Class 11 : Chemistry (In English) – Chapter 7: Redox Reaction
EXPLANATION & SUMMARY
🔷 1. Introduction to Redox Reactions
Chemical reactions involving transfer of electrons are called redox reactions. The term ‘redox’ is a short form of reduction–oxidation reaction.
In every redox reaction:
🔹 One species loses electrons → Oxidation
🔹 Another species gains electrons → Reduction
These processes occur simultaneously.
🔷 2. Classical Definitions
🔸 Oxidation:
Addition of oxygen
Removal of hydrogen
Loss of electrons
Example:
2Mg + O₂ → 2MgO (Magnesium is oxidised)
🔸 Reduction:
Addition of hydrogen
Removal of oxygen
Gain of electrons
Example:
CuO + H₂ → Cu + H₂O (CuO is reduced)
🔷 3. Redox Reaction (Simultaneous Oxidation & Reduction)
Example:
Zn + Cu²⁺ → Zn²⁺ + Cu
Here:
Zn loses electrons → Oxidised
Cu²⁺ gains electrons → Reduced
This is a redox reaction.
🔷 4. Oxidising and Reducing Agents
🔹 Oxidising Agent (Oxidant):
It accepts electrons and gets reduced.
e.g. O₂, Cl₂, KMnO₄
🔹 Reducing Agent (Reductant):
It donates electrons and gets oxidised.
e.g. H₂, Na, Zn
🔷 5. Electron Transfer Concept
Redox reactions can be described in terms of electron transfer:
🔹 Oxidation = Loss of e⁻
🔹 Reduction = Gain of e⁻
Example:
Fe²⁺ + Cu²⁺ → Fe³⁺ + Cu⁺
Fe²⁺ → Fe³⁺ = oxidation (loss of 1 e⁻)
Cu²⁺ → Cu⁺ = reduction (gain of 1 e⁻)
🔷 6. Oxidation Number
Definition: Oxidation number is the charge an atom would carry if all bonds were ionic.
Used to:
✔ Track redox changes
✔ Balance redox reactions
🔸 Rules:
Free element = 0 (e.g. O₂, N₂)
Monoatomic ion = Charge on ion
Oxygen = −2 (except in peroxides = −1)
Hydrogen = +1 (except with metals = −1)
Fluorine = −1 always
Sum of O.N. in neutral compound = 0
In ion = equal to ionic charge
🔷 7. Redox Reactions via Oxidation Number
Change in oxidation number tells us:
✔ Which atom is oxidised
✔ Which is reduced
Example:
Zn + H₂SO₄ → ZnSO₄ + H₂
Zn: 0 → +2 (oxidised)
H⁺: +1 → 0 (reduced)
🔷 8. Types of Redox Reactions
🔸 1. Combination Reactions:
Two or more reactants → One product
e.g. 2H₂ + O₂ → 2H₂O
🔸 2. Decomposition Reactions:
One compound → Simpler substances
e.g. 2KClO₃ → 2KCl + 3O₂
🔸 3. Displacement Reactions:
One element replaces another
e.g. Zn + CuSO₄ → ZnSO₄ + Cu
🔸 4. Disproportionation Reactions:
Same element undergoes both oxidation and reduction
e.g. Cl₂ + H₂O → HCl + HOCl
🔷 9. Balancing Redox Reactions
Two methods:
🔸 A. Oxidation Number Method:
Step-wise process:
Write unbalanced equation
Assign oxidation numbers
Identify changes
Equalise increase and decrease in O.N.
Balance atoms
Balance O using H₂O, H using H⁺
For basic medium: Add OH⁻
🔸 B. Half-Reaction Method:
Steps:
Split into oxidation and reduction half
Balance atoms and charges
Equalise electrons lost/gained
Combine both half-reactions
Example (Acidic medium):
MnO₄⁻ + Fe²⁺ → Mn²⁺ + Fe³⁺
→ MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
→ Fe²⁺ → Fe³⁺ + e⁻
Balance and combine.
🔷 10. Redox Reactions in Electrochemistry
Redox reactions form the basis of:
🔋 Electrochemical cells
✔ Galvanic cells (spontaneous)
✔ Electrolytic cells (non-spontaneous)
e.g. Zn + Cu²⁺ → Zn²⁺ + Cu
Used in batteries, electroplating, etc.
🔷 11. Redox Reactions and Titrations
Redox reactions are used in:
📍 Redox titrations – to determine concentration of oxidising/reducing agents
Examples:
✔ KMnO₄ vs FeSO₄
✔ Iodometry: I₂ formed and titrated with Na₂S₂O₃
Indicators used: self-indicating (like KMnO₄) or starch (in iodometry)
🔷 12. Redox Reactions in Industry
Examples:
🔹 Extraction of metals (Al, Zn used as reducing agents)
🔹 Chlor-alkali process
🔹 Bleaching powder production
🔹 Combustion of fuels
🔷 13. Disproportionation Reaction (Special Type)
In a disproportionation reaction, an element in a single oxidation state gets simultaneously oxidised and reduced.
Example:
2H₂O₂ → 2H₂O + O₂
(O.N. of O: −1 → −2 and 0)
🔷 14. Limitations of Classical Concepts
Earlier definitions (O₂ addition/H₂ removal) fail in ionic or complex reactions. Hence, electron transfer and oxidation number concepts are more universal.
🟢 SUMMARY
Redox reactions are chemical processes where oxidation and reduction occur simultaneously. Oxidation refers to loss of electrons, while reduction is gain of electrons. These reactions involve transfer of electrons between atoms or ions.
Oxidising agents gain electrons (and get reduced), while reducing agents lose electrons (and get oxidised).
Oxidation numbers are useful to identify oxidation/reduction events in complex reactions. Based on this, redox reactions are classified into four main types:
Combination
Decomposition
Displacement
Disproportionation
Balancing redox reactions can be done by two systematic methods:
Oxidation Number Method
Half-Reaction Method
Redox reactions are central to electrochemistry, including batteries, galvanic and electrolytic cells. They are also applied in titrations, where concentrations of unknown solutions are calculated using known redox pairs (e.g. KMnO₄ vs Fe²⁺).
Disproportionation reactions involve a single element undergoing both oxidation and reduction. The classical definitions of redox fail in certain reactions, making oxidation number and electron transfer definitions more accurate and universal.
Redox reactions are vital in industrial processes, metallurgy, environmental chemistry, and biological systems like respiration and photosynthesis.
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QUESTIONS FROM TEXTBOOK
🟦 Q7.1: Assign oxidation number to the underlined elements in each of the following species:
(a) NaH
(b) H₂O₂
(c) NaClO₃
(d) Mg(NO₃)₂
(e) Cr₂O₇²⁻
(f) CH₂Cl₂
(g) CuSO₄
(h) NH₄⁺
(i) HNO₃
(j) KMnO₄
🟢 Answer:
We assign oxidation numbers using standard rules:
(a) H in NaH = −1
(b) O in H₂O₂ = −1
(c) Cl in NaClO₃ = +5
(d) N in NO₃⁻ = +5
(e) Cr in Cr₂O₇²⁻ = +6
(f) Cl in CH₂Cl₂ = −1 (each), H = +1, C = 0
(g) Cu in CuSO₄ = +2
(h) N in NH₄⁺ = −3
(i) N in HNO₃ = +5
(j) Mn in KMnO₄ = +7
🟦 Q7.2: What are the oxidation numbers of the underlined elements in the following compounds?
