Auto Redox Reactions

Types of Chemical Reactions According to Loss or Gain of Electrons

Oxidation

1.   Oxidation as an addition of oxygen

A chemical reaction during which oxygen (O₂) is added to some substance is known as Oxidation e.g.

           

2Cu

+

O2

¾¾¾®

2CuO

2.   Oxidation as a removal of Hydrogen

A chemical reaction which involves removal of hydrogen from a compound is called oxidation. In organic chemistry removal of hydrogen is called dehydrogenation. 

     

4HCl

+

MnO2

¾¾¾®

2MnCl2

+

Cl2

+

2H2O

3.   Oxidation as a loss of electron

According to modern concept, the process in which electrons are lost by an atom or group of atoms or ion is called Oxidation. e.g.

           

Fe	–	2e¯	¾¾¾®	Fe2+
Fe2+	–	1e¯	¾¾¾®	Fe3+

4.   Oxidation as an increase in oxidation number

In terms of oxidation number, the process in which the oxidation number of an element is increased is called Oxidation. In this reaction, carbon has been oxidized because its oxidation number has been increased from 0 to +4. e.g.

           

C

+

O2

¾¾¾®

C+4O2–4

Reduction

1.   Reduction as an addition of hydrogen 

It is a chemical reaction during which hydrogen is added to some substance 

           

Cl2	+	H2	¾¾¾®	2HCl
C2H4	+	H2	¾¾¾®	C2H6

2.   Reduction as a removal of Oxygen (or electronegative atom)

It is a chemical reaction which involves removal of oxygen (electronegative atom) from oxides or other compounds. e.g.

        

ZnO	+	C	¾¾¾®	Zn	+	CO
CuO	+	H2	¾¾¾®	Cu	+	H2O

3.   Reduction as a gain of electron

According to modern concept, the process in which electrons are gained by an atom or group of atoms or ion is called Reduction. e.g.

           

Cl	+	1e–	¾¾¾®	Cl–
Fe3+	+	1e–	¾¾¾®	Fe2+

4.   Reduction as a decrease in oxidation number

In terms of oxidation number, the process in which the oxidation no. of an element is decreased is called Reduction. e.g. In this reaction, reduction of Br₂ occurs because its oxidation number has been decreased from 0 to –1.

H°2

+

Br°2

¾¾¾®

2H+1Br–  1

Oxidizing Agent or Oxidant

1)   Oxidizing agent as Donor of Oxygen

Oxidizing agent is a substance (element or compound) that releases nascent or atomic oxygen either on decomposition or on treatment with other substance.

For example; in following reaction, CuO being a donor of oxygen is acting as oxidizing agent supplying oxygen to H₂ and thus itself reduces to Cu while H₂ being an acceptor of oxygen is acting as reducing agent adding oxygen and thus itself oxidizes to H₂O.

CuO	+	H2	¾¾¾®	Cu	+	H2O
Oxidant		Reductant

Some other examples of oxidant are illustrated by their decomposition or their reactions with other reagents.

O3	¾¾¾®	O2	+	[O]
O2	¾¾¾®	2[O]
HOCl	¾¾¾®	HCl	+	[O]
H2O2	¾¾¾®	H2O	+	[O]
H2SO4	¾¾¾®	SO2	+	[O]	+	H2O
H2SO4	¾¾¾®	H2S	+	4[O]
2HNO3	¾¾¾®	N2O	+	4[O]	+	H2O
2HNO3	¾¾¾®	2NO	+	3[O]	+	H2O
2HNO3	¾¾¾®	2NO2	+	[O]	+	H2O

                                                    

2KMnO4	+	3H2SO4		¾¾¾®K2SO4	+	2MnSO4	+	5[O]	+	H2O
2KMnO4	+	H2O	¾¾¾®	2KOH	+	2MnO2	+	5[O]
K2Cr2O7	+	4H2SO4	¾¾¾®	K2SO4	+	Cr2(SO4)3	+	3[O]	+	4H2O
K2Cr2O7	+	4H2O	¾¾¾®	2KOH	+	2Cr(OH)3	+	5[O]

2)   Oxidizing Agent as Acceptor of Hydrogen

Oxidizing Agent is a substance that removes or accepts hydrogen from a substance.

