Class 11 Chemistry Notes

Chapter 7: Redox Reactions

1. What are Redox Reactions?

A redox reaction is a chemical reaction in which oxidation and reduction occur at the same time.

Example:$$2Mg + O_2 \rightarrow 2MgO$$2Mg+O2​→2MgO

Here,

• Magnesium is oxidised.
• Oxygen is reduced.

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2. Oxidation

Oxidation means any one of the following:

• Addition of oxygen
• Removal of hydrogen
• Addition of an electronegative element
• Removal of an electropositive element
• Loss of electrons
• Increase in oxidation number

Examples

Addition of oxygen$$2Mg + O_2 \rightarrow 2MgO$$2Mg+O2​→2MgO

Loss of hydrogen$$H_2S + Cl_2 \rightarrow S + 2HCl$$H2​S+Cl2​→S+2HCl

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3. Reduction

Reduction means:

• Removal of oxygen
• Addition of hydrogen
• Removal of an electronegative element
• Addition of an electropositive element
• Gain of electrons
• Decrease in oxidation number

Example$$CuO + H_2 \rightarrow Cu + H_2O$$CuO+H2​→Cu+H2​O

Copper oxide loses oxygen, so it is reduced.

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4. Electron Transfer Concept

Oxidation

Loss of electrons

Example$$Na \rightarrow Na^+ + e^-$$Na→Na++e−

Reduction

Gain of electrons

Example$$Cl_2 + 2e^- \rightarrow 2Cl^-$$Cl2​+2e−→2Cl−

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5. Oxidising Agent

An oxidising agent

• Accepts electrons
• Causes oxidation of another substance
• Gets reduced itself

Examples

• Oxygen
• Chlorine
• Potassium permanganate
• Potassium dichromate

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6. Reducing Agent

A reducing agent

• Donates electrons
• Causes reduction of another substance
• Gets oxidised itself

Examples

• Hydrogen
• Carbon
• Zinc
• Magnesium

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7. Oxidation Number

Oxidation number is the imaginary charge assigned to an atom in a compound assuming complete transfer of electrons.

It helps identify oxidation and reduction.

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8. Rules for Oxidation Number

Rule 1

Elements in free state = 0

Examples

• H₂
• O₂
• Cl₂
• Na

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Rule 2

Simple ion

Oxidation number = Ionic charge

Examples

Na⁺ = +1

Mg²⁺ = +2

Cl⁻ = –1

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Rule 3

Oxygen

Usually = −2

Exceptions

• Peroxides = −1
• Superoxides = −½
• OF₂ = +2

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Rule 4

Hydrogen

Usually = +1

Metal hydrides = −1

Examples

NaH

CaH₂

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Rule 5

Fluorine

Always = −1

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Rule 6

Sum of oxidation numbers

Neutral compound = 0

Polyatomic ion = Charge on ion

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9. Oxidation Number Examples

CO₂

Oxygen = −2

Let carbon = x

x + 2(−2)=0

x=+4

Carbon = +4

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H₂SO₄

Hydrogen = +1

Oxygen = −2

Let sulphur = x

2(+1)+x+4(−2)=0

x=+6

Sulphur = +6

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10. Oxidation and Reduction Using Oxidation Number

Increase in oxidation number

→ Oxidation

Decrease in oxidation number

→ Reduction

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11. Types of Redox Reactions

A. Combination Reaction

Two or more substances combine to form one product.

Example$$2Mg + O_2 \rightarrow 2MgO$$2Mg+O2​→2MgO

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B. Decomposition Reaction

One compound breaks into simpler substances.

Example$$2KClO_3 \rightarrow 2KCl + 3O_2$$2KClO3​→2KCl+3O2​

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C. Displacement Reaction

One element replaces another.

Example$$Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$$Zn+CuSO4​→ZnSO4​+Cu

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D. Disproportionation Reaction

The same element undergoes both oxidation and reduction.

Example$$2H_2O_2 \rightarrow 2H_2O + O_2$$2H2​O2​→2H2​O+O2​

Oxygen is both oxidised and reduced.