(a) K₂Cr₂O₇
(b) K₂CrO₄
(c) Cr₂O₃
(d) CrO₃
🟢 Answer:
(a) Cr in K₂Cr₂O₇ = +6
(b) Cr in K₂CrO₄ = +6
(c) Cr in Cr₂O₃ = +3
(d) Cr in CrO₃ = +6
🟦 Q7.3: Name the oxidising agent and the reducing agent in the following reactions:
(a) 2H₂ + O₂ → 2H₂O
(b) CuO + H₂ → Cu + H₂O
(c) MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O
(d) Pb₃O₄ + 8HCl → 3PbCl₂ + Cl₂ + 4H₂O
🟢 Answer:
(a)
Oxidising Agent = O₂
Reducing Agent = H₂
(b)
Oxidising Agent = CuO
Reducing Agent = H₂
(c)
Oxidising Agent = MnO₂
Reducing Agent = HCl
(d)
Oxidising Agent = Pb₃O₄
Reducing Agent = HCl
🟦 Q7.4: What is the oxidation number of phosphorus in the following:
(a) H₃PO₃
(b) HPO₃²⁻
(c) H₄P₂O₇
(d) P₂O₅
🟢 Answer:
(a) P in H₃PO₃ = +3
(b) P in HPO₃²⁻ = +5
(c) P in H₄P₂O₇ = +5
(d) P in P₂O₅ = +5
🟦 Q7.5: Write the name of oxidising and reducing agents in the following redox reactions:
(a) Zn + Cu²⁺ → Zn²⁺ + Cu
(b) 2Na + Cl₂ → 2NaCl
(c) 2Fe²⁺ + Cl₂ → 2Fe³⁺ + 2Cl⁻
🟢 Answer:
(a)
Oxidising Agent = Cu²⁺
Reducing Agent = Zn
(b)
Oxidising Agent = Cl₂
Reducing Agent = Na
(c)
Oxidising Agent = Cl₂
Reducing Agent = Fe²⁺
🟦 Q7.6: Compute the oxidation number of sulphur, chromium and nitrogen in H₂SO₅, Cr₂O₇²⁻ and NO₃⁻.
🟢 Answer:
(a) Sulphur in H₂SO₅ = +6
(b) Chromium in Cr₂O₇²⁻ = +6
(c) Nitrogen in NO₃⁻ = +5
🟦 Q7.7: Identify the substances oxidised and the substances reduced in the following reactions:
(a) 2Na + O₂ → Na₂O₂
(b) CuO + H₂ → Cu + H₂O
🟢 Answer:
(a)
Oxidised: Na
Reduced: O₂
(b)
Oxidised: H₂
Reduced: CuO
🟦 Q7.8: Classify the following as combination, decomposition, displacement and disproportionation reactions:
(a) 2Mg + O₂ → 2MgO
(b) 2H₂O → 2H₂ + O₂
(c) Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag
(d) Cl₂ + 2NaOH → NaCl + NaClO + H₂O
🟢 Answer:
(a) Combination reaction
(b) Decomposition reaction
(c) Displacement reaction
(d) Disproportionation reaction
🟦 Q7.9: Identify the redox reactions out of the following reactions and identify the oxidising and reducing agents in them:
(a) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
(b) 3Mg + N₂ → Mg₃N₂
(c) 4Na + O₂ → 2Na₂O
(d) 2K + 2H₂O → 2KOH + H₂
(e) CuSO₄ + Zn → Cu + ZnSO₄
🟢 Answer:
(a) Not a redox reaction
(b) Redox
Oxidising Agent = N₂
Reducing Agent = Mg
(c) Redox
Oxidising Agent = O₂
Reducing Agent = Na
(d) Redox
Oxidising Agent = H₂O
Reducing Agent = K
(e) Redox
Oxidising Agent = CuSO₄
Reducing Agent = Zn
🟦 Q7.10: Define the terms oxidation and reduction in the classical sense.
🟢 Answer:
📌 Oxidation is defined as:
The addition of oxygen, or
The removal of hydrogen.
📌 Reduction is defined as:
The removal of oxygen, or
The addition of hydrogen.
Example:
H₂S + Cl₂ → 2HCl + S
In this, H₂S is oxidised (loss of H), and Cl₂ is reduced (gain of H).
💡 Q7.11
Why does fluorine not show any positive oxidation state?
✅ Answer
📌 Fluorine is the most electronegative element in the periodic table.
📌 It has an electronic configuration of 1s² 2s² 2p⁵.
📌 Due to its small size and high electronegativity, it can only gain electrons, not lose them.
📌 Hence, it always exhibits an oxidation state of –1, never positive.
💡 Q7.12
Calculate the oxidation number of Cr in Cr₂O₇²⁻.
✅ Answer
Let oxidation number of Cr = x
There are 2 Cr atoms and 7 O atoms.
Oxidation number of O = –2
Total charge = –2
📌 2x + 7(–2) = –2
⇒ 2x – 14 = –2
⇒ 2x = +12
⇒ x = +6
🔷 So, the oxidation number of Cr is +6.
💡 Q7.13
Balance the following redox reaction in acidic medium:
Cr₂O₇²⁻ + I⁻ → Cr³⁺ + I₂
✅ Answer
📌 Step 1: Identify oxidation & reduction
Cr changes from +6 → +3 (Reduction)
I changes from –1 → 0 (Oxidation)
📌 Step 2: Write half reactions
Oxidation: 2I⁻ → I₂ + 2e⁻
Reduction: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
📌 Step 3: Multiply to equalize electrons
Oxidation ×3: 6I⁻ → 3I₂ + 6e⁻
Add both:
Cr₂O₇²⁻ + 14H⁺ + 6I⁻ → 2Cr³⁺ + 7H₂O + 3I₂
🔷 Final Balanced Reaction:
Cr₂O₇²⁻ + 14H⁺ + 6I⁻ → 2Cr³⁺ + 7H₂O + 3I₂
💡 Q7.14
What is the oxidation number of each element in KMnO₄?
✅ Answer
Let oxidation number of Mn = x
K = +1
O = –2
Equation:
+1 + x + 4(–2) = 0
⇒ 1 + x – 8 = 0
⇒ x = +7
🔷 Oxidation numbers:
K = +1, Mn = +7, O = –2
💡 Q7.15
Identify the oxidizing and reducing agents in the reaction:
Zn + Cu²⁺ → Zn²⁺ + Cu
✅ Answer
📌 Zn loses 2 electrons → Oxidation
Zn is the reducing agent
📌 Cu²⁺ gains 2 electrons → Reduction
Cu²⁺ is the oxidizing agent
💡 Q7.16
Balance the following redox reaction in basic medium:
MnO₄⁻ + SO₃²⁻ → MnO₂ + SO₄²⁻
✅ Answer
📌 Oxidation: SO₃²⁻ → SO₄²⁻ + 2e⁻
📌 Reduction: MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻
📌 Multiply and add:
3SO₃²⁻ → 3SO₄²⁻ + 6e⁻
2MnO₄⁻ + 4H₂O + 6e⁻ → 2MnO₂ + 8OH⁻
📌 Add both:
2MnO₄⁻ + 3SO₃²⁻ + 4H₂O → 2MnO₂ + 3SO₄²⁻ + 8OH⁻
💡 Q7.17
Define disproportionation reaction with example.
✅ Answer
📌 A disproportionation reaction is a type of redox reaction where the same species undergoes both oxidation and reduction.
Example:
2H₂O₂ → 2H₂O + O₂
(Oxidation: –1 to 0, Reduction: –1 to –2)
💡 Q7.18
Why is the reaction between H₂ and Cl₂ considered a redox reaction?
✅ Answer
📌 In H₂ + Cl₂ → 2HCl:
H is oxidized: 0 → +1
Cl is reduced: 0 → –1
🔷 Since oxidation and reduction occur simultaneously, it is a redox reaction.
💡 Q7.19
Write the balanced redox reaction for the reaction of hydrogen peroxide with acidified potassium iodide.
✅ Answer
📌 H₂O₂ + 2H⁺ + 2I⁻ → I₂ + 2H₂O
📌 H₂O₂ acts as an oxidizing agent, converting I⁻ → I₂
💡 Q7.20
Find the oxidation number of each element in H₂SO₄.
✅ Answer
Let oxidation number of S = x
H = +1, O = –2
📌 2(+1) + x + 4(–2) = 0
⇒ 2 + x – 8 = 0
⇒ x = +6
🔷 Oxidation Numbers:
H = +1, S = +6, O = –2
💡 Q7.21
Justify that the reaction:
4Na(s) + O₂(g) → 2Na₂O(s)
is a redox reaction.
✅ Answer
📌 Step 1: Assign oxidation numbers
Na in Na (elemental) = 0
O in O₂ = 0
Na in Na₂O = +1
O in Na₂O = –2
📌 Step 2: Identify changes
Na: 0 → +1 → Oxidation
O: 0 → –2 → Reduction
🔷 Conclusion: One species is oxidized and another is reduced, so this is a redox reaction.
💡 Q7.22
What is the oxidation number of Cl in the following?
