Presented by:

TEENA KATHPAL Lecturer Pt. J R Govt Polytechnic College Hoshiarpur

OXIDATION AND REDUCTION

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OXIDATION AND REDUCTION

1. CLASSICAL CONCEPT2. ELECTRONIC CONCEPT

CLASSICAL CONCEPT• OXIDATION :- It is defined as the process

which involves addition of oxygen or removal of hydrogen.

e.g. S + O2 S02 (addition of oxygen)

C + O2 CO2 (addition of oxygen)

H2S+Br2 2HBr+ S (removal of hydrogen)

OXIDISING AGENT

A substance which supplies oxygen or removes hydrogen is called oxidising agent Or oxidiser or oxidant.

e.g. O2 and Br2 act as oxidising agents in the above reactions.

CLASSICAL CONCEPT

• REDUCTION :- It is defined as the process which involves addition of hydrogen or removal of oxygen.

e.g. H2 + Cl22HCl (addition of hydrogen)

H2 + Br2 2HBr (addition of hydrogen)

ZnO + C Zn + CO (removal of oxygen)

REDUCING AGENT

A substance which supplies hydrogen or removes oxygen is celled reducing agent or reductant or reducer.

In the above examples, H2 and C act as reducing agent.

ELECTRONIC CONCEPT

• OXIDATION :- it is defined as the process in which an atom or an ion loses one or more electrons.

• Loss of electrons either increases positive charge or decreases negative charge of the atom or ion.

• Examples showing increase in positive charge

Mg Mg+2 + 2e- Fe+2 Fe+3 + e- Examples showing decrease in negative charge S-2 S + 2e- MnO4

2- MnO4- + e-

The substance which loses electrons is called reducing agent.

Electronic concept

• REDUCTION:- It is defined as the process in which an atom or ion gains one or more electrons .

• The gain of electrons either decreases the positive charge or increases the negative charge of the atom or the ion.

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decrease in positive charge Fe3+ + e- Fe2+

Sn4+ + 2e- Sn2+

increase in negative charge S + 2e- S2-

MnO-4 + e- MnO4

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The substance which gains one or more electrons is called oxidising agent.

Oxidation and reduction are complementary

• Neither oxidation nor reduction can occur by itself. A substance can lose electrons only if there is another substance to gain those electrons. i.e. oxidation occurs at the cost of

reduction and vice versa. So, we can say that both oxidation and reduction go side by side.

This can be explained by taking the example of magnesium when it is burnt in the presence of oxygen-• 2Mg + O2 2MgO• Mg undergoes oxidation as- Mg Mg+2 + 2e- (oxidation)Now, Question arises that where these two electrons have gone?

Actually, these are accepted by oxygen which changes to oxide ion as shown below-

O + 2e- O2- OR

O2 + 4e- 2O2- (reduction)

If we add oxidation and reduction, we get the net reaction as:• 2Mg + O2 2MgO

So, we can say that both oxidation and reduction are complementary to each other.

In the above example, Mg loses electrons and acts as reducing agent and oxygen accepts electrons and act as oxidising agent.

REDOX REACTIONS

• REDOX = RED + OX• i.e. Reduction + Oxidation• So such reactions which involve both

reduction as well as oxidation are called Redox Reactions.

Types of Redox Reactions

• 1. Direct redox reactions.• 2. Indirect redox reactions.

DIRECT REDOX REACTIONS

• The reactions in which both oxidation and reduction take place in the same vessel are called direct redox reactions.

• e.g. Zn rod dipped in CuSO4 Solution.

When a strip of metallic zinc is placed in a solution of CuSO4, after sometime it is noticed that

1. A part of zinc metal plate has dissolved and has lost weight. 2. Reddish metallic copper has started depositing on the zinc plate or is settling down at the bottom of the beaker. 3. The bluish colour of the solution gradually fades away.4. Solution becomes hot after sometime 5. Solution remains electrically neutral.

EXPLANATION:

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Explanation: . When zinc plate is placed in a solution of CuSO4, zinc loses electrons to form Zn2+ ions. The Cu2+ ions gain these electrons to form copper metal as reddish precipitate. As the concentration of Cu2+ ions present in the copper sulphate solution decreases the blue colour of the solution fades away. The overall reaction may be written as:

Electrons are transferred from Zn atom to Cu2+ ion directly. So, this reaction is a direct redox reaction.

INDIRECT REDOX REACTIONS

• The reactions in which both oxidation and reduction take place in different vessels are called indirect redox reactions.

• E.g. ELECTROCHEMICAL CELL

Zinc sulphate solution is taken in a beaker and a zinc rod is dipped in to it. Similarly copper sulphate solution is taken in another beaker and a copper strip is dipped in to it. An inverted U tube containing concentrated solutions of inert electrolytes such as KCl, KNO3 etc., connects the two solutions

Fig: - Electrochemical cell based on redox reaction of zinc and copper sulphate

the following observations are made. There is a flow of electrical current through the external circuit. The zinc rod loses weight, while the copper rod acquires weight. The concentration of ZnSO4 solution increases, while that of CuSO4 solution decreases. The two solutions in beakers remain electrically neutral.

EXPLANATIONZinc is oxidized to Zn2+ ions which go into the solution during the reaction.

The electrons released at Zn move towards Cu electrode through wires where these are accepted by Cu2+ ions of CuSO4 solution. The Cu2+ ions are reduced to metallic copper as:

The above reactions clearly show the overall reaction to be:

Thus, the direction of flow of electrons is from Zn to Cu. The electrode where oxidation occurs is called ANODE and the electrode where reduction occurs is called CATHODE.In electrochemical cells, Anode constitutes negative terminal and cathode constitutes positive terminal.

HALF CELLS AND HALF CELL REACTIONS

• The two vessels of the indirect redox reactions are called two half cells.and the corresponding reactions occuring are called half reactions.

The half cell in which oxidation is taking place is called oxidation half cell and reaction occuring is called oxidation half reaction. The half cell in which reduction is taking place is called reduction half cell and reaction occuring is called reduction half reaction.

SALT BRIDGE and ITS FUNCTIONS• An inverted U tube containing concentrated

solutions of inert electrolytes such as KCl, KNO3 etc., connects the two solutions. The two openings of the U tube are plugged with porous materials like glass wool or cotton.

• FUNCTIONS:1. It completes the circuit.2. It maintains the electrical neutrality of two solutions.

Zn rod loses Zn 2+ ions which move into ZnSO4 solution, so concentration of Zn+2 ions increases in this solution due to which this half cell acquires overall positive charge and oxidation stops. Similarly, in the second beaker due to decrease in concentration of Cu2+ ions, and accumulation of SO4

2- ions, this half cell acquires overall negative charge and reduction stops.

No oxidation- no reduction, cell stops working

Now, starts the function of salt bridge. To neutralise excess Zn 2+ions, an equivalent no. of Cl- ions from salt bridge move into ZnSO4 solution. Similarly to neutralise excess SO4 2- ions, an equivalent no. of K+ ions from salt bridge move into CuSO4 solution.

thus, two solutions become neutral and working of the cell starts again.

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