H2S	+	Cl2	¾¾¾®	2HCl	+	S
H2	+	Cl2	¾¾¾®	2HCl
8[H]	+	2HNO3	¾¾¾®	N2O	+	5H2O
6[H]	+	2HNO3	¾¾¾®	2NO	+	4H2O
2[H]	+	2HNO3	¾¾¾®	2NO2	+	2H2O
2[H]	+	H2SO4	¾¾¾®	SO2	+	2H2O
8[H]	+	H2SO4	¾¾¾®	H2S	+	4H2O

3)   Oxidizing Agent as Electron Acceptor

A substance that accepts or gains one or more electrons is called an oxidizing agent or oxidant. Thus it is a substance that oxidizes the other substance (by removing electrons from it) while itself gets reduced (by accepting electrons from the other substance). Stated differently, oxidizing agent undergoes decrease in oxidation number. e.g. zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas. In this redox reaction, H₂SO₄ acts as an oxidizing agent accepting electrons (undergoing increase in oxidation number) form zinc and thus reduces to H₂ gas while zinc acts as reducing agent by donating electrons (undergoing decreases in oxidation number) and thus oxidizes to Zn²⁺ ions.[ SO₄^2– ions being spectator ions, do not appear in net equation].

                       

	Zn(s)			¾¾¾®	Zn2+	+	2e–	(Oxidation; Reducing agent)
	2H+(aq)	+	2e–	¾¾¾®	H2(g)			(Reduction; Oxidizing agent)
	Zn(s)	+	2H+(aq)	¾¾¾®	Zn2+(aq)	+	H2(g)	(Net redox reaction)

4)   Identification Test for Oxidizing Agent

The resulting colour change of colourless to brown by the addition of KI (a reductant) to an oxidizing agent is a test for oxidizing agent.

5)   Examples of Oxidizing Agent

Following are the examples of Oxidant:

1	All reactive non-metals	e.g	O2, O3, F2, Cl2, Br2
2	All oxyacids(except oxalic acid)	e.g	H2SO4, HNO3, HOCl
3	All oxysalts of transition metals	e.g	KMnO4, K2Cr2O7,
4	Organic mild oxidizing agent	e.g	Tollen’s reagent, Fehling’s solution, Benedict’s reagent
5	Miscellaneous oxidizing agent	e.g	H2O2 ,SO2

Reducing Agent or Reductant

1)   Reducing Agent as Acceptor of Oxygen

Reducing agent is a substance (element or compound) that accepts nascent/atomic oxygen (released by oxidizing agent). e.g. in following reaction, carbon being acceptor of oxygen is acting as reducing agent receiving oxygen from zinc oxide and thus itself oxidizes to CO while zinc oxide being donor of oxygen is behaving as oxidizing agent supplying oxygen and thus itself reduces to zinc.

           

ZnO	+	C	¾¾¾®	CO	+	Zn
Oxidant		Reductant

Some other examples of reducing agent are:

SiO2	+	4Na	¾¾¾®	2Na2O	+	Si
Fe2O3	+	2Al	¾¾¾®	Al2O3	+	2Fe
CO2	+	2Mg	¾¾¾®	2MgO	+	C
CuO	+	H2	¾¾¾®	H2O	+	Cu

2    Reducing Agent as Donor of Hydrogen

Reducing agent is a substance (element or compound) that releases nascent or atomic hydrogen either on decomposition or on treatment with other substance.

H2S			¾¾¾®	S	+	2[H]
H2O2			¾¾¾®	O2	+	2[H]
2HNO3	+	Cu	¾¾¾®	Cu(NO3)2	+	2[H]
H2SO4	+	Zn	¾¾¾®	ZnSO4	+	2[H]

3)   Reducing Agent as Electron Donor

A substance that donates or loses one or more electrons is called a reducing agent or reductant. Thus it is a substance that reduces the other substance (by supplying electrons to it) while itself gets oxidized (by losing electrons). Stated differently, reducing agent undergoes increase in oxidation number. e.g. zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas. In this redox reaction, H₂SO₄ acts as an oxidizing agent accepting electrons (undergoing increase in oxidation number) form zinc and thus reduces to H₂ gas while zinc acts as reducing agent by donating electrons (undergoing decreases in oxidation number) and thus oxidizes to Zn²⁺ ions.[ SO₄^2– ions being spectator ions, do not appear in net equation].