(a) Cl₂O₇
(b) ClO₄⁻
(c) ClO₃⁻
(d) ClO₂⁻
(e) ClO⁻
✅ Answer
Let x be the oxidation number of Cl
O = –2
(a) Cl₂O₇:
2x + 7(–2) = 0
⇒ 2x – 14 = 0
⇒ x = +7
(b) ClO₄⁻:
x + 4(–2) = –1
⇒ x – 8 = –1
⇒ x = +7
(c) ClO₃⁻:
x + 3(–2) = –1
⇒ x – 6 = –1
⇒ x = +5
(d) ClO₂⁻:
x + 2(–2) = –1
⇒ x – 4 = –1
⇒ x = +3
(e) ClO⁻:
x – 2 = –1
⇒ x = +1
💡 Q7.23
Identify the oxidant and reductant in the following reaction:
MnO₄⁻ + C₂O₄²⁻ → Mn²⁺ + CO₂
✅ Answer
📌 Mn in MnO₄⁻ = +7
Mn in Mn²⁺ = +2 → Reduction (MnO₄⁻ is oxidant)
📌 C in C₂O₄²⁻ = +3
C in CO₂ = +4 → Oxidation (C₂O₄²⁻ is reductant)
🔷 Oxidant: MnO₄⁻
🔷 Reductant: C₂O₄²⁻
💡 Q7.24
What is meant by the term auto-oxidation?
✅ Answer
📌 Auto-oxidation is a process in which a substance undergoes simultaneous oxidation and reduction.
📌 Example:
H₂O₂ → H₂O + O₂
Here, oxygen in H₂O₂ has oxidation number –1
Becomes –2 in H₂O → Reduction
Becomes 0 in O₂ → Oxidation
🔷 Hence, it’s called auto-oxidation or disproportionation.
💡 Q7.25
Balance the following redox reaction in acidic medium:
MnO₄⁻ + Fe²⁺ → Mn²⁺ + Fe³⁺
✅ Answer
📌 Oxidation: Fe²⁺ → Fe³⁺ + e⁻
📌 Reduction: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
📌 Multiply oxidation half by 5:
5Fe²⁺ → 5Fe³⁺ + 5e⁻
📌 Add both:
MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
💡 Q7.26
Assign oxidation numbers to each element in HNO₃.
✅ Answer
Let oxidation number of N = x
H = +1, O = –2
📌 +1 + x + 3(–2) = 0
⇒ 1 + x – 6 = 0
⇒ x = +5
🔷 Oxidation Numbers:
H = +1, N = +5, O = –2
💡 Q7.27
Differentiate between oxidation and reduction in terms of electron transfer.
✅ Answer
📌 Oxidation:
Loss of electrons → Increase in oxidation number
Example: Na → Na⁺ + e⁻
📌 Reduction:
Gain of electrons → Decrease in oxidation number
Example: Cl₂ + 2e⁻ → 2Cl⁻
💡 Q7.28
What is the oxidation number of Mn in Mn₃O₄?
✅ Answer
Let x = oxidation number of Mn
Mn₃O₄ = 3Mn + 4O → net charge = 0
3x + 4(–2) = 0
⇒ 3x – 8 = 0
⇒ x = +8/3
🔷 Oxidation state of Mn is fractional: +8/3
(average of Mn⁺² and Mn⁺⁴)
💡 Q7.29
Balance the redox reaction:
Cl₂ → Cl⁻ + ClO₃⁻
✅ Answer
📌 Oxidation: Cl₂ → ClO₃⁻
📌 Reduction: Cl₂ → Cl⁻
📌 Balanced Equation:
3Cl₂ + 3H₂O → 5Cl⁻ + ClO₃⁻ + 6H⁺
💡 Q7.30
Which oxidation state is exhibited by nitrogen in the following compounds?
(a) N₂
(b) NO
(c) NO₂
(d) NH₃
✅ Answer
(a) N₂: 0 (elemental form)
(b) NO: +2
(c) NO₂: +4
(d) NH₃: –3
💡 Q7.31
Balance the following redox reaction using the ion-electron method:
Cr₂O₇²⁻ + SO₂ → Cr³⁺ + SO₄²⁻ (in acidic medium)
✅ Answer
📌 Oxidation:
SO₂ → SO₄²⁻
S: +4 → +6
Change = 2 e⁻ lost
📌 Reduction:
Cr₂O₇²⁻ → Cr³⁺
Cr: +6 → +3
Change = 3 e⁻ per Cr, total 6 e⁻ (2 Cr)
📌 Balance electrons:
Multiply oxidation by 3 and reduction by 1
🔁 Balanced Equation:
Cr₂O₇²⁻ + 3SO₂ + 2H₂O → 2Cr³⁺ + 3SO₄²⁻ + 4H⁺
💡 Q7.32
Which among the following does not show disproportionation reaction?
(A) Cl₂
(B) F₂
(C) Br₂
(D) I₂
✅ Answer
📌 Fluorine (F₂) is the most electronegative element and cannot show positive oxidation state.
So, it cannot undergo disproportionation.
✔️ Correct Answer: (B) F₂
💡 Q7.33
Name a compound in which nitrogen shows +1 oxidation state.
✅ Answer
📌 Example: N₂O (Nitrous oxide or laughing gas)
Let x = oxidation number of N
2x + (–2) = 0
⇒ x = +1
✔️ Nitrogen in N₂O has oxidation state +1
💡 Q7.34
Write two examples of disproportionation reactions.
✅ Answer
📌 Example 1:
2H₂O₂ → 2H₂O + O₂
(Oxidation: –1 to 0, Reduction: –1 to –2)
📌 Example 2:
Cl₂ + H₂O → HCl + HOCl
(Cl: 0 → –1 and +1)
💡 Q7.35
What is meant by a redox couple?
✅ Answer
📌 A redox couple refers to an oxidized and a reduced form of the same species involved in a redox reaction.
📌 Examples:
Fe³⁺ / Fe²⁺
Cu²⁺ / Cu
🔷 A redox couple helps define electrode potentials.
💡 Q7.36
What is a reducing agent?
✅ Answer
📌 A reducing agent is a substance that donates electrons to another species and gets oxidized in the process.
📌 Example:
Zn + Cu²⁺ → Zn²⁺ + Cu
Zn acts as reducing agent.
💡 Q7.37
Identify the redox changes in the following reaction:
Zn + CuSO₄ → ZnSO₄ + Cu
✅ Answer
📌 Zn: 0 → +2 (Oxidation)
📌 Cu²⁺: +2 → 0 (Reduction)
🔷 Zn is oxidized → reducing agent
🔷 Cu²⁺ is reduced → oxidizing agent
💡 Q7.38
Name a compound in which oxygen has a positive oxidation state.
✅ Answer
📌 Example: OF₂ (Oxygen difluoride)
Oxygen = +2
Fluorine = –1 (being more electronegative)
📌 1x + 2(–1) = 0
⇒ x = +2
✔️ So, oxygen has +2 oxidation state in OF₂
💡 Q7.39
Which species undergoes oxidation in the reaction:
H₂S + Cl₂ → S + HCl
✅ Answer
📌 Sulfur in H₂S: –2
Sulfur in S: 0 → Oxidation
📌 Chlorine: 0 → –1 → Reduction
✔️ Oxidized species: H₂S (Sulfur)
💡 Q7.40
Why is H₂O₂ both an oxidizing and a reducing agent?
✅ Answer
📌 Oxidation number of O in H₂O₂ = –1
📌 Acts as oxidizing agent:
H₂O₂ + 2H⁺ + 2e⁻ → 2H₂O
(–1 to –2)
📌 Acts as reducing agent:
H₂O₂ → O₂ + 2H⁺ + 2e⁻
(–1 to 0)
✔️ Hence, H₂O₂ shows dual behavior.
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OTHER IMPORTANT QUESTIONS FOR EXAMS
(CBSE MODEL QUESTIONS PAPER)
ESPECIALLY MADE FROM THIS LESSON ONLY
🔵 Q1. What is the oxidation number of chlorine in NaClO₄?
(A) +7
(B) +5
(C) +3
(D) +1
✅ Answer: (A) +7
🔵 Q2. Identify the reducing agent in the reaction:
Zn + CuSO₄ → ZnSO₄ + Cu
(A) Zn
(B) Cu²⁺
(C) SO₄²⁻
(D) Cu
✅ Answer: (A) Zn
🔵 Q3. Which of the following is not a redox reaction?
(A) H₂ + Cl₂ → 2HCl
(B) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
(C) CuO + H₂ → Cu + H₂O
(D) Fe₂O₃ + 3CO → 2Fe + 3CO₂
✅ Answer: (B) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
🔵 Q4. The equivalent weight of a substance in a redox reaction depends on:
(A) Molecular weight only
(B) Atomic number
(C) Number of electrons lost or gained
(D) Volume of gas evolved
✅ Answer: (C) Number of electrons lost or gained
🔵 Q5. Oxidation involves:
(A) Gain of electrons
(B) Gain of protons
(C) Loss of protons
(D) Loss of electrons
✅ Answer: (D) Loss of electrons
🔵 Q6. In acidic medium, the oxidation number of Mn in MnO₄⁻ changes to what value in Mn²⁺?
