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Oxidation and Reduction-Oxidising and Reducing agents - definitions, examples, Practice problems

Have you ever wondered why packets of chips are packed air-tight?
Decomposition of fats or oils leads to undesirable flavour and odour formation by either oxidation or hydrolysis or both, which is termed rancidity. 
To prevent rancidity, N2 gas is used for food packaging. N2 gas is inert in nature (non-reactive) which helps to preserve the odour & taste of the food. As N2 is also colourless, odourless, and tasteless, So, it not going to change the colour and flavour of the food material.


Table of contents

Oxidation & Reduction

Oxidation 

Reduction

Addition of oxygen to an element or compound

S+O2SO2

Removal of oxygen from a compound

FeO+C⟶Fe+CO

Removal of hydrogen from any compound

2H2S+O2⟶2S +2H2O

Addition of hydrogen to an element or compound

CH2=CH2+H2CH3CH3

Increase in oxidation number of element

  • By the addition of a more electronegative element 

Mg+Cl2MgCl2

  • By removal of a more electropositive element

2K4[Fe(CN)6]+H2O2

2K3[Fe(CN)6]+2KOH

Decrease in oxidation number of element

  • By the removal of a more electronegative element 

2FeCl3+H2⟶2FeCl2+2HCl

  • By the addition of a more electropositive element

2HgCl2+SnCl2

Hg2Cl2+SnCl4

Removal of electron

Na⟶Na++e-

Mg⟶Mg2++2e-

Addition of electron

Fe3++e-⟶Fe2+

Oxidation and reduction always take place simultaneously. Overall reactions which involve simultaneous oxidation and reduction are called Redox reactions.


Oxidising & Reducing agent

Substances that are oxidized is called reducing agent and substances that are reduced is called oxidizing agent.


Oxidizing agent / Oxidant

A compound that can oxidize others and reduce itself during the chemical reaction or a reagent that gains electrons in a redox reaction are called an oxidant.

List of important oxidizing agents 

Potassium permanganate (KMnO4) Potassium Dichromate (K2Cr2O7), Nitric Acid (HNO3), Hydrogen Peroxide (H2O2), Sulphuric Acid (H2SO4), etc

E.g,- 2Fe2++ H2O2+2H+⟶2Fe3++ 2H2O

Iron’s oxidation state changes from +2 to +3 and oxygen’s oxidation state changes from -1 to -2

( H2O2 acts as O.A)

Reducing agents / Reductants

A compound that can reduce others and oxidize itself during the chemical reaction or a reagent that loses electrons in a redox reaction is called as a reductant.

List of important reducing agents 

All metals (Na,K,Zn etc)), few non-metals (H2, C etc). Normally Cs is the strongest reducing agent but in an aqueous medium, Li is the strongest reducing agent.

E.g- Na +H2O⟶NaOH+H2

Sodium’s oxidation state changes from 0 to +1 and hydrogen’s oxidation state changes from +1 to 0.

(Na acts as R.A)

Identifying the oxidants and reductants 

Case 1: If an element is in its maximum oxidation state in a compound, it can function only as 

an oxidizing agent. 

Ex. Potassium Dichromate (K2Cr2O7), Nitric Acid (HNO3), Sulphuric Acid (H2SO4), etc. 

Case 2: If an element is in its minimum possible oxidation state in a compound, it can 

function only as a reducing agent. 

Ex. Sodium Thiosulphate (Na2S2O3), Stannous Chloride (SnCl2), etc

Case 3: If an element is in its intermediate oxidation state in a compound it can function both as an oxidizing agent as well as a reducing agent.

 Ex. Hydrogen peroxide (H2O2), Nitrous Acid (HNO2) etc.

Practice problems

Q 1. When KMnO4 is converted into K2MnO4 the change in oxidation number of Mn is 

a. 0
b. 1
c. 2
d. 4

Answer: (B)

oxidation number of Mn in KMnO4 is +7.

oxidation number of Mn in K2MnO4 is +6.

So the difference in the oxidation state of Mn = (+7) - (+6)

= +1

Q 2. Select incorrect statement

a. All decomposition reactions are redox reactions
b. Oxidation is the addition of electronegative element 
c. Removal of the electron is oxidation
d. None of these

Answer: (A)

All decomposition reactions are not redox reactions

CaCO3CaO + CO2

Oxidation state of Ca in reactant and product is +2.

Oxidation state of C in reactant and product is +4.

Oxidation state of O in reactant and product is -2.

Q 3. Which can act as both oxidizing and reducing agent?

a. KMnO4
b. H2SO4
c. H2S
d. H2O2

Answer: (D)

Solution: If an element is in its intermediate oxidation state in a compound it can function both as an oxidizing agent as well as a reducing agent e.g. H2O2, HNO2 etc.

In case of H2O2 O.S of oxygen is -1, it can gain and lose an electron.

2Fe2++ H2O2+2H+⟶2Fe3++ 2H2O ( H2O2 as O.A)

I2+ H2O2+2OH-⟶2I-+ 2H2O+O2 ( H2O2 as R.A)

Q 4. In the reaction 2HgCl2+SnCl2⟶Hg2Cl2+SnCl4

a. HgCl2 is oxidized and SnCl2 is reduced
b. SnCl2 is oxidized and HgCl2 is reduced 
c. HgCl2 is both oxidized and reduced
d. HgCl2 is neither oxidized nor reduced. 

Answer: (B)

O.S of Hg in HgCl2 is +2

O.S of Hg in Hg2Cl2 is +1

 O.S of Hg in SnCl2 is +2

O.S of Hg in SnCl4 is +4. So, SnCl2 is oxidized and HgCl2 is reduced 

Frequently asked questions-FAQs

Q 1. What is the major difference between oxidation and reduction?
Answer:
Oxidation: loss of electrons and Reduction: gain of electrons.

Q 2. What is an example of slow oxidation?
Answer:
Iron rusting and wood-rotting are good examples of gradual oxidation.

Q 3. What is the oxidation state?
Answer:
A chemical compound’s degree of oxidation (loss of electrons) of an atom. A real or imaginary charge is present on an element when it goes from its elemental free state to a combined state.

Q 4. What is rancidity?
Answer:
Food turns rancid when the fats and oils present in food items get oxidized and the taste and odour of the food change unfavourable to consume.

Q 5. Which gas is used in packaging food items?
Answer:
N2 gas is used to prevent food material from getting oxidized. Nitrogen doesn't react with foods or affect the flavour or texture, so they stay fresher longer. The high bond energy of N2 prevents its decomposition and as a result, it stays chemically unreactive. 

Q 6. Why are oxidation and reductions important?
Answer:
Oxidation-reduction (redox) reactions are significant because they are the main natural or biological and artificial energy sources on this planet. Oxidation of molecules usually releases large amounts of energy by removing hydrogen and replacing it with oxygen.

Related topics:

Oxidation number

Balancing of Redox equation

Types of redox reactions

Equivalent Weight 

Fractional oxidation state

Electronegativity

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