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12. Balancing Redox Reactions

Two methods

1. Oxidation Number Method
2. Half Reaction Method

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13. Important Differences

Oxidation	Reduction
Loss of electrons	Gain of electrons
Increase in oxidation number	Decrease in oxidation number
Addition of oxygen	Removal of oxygen
Removal of hydrogen	Addition of hydrogen

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14. Quick Revision Table

Term	Meaning
Oxidation	Loss of electrons
Reduction	Gain of electrons
Oxidising agent	Electron acceptor
Reducing agent	Electron donor
Oxidation Number	Imaginary charge on an atom
Redox Reaction	Oxidation and reduction together

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15. Important Exam Questions

1. Define oxidation and reduction.
2. What is a redox reaction?
3. Define oxidising and reducing agents.
4. State the rules for assigning oxidation numbers.
5. Find the oxidation number of sulphur in H₂SO₄.
6. Explain disproportionation reaction with one example.
7. Differentiate oxidation and reduction.
8. Explain oxidation in terms of electron transfer.
9. Explain the oxidation number method.
10. Classify different types of redox reactions.

1. Multiple Choice Questions (MCQs)

Basic MCQs

1. Oxidation is the process of:
   a) Gain of electrons
   b) Loss of electrons
   c) Gain of neutrons
   d) Gain of protons
2. Reduction involves:
   a) Increase in oxidation number
   b) Gain of electrons
   c) Addition of oxygen
   d) Removal of hydrogen
3. Which of the following is always an oxidising agent?
   a) Electron donor
   b) Electron acceptor
   c) Proton donor
   d) Hydrogen donor
4. Which of the following is always reduced?
   a) Oxidising agent
   b) Reducing agent
   c) Catalyst
   d) Solvent
5. In Mg + Cl₂ → MgCl₂, magnesium is:
   a) Reduced
   b) Oxidised
   c) Catalyst
   d) Neutral
6. Which element always has oxidation number –1?
   a) Oxygen
   b) Chlorine
   c) Fluorine
   d) Hydrogen
7. Oxidation number of oxygen in H₂O₂ is:
   a) –2
   b) –1
   c) +2
   d) 0
8. Oxidation number of hydrogen in NaH is:
   a) +1
   b) –1
   c) 0
   d) +2
9. Which is a disproportionation reaction?
   a) Zn + CuSO₄
   b) 2H₂O₂ → 2H₂O + O₂
   c) Mg + O₂
   d) CaCO₃ → CaO + CO₂
10. Which reaction is not a redox reaction?
    a) Zn + CuSO₄
    b) H₂ + Cl₂
    c) CaCO₃ → CaO + CO₂
    d) Fe₂O₃ + Al

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2. Assertion–Reason Questions

Assertion: Oxidation and reduction always occur together.
Reason: Electron lost by one species is gained by another.

Assertion: Oxygen always has oxidation number –2.
Reason: Oxygen forms peroxides.

Assertion: Fluorine never shows positive oxidation state.
Reason: Fluorine is the most electronegative element.

Assertion: Oxidising agents gain electrons.
Reason: They themselves undergo reduction.

Assertion: Hydrogen peroxide is both an oxidising and reducing agent.
Reason: Oxygen in H₂O₂ has oxidation number –1.

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3. Fill in the Blanks

1. Oxidation is the ________ of electrons.
2. Reduction is the ________ of electrons.
3. Oxidising agent always ________ electrons.
4. Reducing agent always ________ electrons.
5. Oxidation number of elemental oxygen is ________.
6. Oxidation number of fluorine is always ________.
7. Hydrogen shows oxidation number ________ in metal hydrides.
8. Oxidation number of oxygen in OF₂ is ________.
9. Redox reactions involve simultaneous ________ and ________.
10. Increase in oxidation number indicates ________.
11. Decrease in oxidation number indicates ________.
12. Potassium permanganate is a strong ________ agent.
13. Zinc is a good ________ agent.
14. Oxidation number of sulphur in H₂SO₄ is ________.
15. Oxidation number of carbon in CO₂ is ________.

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4. True or False

1. Oxidation means gain of electrons.
2. Reduction means gain of electrons.
3. Oxygen always has oxidation number –2.
4. Fluorine always has oxidation number –1.
5. Hydrogen peroxide is a peroxide.
6. Oxidation number of free elements is zero.
7. Oxidation number of Na⁺ is +1.
8. Oxidising agents donate electrons.
9. Reduction decreases oxidation number.
10. All decomposition reactions are redox reactions.