(A) +4
(B) +6
(C) +2
(D) 0
✅ Answer: (C) +2
🔵 Q7. Assertion (A): Oxidation number of carbon in CH₄ is –4.
Reason (R): Carbon is more electronegative than hydrogen.
(A) A and R both are correct, and R is the correct explanation of A.
(B) A and R both are correct, but R is not the correct explanation of A.
(C) A is correct, R is incorrect.
(D) A is incorrect, R is correct.
✅ Answer: (C) A is correct, R is incorrect.
🔵 Q8. Which of the following reactions is a disproportionation reaction?
(A) Cl₂ + 2NaBr → 2NaCl + Br₂
(B) 2F₂ + 2OH⁻ → OF₂ + 2F⁻ + H₂O
(C) Zn + HCl → ZnCl₂ + H₂
(D) Fe + CuSO₄ → FeSO₄ + Cu
✅ Answer: (B) 2F₂ + 2OH⁻ → OF₂ + 2F⁻ + H₂O
🔵 Q9. What is the oxidation number of Fe in K₄[Fe(CN)₆]?
(A) +3
(B) +2
(C) +4
(D) 0
✅ Answer: (B) +2
🔵 Q10. What is the change in oxidation number of sulfur in the reaction:
S₈ + 12O₂ → 8SO₃
(A) 0 to +6
(B) 0 to +4
(C) –2 to +6
(D) +2 to +6
✅ Answer: (A) 0 to +6
🔵 Q11. Very Short Answer: Define redox reaction.
✅ Answer: A redox reaction is a chemical reaction in which oxidation and reduction occur simultaneously.
🔵 Q12. Very Short Answer: What is meant by oxidation number?
✅ Answer: Oxidation number is the hypothetical charge assigned to an atom assuming that the bonding is purely ionic.
🔵 Q13. Very Short Answer: Name one oxidizing agent used in acidic medium.
✅ Answer: Potassium permanganate (KMnO₄)
🔵 Q14. Very Short Answer: Give one example of a disproportionation reaction.
✅ Answer: 2H₂O₂ → 2H₂O + O₂
🔵 Q15. Very Short Answer: What is the oxidation number of nitrogen in NO₃⁻?
✅ Answer: +5
🔵 Q16. Case-based MCQ:
Case: Consider the redox reaction:
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
Q: What is the change in oxidation number of Cr?
(A) 0 to +3
(B) +6 to +3
(C) +3 to +6
(D) –2 to 0
✅ Answer: (B) +6 to +3
🔵 Q17. Case-based MCQ:
Case: In the reaction Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag,
Q: What role does copper play?
(A) Reducing agent
(B) Oxidizing agent
(C) Catalyst
(D) Neutral compound
✅ Answer: (A) Reducing agent
🔵 Q18. Case-based MCQ:
Case: The following reaction occurs in acidic solution:
MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
Q: How many electrons are transferred in total?
(A) 3
(B) 4
(C) 5
(D) 6
✅ Answer: (C) 5
🔷 Q19. Write the oxidation half-reaction and the reduction half-reaction for the reaction:
Zn + Cu²⁺ → Zn²⁺ + Cu
📌 Answer:
🔹 Oxidation half-reaction:
Zn → Zn²⁺ + 2e⁻
🔹 Reduction half-reaction:
Cu²⁺ + 2e⁻ → Cu
🔷 Q20. Calculate the oxidation number of Mn in MnO₄⁻.
📌 Answer:
Let oxidation number of Mn = x
Oxygen = –2 × 4 = –8
Net charge on MnO₄⁻ = –1
So,
x – 8 = –1
⇒ x = +7
✅ Oxidation number of Mn = +7
🔷 Q21. What is disproportionation reaction? Give an example.
📌 Answer:
🔹 A disproportionation reaction is a type of redox reaction in which a single substance undergoes both oxidation and reduction.
🔹 Example:
2H₂O₂ → 2H₂O + O₂
Here, O in H₂O₂ has oxidation number –1. It gets oxidised to 0 in O₂ and reduced to –2 in H₂O.
🔷 Q22. Differentiate between oxidising agent and reducing agent.
📌 Answer:
🔹 Oxidising agent: Accepts electrons and gets reduced.
🔹 Reducing agent: Donates electrons and gets oxidised.
🟢 Example:
In the reaction: CuO + H₂ → Cu + H₂O
• CuO is oxidising agent
• H₂ is reducing agent
🔷 Q23. What is meant by the term oxidation number? How is it different from valency?
📌 Answer:
🔹 Oxidation number: It is the apparent charge assigned to an atom in a molecule/ion.
🔹 Valency: It is the combining capacity of an element.
🟢 Difference:
Oxidation number can be positive or negative.
Valency is always positive and whole number.
Oxidation number varies with compound; valency is fixed for most elements.
🔶 Q24. Balance the redox reaction:
Fe²⁺ + Cr₂O₇²⁻ + H⁺ → Fe³⁺ + Cr³⁺ + H₂O (acidic medium)
📌 Answer:
🔹 Step 1: Oxidation half (Fe²⁺ → Fe³⁺)
Fe²⁺ → Fe³⁺ + e⁻
🔹 Step 2: Reduction half (Cr₂O₇²⁻ → Cr³⁺)
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
🔹 Step 3: Multiply oxidation by 6:
6Fe²⁺ → 6Fe³⁺ + 6e⁻
🔹 Step 4: Add both:
Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 7H₂O + 6Fe³⁺
✅ Final balanced redox equation.
🔶 Q25. Explain the concept of redox reactions in terms of electron transfer. Give an example.
📌 Answer:
🔹 Redox reactions involve transfer of electrons:
• Oxidation: Loss of electrons
• Reduction: Gain of electrons
🟢 Example:
Zn + Cu²⁺ → Zn²⁺ + Cu
• Zn is oxidised (loses 2e⁻)
• Cu²⁺ is reduced (gains 2e⁻)
✅ Hence, electron transfer governs redox processes.
🔶 Q26. Determine the oxidation number of N in NO₃⁻ and NH₄⁺.
📌 Answer:
🔹 In NO₃⁻:
Let x be oxidation number of N
x + 3(–2) = –1
⇒ x – 6 = –1
⇒ x = +5
🔹 In NH₄⁺:
x + 4(+1) = +1
⇒ x + 4 = +1
⇒ x = –3
✅ Oxidation numbers:
• N in NO₃⁻ = +5
• N in NH₄⁺ = –3
🔶 Q27. Define equivalent weight of an oxidising agent. Give its formula.
📌 Answer:
🔹 Equivalent weight of an oxidising agent =
(Molecular weight) / (Number of electrons gained per molecule)
🟢 Example: KMnO₄ in acid medium
Mn⁷⁺ → Mn²⁺ (gain of 5e⁻)
Equivalent weight = Molecular weight / 5
✅ Used in redox titrations.
🔶 Q28. Identify the oxidised and reduced species in the following reaction:
Cl₂ + 2NaOH → NaCl + NaClO + H₂O
📌 Answer:
Cl₂ → NaCl → Cl⁻ (oxidation number 0 to –1) → reduction
Cl₂ → NaClO → Cl⁺ (oxidation number 0 to +1) → oxidation
✅ So, Cl₂ is both oxidised and reduced → disproportionation reaction
🔷 Q29. Case-Based Question
Read the passage and answer the questions:
Hydrogen peroxide (H₂O₂) is a powerful oxidising as well as reducing agent. It decomposes to form water and oxygen. In acidic medium, it can oxidise iodide to iodine, whereas in basic medium, it undergoes disproportionation.
(i) Write the redox reaction of H₂O₂ with KI in acidic medium.
(ii) Identify the oxidised and reduced species.
(iii) What is the oxidation number of O in H₂O₂?
(iv) Write the reaction of H₂O₂ undergoing disproportionation in basic medium.