           

Zn(s)			¾¾¾®	Zn2+	+	2e–	(Oxidation; Reducing agent)
2H+(aq)	+	2e–	¾¾¾®	H2(g)			(Reduction; Oxidizing agent)
Zn(s)	+	2H+(aq)	¾¾¾®	Zn2+(aq)	+	H2(g)	(Net redox reaction)

4)   Identification Test for Reducing Agent

 The resulting colour change of colourless to brown by the addition of KI (a reductant) to an oxidizing agent is a test for oxidizing agent.

5)   Examples of Reducing Agent

Following are the examples of Reductant:

1	Non-reactive non-metals	e.g	C, H2
2	All metals	e.g	Li, K, Na, Al, Mg, Zn etc
3	Few acids (binary acids)	e.g	HCl, HBr, HI, H2S, H2C2O4 etc
4	Ionic Hydrides	e.g	NaH, CaH2 etc
5	Complex Hydrides	e.g	LiAlH4, NaBH4,
6	Miscellaneous	e.g	SO2, H2O2, CO, FeSO4, Mohr’s salt, etc
7.	Organic reducing agent	e.g	Formaldehyde, glucose etc

Types of Chemical Reactions According to Electron Transfer

There are two types of chemical reactions based on oxidation and reduction:

1.         Non- Redox Reaction

2.         Redox Reaction or Oxidation-Reduction Reaction (ORR)

2.         Auto Redox Reaction or Self Oxidation-Reduction Reaction (ARR)

Non- Redox Reaction

Definition

The chemical reaction in which there is no electron transfer i.e. no substance is oxidized or reduced not undergoing change in oxidation number is called non-redox reaction.

Examples

1.         Precipitation reactions

2.         Hydrolysis

3.         Neutralization

4.         Some molecular addition reactions

5.         Acid displacement reactions

6.         Some decomposition reactions

7.         All double decomposition reactions

Redox Reaction or Oxidation-Reduction Reaction or Reduction-Oxidation Reaction

Definition of Oxidation-Reduction (Redox) Reaction (ORR)

Oxidation and reduction always occur simultaneously during a chemical reaction. The chemical reactions in which oxidation and reduction occur simultaneously are called oxidation-reduction reactions (ORR) or redox reactions. In terms of electron transfer, a redox reaction is defined as the process in which electrons are transferred from one substance (reducing agent) to another (oxidizing agent).

Oxidation-reduction reactions involve two opposing but complementary processes. These processes can never occur singly .i.e. every oxidation must necessarily be accompanied by its opposing process reduction and vice versa.

For Example

The formation of hydrogen bromide by the addition reaction between H₂ gas and Br₂ is an example of redox reaction. In this reaction H₂ has been oxidized because its oxidation number has been increased so H₂ is a reducing agent while Br₂ has been reduced because its oxidation number has been decreased so Br₂ is an oxidizing agent. Thus it is a Redox Reaction during which oxidation and reduction takes place simultaneously.

General Examples of Redox Reactions

Following are the examples of Redox reactions:

1	All combustion reactions
2	All addition reactions	(except some molecular addition reactions)
3	All decomposition reactions
4	All displacement reactions
5	All electrolysis reactions	(electrolytic decomposition
6	Metal-water reactions
7	Metal-Base reactions
8	Metal-acid reactions
9	Non-metal-acids reactions
10	Oxides-Hydrogen reactions
11	Oxides-carbon reactions
12	Oxides-aluminium reactions
13	Oxides-sodium reactions
14	Oxides-magnesium reactions
15	Photosynthesis
16	Water gas formation	(from coal or natural gas)
17	Halogen acid-MnO2 reactions
18	Corrosion of metals	(e.g. rusting of iron)
19	KMnO4-FeSO4 reactions
20	KMnO4-Mohr’s salt reactions
21	KMnO4-oxalic acids reactions
22	KMnO4-chlorides reactions
23	KMnO4-Iodides reactions
24	KMnO4-H2O2 reactions
25	KMnO4-nitrites reactions
26	KMnO4-sulphites reactions
27	K2Cr2O7-reductant reactions