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5. Match the Following

Column A	Column B
Oxidation	Loss of electrons
Reduction	Gain of electrons
Oxidising agent	Electron acceptor
Reducing agent	Electron donor
Disproportionation	Same element oxidised and reduced

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6. Very Short Answer Questions (1 Mark)

1. Define oxidation.
2. Define reduction.
3. What is oxidation number?
4. What is an oxidising agent?
5. What is a reducing agent?
6. Define redox reaction.
7. Give one example of oxidation.
8. Give one example of reduction.
9. Name one oxidising agent.
10. Name one reducing agent.

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7. Short Answer Questions (2–3 Marks)

1. Explain oxidation in terms of electron transfer.
2. Explain reduction using oxidation number.
3. Write any four rules for oxidation number.
4. Differentiate oxidising and reducing agents.
5. Explain disproportionation reaction with an example.
6. Explain combination redox reactions.
7. Explain decomposition redox reactions.
8. Explain displacement reactions.
9. Why do oxidation and reduction always occur together?
10. Explain electron transfer in NaCl formation.

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8. Long Answer Questions (5 Marks)

1. Explain oxidation number with all rules and suitable examples.
2. Explain different types of redox reactions.
3. Describe oxidation and reduction according to:
   • Classical concept
   • Electron transfer concept
   • Oxidation number concept
4. Explain balancing of redox reactions by oxidation number method.
5. Explain balancing of redox reactions by half reaction method.

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9. Numerical / Oxidation Number Questions

Find oxidation number of the indicated element in:

1. H₂SO₄
2. KMnO₄
3. K₂Cr₂O₇
4. Na₂S₂O₃
5. NH₄Cl
6. HNO₃
7. HClO₄
8. Fe₂O₃
9. MnO₂
10. Cu₂O
11. ClO₃⁻
12. SO₄²⁻
13. Cr₂O₇²⁻
14. CO₃²⁻
15. H₂O₂
16. OF₂
17. KO₂
18. Na₂O₂
19. KClO₃
20. H₃PO₄

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10. Identify the Type of Redox Reaction

Classify each as:

• Combination
• Decomposition
• Displacement
• Disproportionation

1. 2Mg + O₂ → 2MgO
2. 2KClO₃ → 2KCl + 3O₂
3. Zn + CuSO₄ → ZnSO₄ + Cu
4. 2H₂O₂ → 2H₂O + O₂
5. Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O

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11. Identify the Oxidising and Reducing Agents

For each reaction:

1. Zn + CuSO₄ → ZnSO₄ + Cu
2. Mg + O₂ → MgO
3. Fe₂O₃ + Al → Al₂O₃ + Fe
4. H₂ + Cl₂ → HCl
5. CuO + H₂ → Cu + H₂O

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12. HOTS (Higher Order Thinking Questions)

1. Why can hydrogen peroxide act as both an oxidising and a reducing agent?
2. Why does fluorine never undergo disproportionation?
3. Why are oxidation numbers sometimes fractional?
4. Why are redox reactions called electron-transfer reactions?
5. Explain why NaH is considered a reducing agent.

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13. Case-Based Questions

Case Study 1

A student places a zinc strip in copper sulphate solution. After some time, the blue colour fades and reddish-brown copper deposits on the zinc strip.

Answer:

1. Which metal is oxidised?
2. Which ion is reduced?
3. Name the oxidising agent.
4. Name the reducing agent.
5. Write the balanced reaction.

Case Study 2

Hydrogen peroxide decomposes into water and oxygen.

Answer:

1. Name the type of reaction.
2. Find oxidation number of oxygen in H₂O₂.
3. Which oxygen atoms are oxidised?
4. Which oxygen atoms are reduced?
5. Why is H₂O₂ unique?

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14. One-Word Questions

1. Electron donor = ________
2. Electron acceptor = ________
3. Increase in oxidation number = ________
4. Decrease in oxidation number = ________
5. Oxygen in peroxide = ________
6. Oxygen in superoxide = ________
7. Hydrogen in NaH = ________
8. Oxidation number of free element = ________