📌 Answer:
(i) 2KI + H₂O₂ + H₂SO₄ → I₂ + K₂SO₄ + 2H₂O
(ii)
🔹 I⁻ → I₂ (oxidation)
🔹 H₂O₂ → H₂O (reduction)
(iii)
🔹 In H₂O₂, oxidation number of O = –1
(iv)
🔹 2H₂O₂ → 2H₂O + O₂
(This is a disproportionation reaction: O is both oxidised and reduced)
🔷 Q30. Case-Based Question
The oxidation number concept helps in balancing redox reactions and identifying oxidising/reducing agents.
Consider the reaction:
Fe₂O₃ + CO → Fe + CO₂
(i) Assign oxidation numbers to all atoms.
(ii) Identify the oxidising and reducing agents.
(iii) Balance the redox reaction using oxidation number method.
(iv) Classify the type of redox process.
📌 Answer:
(i)
🔹 Fe in Fe₂O₃ = +3
🔹 O = –2
🔹 C in CO = +2
🔹 C in CO₂ = +4
🔹 Fe in Fe = 0
(ii)
🔹 Fe₂O₃ is oxidising agent
🔹 CO is reducing agent
(iii)
Fe₂O₃ + 3CO → 2Fe + 3CO₂
(iv)
This is a redox reaction involving oxidation of carbon and reduction of iron.
🔷 Q31. Case-Based Question
A redox titration involves KMnO₄ and FeSO₄ in acidic medium. KMnO₄ acts as oxidising agent and Fe²⁺ is reducing agent.
(i) Write the balanced redox reaction.
(ii) What is the role of H₂SO₄?
(iii) Calculate the n-factor of KMnO₄ in this reaction.
(iv) What colour change is observed at endpoint?
📌 Answer:
(i)
MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
(ii)
🔹 H₂SO₄ provides acidic medium necessary for KMnO₄ to act as a strong oxidising agent.
(iii)
🔹 Mn⁷⁺ → Mn²⁺ (change = 5)
✅ n-factor = 5
(iv)
🔹 Purple (KMnO₄) to Colourless (Mn²⁺)
At endpoint, a permanent pink colour appears due to excess KMnO₄.
🔶 Q32. Long Answer (5 Marks)
Balance the following redox reaction by ion-electron method in acidic medium:
Cr₂O₇²⁻ + Fe²⁺ + H⁺ → Cr³⁺ + Fe³⁺ + H₂O
📌 Answer:
🔹 Step 1: Write half-reactions
Oxidation: Fe²⁺ → Fe³⁺ + e⁻
Reduction: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
🔹 Step 2: Multiply oxidation half by 6
6Fe²⁺ → 6Fe³⁺ + 6e⁻
🔹 Step 3: Add both
Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 7H₂O + 6Fe³⁺
✅ Final Balanced Redox Equation
🔶 Q33. Explain the types of redox reactions with one example each.
📌 Answer:
🔹 1. Combination Reaction:
H₂ + Cl₂ → 2HCl
🔹 2. Decomposition Reaction:
2H₂O₂ → 2H₂O + O₂
🔹 3. Displacement Reaction:
Zn + CuSO₄ → ZnSO₄ + Cu
🔹 4. Disproportionation Reaction:
Cl₂ + 2NaOH → NaCl + NaClO + H₂O
✅ These are all types of redox reactions involving oxidation and reduction.
🔶 Q34. Balance the reaction using ion-electron method in basic medium:
MnO₄⁻ + I⁻ → MnO₂ + IO₃⁻
📌 Answer:
🔹 Step 1: Oxidation half
I⁻ → IO₃⁻ (I: –1 to +5) → Loss of 6e⁻
🔹 Step 2: Reduction half
MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻
🔹 Step 3: LCM of electrons = 6
Multiply oxidation half by 1
Multiply reduction half by 2
🔹 Step 4: Add both
2MnO₄⁻ + I⁻ + 4H₂O → 2MnO₂ + IO₃⁻ + 4OH⁻
✅ Final Balanced Equation in basic medium
🔶 Q35. Differentiate between the following:
(i) Oxidation and reduction
(ii) Redox and non-redox reactions
(iii) Oxidising and reducing agents
📌 Answer:
(i) Oxidation vs Reduction
🔹 Oxidation = Loss of electrons
🔹 Reduction = Gain of electrons
(ii) Redox vs Non-Redox
🔹 Redox: Electron transfer occurs (e.g. Zn + Cu²⁺ → Zn²⁺ + Cu)
🔹 Non-redox: No electron transfer (e.g. NaCl + AgNO₃ → AgCl + NaNO₃)
(iii) Oxidising vs Reducing Agents
🔹 Oxidising Agent: Gains electrons (gets reduced)
🔹 Reducing Agent: Loses electrons (gets oxidised)
✅ These distinctions help in identifying redox processes clearly.
————————————————————————————————————————————————————————————————————————————
NEET QUESTIONS FROM THIS LESSON
Q1. In which of the following compounds does nitrogen exhibit the highest oxidation state?
(A) NH₃
(B) N₂O
(C) HNO₃
(D) NO₂
Answer: (C) HNO₃
Year: 2025 | Set: Z
Q2. Which of the following is a redox reaction?
(A) AgNO₃ + NaCl → AgCl + NaNO₃
(B) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
(C) Zn + CuSO₄ → ZnSO₄ + Cu
(D) NaOH + HCl → NaCl + H₂O
Answer: (C) Zn + CuSO₄ → ZnSO₄ + Cu
Year: 2024 | Set: 2
Q3. In the reaction:
H₂S + Cl₂ → 2HCl + S
The reducing agent is:
(A) Cl₂
(B) HCl
(C) H₂S
(D) S
Answer: (C) H₂S
Year: 2024 | Set: 3
Q4. What is the oxidation number of chromium in Cr₂O₇²⁻?
(A) +3
(B) +6
(C) +2
(D) +7
Answer: (B) +6
Year: 2023 | Set: 1
Q5. Which element acts both as oxidising and reducing agent in the reaction:
2H₂O₂ → 2H₂O + O₂
(A) Hydrogen
(B) Oxygen
(C) Both H and O
(D) None
Answer: (B) Oxygen
Year: 2023 | Set: Z
Q6. Which of the following is not a redox reaction?
(A) Zn + 2HCl → ZnCl₂ + H₂
(B) CuO + H₂ → Cu + H₂O
(C) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
(D) 2Na + Cl₂ → 2NaCl
Answer: (C) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
Year: 2022 | Set: 1
Q7. In the disproportionation reaction:
Cl₂ + 2NaOH → NaCl + NaClO + H₂O
The oxidation state of Cl in NaClO is:
(A) -1
(B) 0
(C) +1
(D) +3
Answer: (C) +1
Year: 2022 | Set: 2
Q8. Which of the following species acts as both oxidising and reducing agent?
(A) H₂O
(B) H₂S
(C) H₂O₂
(D) Cl₂
Answer: (C) H₂O₂
Year: 2021 | Set: 3
Q9. Which of the following has the lowest oxidation number for nitrogen?
(A) NO
(B) NO₂
(C) NH₃
(D) N₂O
Answer: (C) NH₃
Year: 2021 | Set: Z
Q10. In the reaction:
MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
How many electrons are transferred per MnO₄⁻ ion?
(A) 2
(B) 3
(C) 4
(D) 5
Answer: (D) 5
Year: 2020 | Set: A
Q11. Which of the following pairs undergoes redox reaction?
(A) HCl + NaOH
(B) BaCl₂ + Na₂SO₄
(C) Fe + CuSO₄
(D) AgNO₃ + KCl
Answer: (C) Fe + CuSO₄
Year: 2020 | Set: B
Q12. Oxidation number of oxygen in OF₂ is:
(A) -2
(B) -1
(C) +1
(D) +2
Answer: (C) +2
Year: 2019 | Set: 1
Q13. The equivalent weight of KMnO₄ in acidic medium is:
(A) M/2
(B) M/3
(C) M/5
(D) M/1
Answer: (C) M/5
Year: 2019 | Set: Z
Q14. In acidic medium, the oxidation number of Cl in ClO₄⁻ is:
(A) +7
(B) +5
(C) +3
(D) +1
Answer: (A) +7
Year: 2018 | Set: A
Q15. In the reaction:
Cl₂ + 2NaI → 2NaCl + I₂
Cl₂ acts as:
(A) Oxidising agent
(B) Reducing agent
(C) Catalyst
(D) Acid
Answer: (A) Oxidising agent
Year: 2018 | Set: 2
Q16. Which of the following is correct for a redox reaction?
