).

15. Redox Reaction

Q1 The correct order of the increasing oxidation states of nitrogen in NO, N 2 O, NO 2 and N 2 O 3 is: (A) N 2 O < NO < N 2 O 3 < NO 2 (B) N 2 O < N 2 O 3 < NO < NO 2 (C) NO 2 < NO < N 2 O 3 < N 2 O (D) NO 2 < N 2 O 3 < NO < N 2 O

A1 A Oxidation states of N in NO, N 2 O, NO 2 and N 2 O 3 are:

Q2 Oxidation number of oxygen in K 2 O, K 2 O 2 and KO 2 , respectively, is:

(A) +1, +2 and +4 (B) +1, +4 and +

(C) –2, –1 and (D) −2, −1 and

A2 (C) K 2 O Oxide K 2 O 2 Peroxide KO 2 Superoxide

Redox Reaction 16.

Oxidation: The process in which oxidation number of element increases is known as oxidation. e.g.  (^)     

 (^)    

Reduction: The process in which oxidation number of element decreases is known as reduction.

 (^) ^       (^)      

Note: Alone oxidation or alone reduction never takes place means both process are carried out simultaneously and hence the reaction is collectively known as Redox reaction. e.g.

   

(i)

Mg is oxidized. O 2 is reduced.

   

(ii)

H 2 is oxidized. Cl 2 is reduced.

   

(iii)

O is oxidised. H is reduced.