(A) Electrons are gained and lost
(B) No electron is transferred
(C) Only reduction occurs
(D) Only oxidation occurs
Answer: (A) Electrons are gained and lost
Year: 2017 | Set: X
Q17. The change in oxidation number of Mn in KMnO₄ to Mn²⁺ is:
(A) 5
(B) 4
(C) 2
(D) 7
Answer: (A) 5
Year: 2017 | Set: Z
Q18. Which compound shows +3 oxidation state for iron?
(A) Fe₂O₃
(B) FeO
(C) Fe₃O₄
(D) FeCl₂
Answer: (A) Fe₂O₃
Year: 2016 | Set: Y
Q19. In acidic medium, the number of electrons accepted per MnO₄⁻ is:
(A) 3
(B) 4
(C) 5
(D) 2
Answer: (C) 5
Year: 2016 | Set: 3
Q20. Which of the following statements is incorrect?
(A) Redox reactions involve electron transfer
(B) Oxidation means gain of electrons
(C) Reducing agent loses electrons
(D) Oxidising agent gains electrons
Answer: (B) Oxidation means gain of electrons
Year: 2015 | Set: A
Q21. Disproportionation is shown by:
(A) NaCl
(B) Cl₂
(C) HCl
(D) NaOH
Answer: (B) Cl₂
Year: 2015 | Set: 1
Q22. What is the oxidation state of S in H₂SO₄?
(A) +6
(B) +4
(C) +2
(D) 0
Answer: (A) +6
Year: 2014 | Set: Z
Q23. Which one is a redox couple?
(A) Fe³⁺/Fe²⁺
(B) HCl/H₂O
(C) O₂/H₂O₂
(D) H₂/Cl₂
Answer: (A) Fe³⁺/Fe²⁺
Year: 2014 | Set: A
Q24. The equivalent weight of an oxidising agent is:
(A) Molecular mass / n-factor
(B) Atomic number × 2
(C) Valency × Mass
(D) Molecular weight × n-factor
Answer: (A) Molecular mass / n-factor
Year: 2013 | Set: Y
Q25. Which of the following is the correct reducing agent?
(A) KMnO₄
(B) H₂O₂
(C) SO₂
(D) Cl₂
Answer: (C) SO₂
Year: 2013 | Set: 1
Q26. Which among the following is the best oxidising agent?
(A) Cl₂
(B) H₂O₂
(C) F₂
(D) O₃
Answer: (C) F₂
Year: 2012 | Set: Z
Q27. In which of the following does oxidation number of nitrogen become zero?
(A) NO₂
(B) NO
(C) N₂
(D) NH₃
Answer: (C) N₂
Year: 2012 | Set: 2
Q28. Oxidation number of Fe in K₄[Fe(CN)₆] is:
(A) +3
(B) +2
(C) +4
(D) 0
Answer: (B) +2
Year: 2011 | Set: Y
Q29. Which of the following is not involved in redox reaction?
(A) Combustion
(B) Respiration
(C) Neutralisation
(D) Rusting
Answer: (C) Neutralisation
Year: 2011 | Set: A
Q30. In the reaction:
2Fe²⁺ + Cl₂ → 2Fe³⁺ + 2Cl⁻
Cl₂ is:
(A) Reducing agent
(B) Oxidising agent
(C) Catalyst
(D) None
Answer: (B) Oxidising agent
Year: 2010 | Set: 1
Q31. Oxidation number of O in KO₂ is:
(A) -1
(B) -0.5
(C) -2
(D) -1/2
Answer: (D) -1/2
Year: 2010 | Set: Z
Q32. Which reaction involves disproportionation?
(A) H₂ + Cl₂ → 2HCl
(B) 2H₂O₂ → 2H₂O + O₂
(C) NaOH + HCl → NaCl + H₂O
(D) CH₄ + O₂ → CO₂ + H₂O
Answer: (B) 2H₂O₂ → 2H₂O + O₂
Year: 2009 | Set: Y
Q33. Oxidising agent in reaction:
Zn + H₂SO₄ → ZnSO₄ + H₂ is:
(A) Zn
(B) H₂SO₄
(C) H⁺
(D) SO₄²⁻
Answer: (C) H⁺
Year: 2009 | Set: 2
Q34. Which species has highest oxidation number?
(A) KMnO₄
(B) K₂Cr₂O₇
(C) HClO₄
(D) MnO₂
Answer: (C) HClO₄
Year: 2008 | Set: X
Q35. In the redox reaction, oxidation and reduction:
(A) Occur simultaneously
(B) Occur independently
(C) Do not occur
(D) Are catalysed always
Answer: (A) Occur simultaneously
Year: 2008 | Set: Z
Q36. In which of the following is oxidation state of nitrogen +5?
(A) NO₂
(B) NO
(C) N₂O
(D) HNO₃
Answer: (D) HNO₃
Year: 2007 | Set: A
Q37. The oxidising agent in photosynthesis is:
(A) CO₂
(B) H₂O
(C) Light
(D) Chlorophyll
Answer: (B) H₂O
Year: 2007 | Set: 1
Q38. Which of the following reactions shows reduction?
(A) Fe³⁺ → Fe²⁺
(B) Zn²⁺ → Zn
(C) Cu → Cu²⁺
(D) H₂O → H₂
Answer: (A) Fe³⁺ → Fe²⁺
Year: 2006 | Set: B
Q39. What is the oxidation number of P in H₃PO₄?
(A) +3
(B) +5
(C) +4
(D) +2
Answer: (B) +5
Year: 2006 | Set: Z
Q40. Which among these has nitrogen in highest oxidation state?
(A) N₂O
(B) NO₂
(C) NO
(D) HNO₃
Answer: (D) HNO₃
Year: 2005 | Set: X
Q41. Redox reactions are involved in:
(A) Combustion
(B) Photosynthesis
(C) Respiration
(D) All of these
Answer: (D) All of these
Year: 2005 | Set: A
Q42. Oxidation state of S in Na₂S₂O₃ is:
(A) +2
(B) +3
(C) +6 and 0 (average +2)
(D) +2
Answer: (C) +6 and 0 (average +2)
Year: 2004 | Set: Y
Q43. The redox reaction is best represented by:
(A) HCl + NaOH → NaCl + H₂O
(B) Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag
(C) AgNO₃ + NaCl → AgCl + NaNO₃
(D) Na₂CO₃ + HCl → NaCl + H₂O + CO₂
Answer: (B) Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag
Year: 2004 | Set: 1
Q44. The compound which shows disproportionation is:
(A) Cl₂
(B) NaCl
(C) HCl
(D) NaOH
Answer: (A) Cl₂
Year: 2003 | Set: Z
Q45. Identify the reducing agent:
Zn + Cu²⁺ → Zn²⁺ + Cu
(A) Cu²⁺
(B) Zn²⁺
(C) Zn
(D) Cu
Answer: (C) Zn
Year: 2003 | Set: A
Q46. Which of these is not a redox reaction?
(A) Zn + H₂SO₄
(B) HCl + NaOH
(C) Cu + AgNO₃
(D) H₂ + O₂
Answer: (B) HCl + NaOH
Year: 2002 | Set: Y
Q47. In the redox reaction:
Fe + CuSO₄ → FeSO₄ + Cu,
the oxidised species is:
(A) Fe
(B) Cu
(C) CuSO₄
(D) SO₄²⁻
Answer: (A) Fe
Year: 2002 | Set: 2
Q48. The oxidation number of P in H₃PO₂ is:
(A) +1
(B) +3
(C) +5
(D) +2
Answer: (B) +3
Year: 2001 | Set: 1
Q49. Which of the following involves oxidation only?
(A) Loss of electron
(B) Gain of hydrogen
(C) Gain of electron
(D) Loss of oxygen
Answer: (A) Loss of electron
Year: 2001 | Set: Z
Q50. Oxidation state of S in H₂S₂O₇ is:
(A) +6
(B) +7
(C) +5
(D) +3
Answer: (A) +6
Year: 2001 | Set: A
————————————————————————————————————————————————————————————————————————————
JEE MAINS QUESTIONS FROM THIS LESSON
Q1. Which of the following is not a redox reaction?
(A) 2Na + Cl₂ → 2NaCl
(B) H₂ + Cl₂ → 2HCl
(C) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
(D) CuO + H₂ → Cu + H₂O
Answer: (C)
Year: 2023 | Shift 1
Q2. In the reaction: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O, the reducing agent is:
(A) H⁺
(B) MnO₄⁻
(C) H₂O
(D) Mn²⁺
Answer: (B)
Year: 2022 | Shift 2
Q3. In which of the following reactions is the oxidation number of nitrogen increased?
(A) NH₃ → NO₂
(B) NO → NO₂
(C) NH₄⁺ → N₂
(D) N₂H₄ → NH₃
Answer: (A)
Year: 2022 | Shift 1
Q4. The oxidation number of Cr in Cr₂O₇²⁻ is:
(A) +6
(B) +7
(C) +3
(D) +2
Answer: (A)
Year: 2021 | Shift 2
Q5. In the redox reaction, identify the oxidizing agent: Zn + CuSO₄ → ZnSO₄ + Cu
(A) Zn
(B) Cu²⁺
(C) SO₄²⁻
(D) Cu
Answer: (B)
Year: 2021 | Shift 1
Q6. Which element shows variable oxidation states due to unpaired d-electrons?
(A) Calcium
(B) Potassium
(C) Chromium
(D) Barium
Answer: (C)
Year: 2020 | Shift 2
Q7. What is the oxidation number of Fe in K₄[Fe(CN)₆]?
(A) +3
(B) +2
(C) 0
(D) +4
Answer: (B)
Year: 2020 | Shift 1
Q8. Which of the following has both oxidation and reduction occurring?
(A) NaOH + HCl → NaCl + H₂O
(B) 2H₂O₂ → 2H₂O + O₂
(C) AgNO₃ + NaCl → AgCl + NaNO₃
(D) CaCO₃ → CaO + CO₂
Answer: (B)
Year: 2019 | Shift 2
Q9. In acidic medium, the change in oxidation number of sulphur in H₂S to H₂SO₄ is:
(A) +6 to -2
(B) -2 to +6
(C) 0 to +6
(D) +2 to +4
Answer: (B)
Year: 2019 | Shift 1
Q10. Disproportionation reaction is:
(A) Redox reaction where same element is both oxidized and reduced
(B) Only reduction occurs
(C) Only oxidation occurs
(D) Neither oxidation nor reduction
Answer: (A)
Year: 2018 | Paper 1
Q11. Oxidation number of O in KO₂ is:
(A) -2
(B) -1
(C) -½
(D) -1⁄3
Answer: (C)
Year: 2018 | Paper 2
Q12. Which of the following represents a redox reaction?
(A) AgNO₃ + NaCl → AgCl + NaNO₃
(B) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
(C) H₂ + Cl₂ → 2HCl
(D) NaOH + HCl → NaCl + H₂O
Answer: (C)
Year: 2017 | Paper 2
Q13. Which of the following compounds contains nitrogen with oxidation number +5?
(A) NH₃
(B) N₂
(C) NO₂
(D) HNO₃
Answer: (D)
Year: 2017 | Paper 1
Q14. In a redox reaction, the element undergoing reduction is:
(A) One gaining electrons
(B) One losing electrons
(C) One losing protons
(D) One gaining neutrons
Answer: (A)
Year: 2016 | Set 2
Q15. What is the oxidation number of S in Na₂S₂O₃?
(A) +6
(B) +2
(C) +3
(D) Average of +2 and +6
Answer: (D)
Year: 2016 | Set 1
Q16. Oxidation number of Mn in KMnO₄ is:
(A) +2
(B) +4
(C) +6
(D) +7
Answer: (D)
Year: 2015 | Set 3
Q17. In the reaction: Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O, Cl₂ undergoes:
(A) Only reduction
(B) Only oxidation
(C) Disproportionation
(D) Photochemical change
Answer: (C)
Year: 2015 | Set 1
Q18. The equivalent weight of a reducing agent is calculated as:
(A) Molecular weight / number of electrons lost
(B) Molecular weight / number of electrons gained
(C) Atomic weight × valency
(D) Molecular weight × valency
Answer: (A)
Year: 2014 | Set 2
Q19. Which is the correct oxidizing agent in this reaction: Zn + H₂SO₄ → ZnSO₄ + H₂?
(A) Zn
(B) SO₄²⁻
(C) H⁺
(D) H₂
Answer: (C)
Year: 2014 | Set 1
Q20. The oxidation number of nitrogen in NO₃⁻ is:
(A) +1
(B) +3
(C) +5
(D) -1
Answer: (C)
Year: 2013 | Paper 1
Q21. The oxidation state of carbon in CH₄ is:
(A) +4
(B) 0
(C) -4
(D) +2
Answer: (C)
Year: 2012 | Paper 1
Q22. Identify the redox pair in the following: Fe³⁺ + e⁻ ⇌ Fe²⁺
(A) Fe³⁺/Fe
(B) Fe²⁺/Fe³⁺
(C) Fe/Fe²⁺
(D) Fe³⁺/FeO
Answer: (B)
Year: 2012 | Paper 2
Q23. Which of the following is NOT a redox change?
(A) H₂ → 2H⁺ + 2e⁻
(B) Cl₂ → 2Cl⁻
(C) NaOH + HCl → NaCl + H₂O
(D) Zn + CuSO₄ → Cu + ZnSO₄
Answer: (C)
Year: 2011 | Paper 1
Q24. In the disproportionation of H₂O₂, products are:
(A) H₂O + H₂
(B) H₂O + O₂
(C) H₂ + O₂
(D) H₂O₂ + H⁺
Answer: (B)
Year: 2010 | Paper 2
Q25. Which of the following undergoes self-oxidation and self-reduction?
(A) H₂O₂
(B) NH₃
(C) Cl₂
(D) HNO₃
Answer: (C)
Year: 2010 | Paper 1
Q26. In which of the following reactions does hydrogen act as an oxidizing agent?
(A) H₂ + F₂ → 2HF
(B) H₂ + Cl₂ → 2HCl
(C) H₂ + O₂ → H₂O
(D) None of these
Answer: (A)
Year: 2009 | Paper 1
Q27. Oxidation number of Cl in ClO₃⁻ is:
(A) +5
(B) +7
(C) +3
(D) +1
Answer: (A)
Year: 2009 | Paper 2
Q28. The compound in which nitrogen has lowest oxidation state is:
(A) NH₄Cl
(B) NO₂
(C) N₂O
(D) HNO₃
Answer: (A)
Year: 2008 | Paper 1
Q29. Disproportionation involves:
(A) One species oxidized, another reduced
(B) Same species both oxidized and reduced
(C) Only reduction
(D) Only oxidation
Answer: (B)
Year: 2008 | Paper 2
Q30. Oxidizing agent is one that:
(A) Loses electrons
(B) Gains electrons
(C) Is oxidized
(D) Forms complexes
Answer: (B)
Year: 2007 | Set 1
Q31. In a redox reaction, reducing agent:
(A) Is reduced
(B) Is oxidized
(C) Gains electrons
(D) Forms acid
Answer: (B)
Year: 2007 | Set 2
Q32. What is the oxidation number of oxygen in OF₂?
(A) -2
(B) -1
(C) 0
(D) +2
Answer: (D)
Year: 2006 | Set 2
Q33. The change in oxidation number of Mn in MnO₂ to Mn²⁺ is:
(A) 2
(B) 4
(C) 3
(D) 5
Answer: (C)
Year: 2006 | Set 1
Q34. In which reaction does Cl show both +1 and -1 oxidation state?
(A) Cl₂ + H₂O
(B) Cl₂ + NaOH (cold)
(C) Cl₂ + NaOH (hot)
(D) Cl₂ + HCl
Answer: (C)
Year: 2005 | Set 1
Q35. Oxidation number of sulphur in H₂SO₄ is:
(A) +2
(B) +4
(C) +6
(D) +8
Answer: (C)
Year: 2005 | Set 2
Q36. Which of the following is disproportionation reaction?
(A) 2H₂O₂ → 2H₂O + O₂
(B) Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag
(C) 2Na + Cl₂ → 2NaCl
(D) HCl + NaOH → NaCl + H₂O
Answer: (A)
Year: 2004 | Set 1
Q37. In acidic medium, the equivalent weight of KMnO₄ is:
(A) M/1
(B) M/3
(C) M/5
(D) M/7
Answer: (C)
Year: 2004 | Set 2
(Where M = molar mass)
Q38. Which element has variable oxidation states and shows colored compounds?
(A) Zn
(B) Cu
(C) Mg
(D) Na
Answer: (B)
Year: 2003 | Set 1
Q39. In disproportionation reaction of Cl₂ in hot NaOH, the products are:
(A) Cl⁻ and ClO⁻
(B) Cl⁻ and ClO₃⁻
(C) Cl₂ and Cl⁻
(D) Cl⁻ and Cl₂
Answer: (B)
Year: 2003 | Set 2
Q40. The sum of oxidation numbers in a neutral molecule is:
(A) Zero
(B) Positive
(C) Negative
(D) Depends on molecule
Answer: (A)
Year: 2002 | Set 1
Q41. Which of the following has oxidation number of P as +3?
(A) H₃PO₄
(B) PCl₃
(C) H₃PO₂
(D) P₂O₅
Answer: (B)
Year: 2002 | Set 2
Q42. Oxidation number of N in NH₄NO₃ is:
(A) -3 and +3
(B) -3 and +5
(C) -1 and +3
(D) +3 and +5
Answer: (B)
Year: 2001 | Set 1
Q43. Which of the following is correct for a reducing agent?
(A) Gains electrons
(B) Oxidized during reaction
(C) Is inert
(D) Forms salts only
Answer: (B)
Year: 2001 | Set 2
Q44. In redox titration, indicator used is:
(A) Litmus
(B) Starch
(C) Methyl orange
(D) Self-indicator like KMnO₄
Answer: (D)
Year: 2000 | Paper 1
Q45. The species oxidized in following reaction: Cu + HNO₃ → Cu(NO₃)₂ + NO + H₂O
(A) H⁺
(B) NO₃⁻
(C) Cu
(D) NO
Answer: (C)
Year: 2000 | Paper 2
Q46. In acidic medium, MnO₄⁻ changes to:
(A) MnO₂
(B) Mn³⁺
(C) Mn²⁺
(D) Mn
Answer: (C)
Year: 1999 | Set 1
Q47. In redox reactions, electrons are:
(A) Always lost
(B) Always gained
(C) Transferred from reducing agent to oxidizing agent
(D) Shared
Answer: (C)
Year: 1999 | Set 2
Q48. Redox reaction where oxidation and reduction take place simultaneously is called:
(A) Acid-base reaction
(B) Neutralization
(C) Disproportionation
(D) Redox
Answer: (D)
Year: 1998 | Set 1
Q49. Oxidation number of carbon in CH₃OH is:
(A) -2
(B) -4
(C) 0
(D) +1
Answer: (A)
Year: 1998 | Set 2
Q50. Oxidation of Na to Na⁺ involves:
(A) Gain of electron
(B) Loss of proton
(C) Loss of electron
(D) Gain of neutron
Answer: (C)
Year: 1997 | Paper 1
————————————————————————————————————————————————————————————————————————————
JEE ADVANCED QUESTIONS FROM THIS LESSON
🧪 Paper 1: Questions 1 to 17
Q1. Which of the following pairs are both redox reactions?
(A) Combustion and neutralisation
(B) Decomposition of KClO₃ and photosynthesis
(C) Dissociation of NaCl and boiling of water
(D) Sublimation and melting
Answer: (B)
Year: 2024 | Paper: 1 | Set: 2
Q2. The oxidation number of Mn in KMnO₄ is:
(A) +7
(B) +2
(C) +6
(D) +4
Answer: (A)
Year: 2023 | Paper: 1 | Set: 1
Q3. In the reaction:
Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O,
chlorine is simultaneously:
(A) Oxidised and reduced
(B) Only oxidised
(C) Only reduced
(D) Neither oxidised nor reduced
Answer: (A)
Year: 2023 | Paper: 1 | Set: 3
Q4. What is the oxidation number of Fe in Fe(CN)₆³⁻?
(A) +2
(B) +3
(C) +4
(D) 0
Answer: (B)
Year: 2022 | Paper: 1 | Set: 1
Q5. Identify the species undergoing oxidation in the following reaction:
Zn + CuSO₄ → ZnSO₄ + Cu
(A) Zn
(B) Cu²⁺
(C) SO₄²⁻
(D) Cu
Answer: (A)
Year: 2022 | Paper: 1 | Set: 2
Q6. Which of the following statements is correct about disproportionation reactions?
(A) Same element is oxidised and reduced
(B) Different elements are oxidised and reduced
(C) Oxidation occurs only
(D) Reduction occurs only
Answer: (A)
Year: 2021 | Paper: 1 | Set: 2
Q7. The change in oxidation number of Cr in Cr₂O₇²⁻ to Cr³⁺ is:
(A) 6 to 3
(B) 6 to 2
(C) 5 to 2
(D) 6 to 4
Answer: (A)
Year: 2020 | Paper: 1 | Set: 1
Q8. Which of the following is NOT a redox reaction?
(A) CaCO₃ → CaO + CO₂
(B) Zn + H₂SO₄ → ZnSO₄ + H₂
(C) CuO + H₂ → Cu + H₂O
(D) 2Na + Cl₂ → 2NaCl
Answer: (A)
Year: 2019 | Paper: 1 | Set: 3
Q9. The equivalent weight of an oxidising agent depends on:
(A) Number of electrons gained
(B) Its molar mass
(C) Oxidation number
(D) Its basicity
Answer: (A)
Year: 2019 | Paper: 1 | Set: 2
Q10. Identify the reducing agent in the reaction:
2H₂S + SO₂ → 3S + 2H₂O
(A) SO₂
(B) S
(C) H₂S
(D) H₂O
Answer: (C)
Year: 2018 | Paper: 1 | Set: 1
Q11. Oxidation state of sulphur in H₂SO₄ is:
(A) +6
(B) +4
(C) +2
(D) +8
Answer: (A)
Year: 2018 | Paper: 1 | Set: 2
Q12. Which species acts as both oxidising and reducing agent?
(A) H₂O₂
(B) NaCl
(C) H₂
(D) KMnO₄
Answer: (A)
Year: 2017 | Paper: 1 | Set: 1
Q13. In acidic medium, which of the following shows disproportionation?
(A) Cl₂
(B) Br₂
(C) I₂
(D) F₂
Answer: (A)
Year: 2016 | Paper: 1 | Set: 2
Q14. Identify the oxidising agent:
PbO₂ + HCl → PbCl₂ + Cl₂ + H₂O
(A) HCl
(B) Cl₂
(C) PbO₂
(D) H₂O
Answer: (C)
Year: 2015 | Paper: 1 | Set: 2
Q15. In which of the following reactions does nitrogen get oxidised?
(A) NH₃ → NO
(B) NO₂ → NO
(C) NH₄⁺ → NH₃
(D) N₂ → NH₃
Answer: (A)
Year: 2015 | Paper: 1 | Set: 1
Q16. Which is the correct combination of oxidised and reduced species?
2Fe²⁺ + Cl₂ → 2Fe³⁺ + 2Cl⁻
(A) Fe²⁺ oxidised, Cl₂ reduced
(B) Fe³⁺ oxidised, Cl⁻ reduced
(C) Fe²⁺ reduced, Cl₂ oxidised
(D) Fe³⁺ reduced, Cl⁻ oxidised
Answer: (A)
Year: 2014 | Paper: 1 | Set: 1
Q17. Which is NOT true for redox reactions?
(A) They involve electron transfer
(B) Always involve oxygen
(C) Oxidation and reduction occur together
(D) Oxidising and reducing agents participate
Answer: (B)
Year: 2014 | Paper: 1 | Set: 3
🔬 Paper 2: Questions 18 to 34
Q18. In acidic medium, MnO₄⁻ oxidises Fe²⁺ to Fe³⁺. The molar ratio of Fe²⁺:MnO₄⁻ is:
(A) 5:1
(B) 1:5
(C) 2:1
(D) 1:2
Answer: (A)
Year: 2024 | Paper: 2 | Set: 1
Q19. Which pair undergoes a redox reaction?
(A) Zn and HCl
(B) NaCl and H₂O
(C) CO₂ and H₂O
(D) CH₄ and N₂
Answer: (A)
Year: 2023 | Paper: 2 | Set: 2
Q20. In reaction:
Cl₂ + 2I⁻ → 2Cl⁻ + I₂,
the reducing agent is:
(A) I⁻
(B) Cl₂
(C) Cl⁻
(D) I₂
Answer: (A)
Year: 2023 | Paper: 2 | Set: 3
Q21. Number of electrons exchanged in the reaction:
Cr₂O₇²⁻ + 6Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O
(A) 6
(B) 12
(C) 8
(D) 10
Answer: (B)
Year: 2022 | Paper: 2 | Set: 1
Q22. Oxidation number of nitrogen in NO₂⁻ is:
(A) +3
(B) +2
(C) +1
(D) +4
Answer: (D)
Year: 2022 | Paper: 2 | Set: 2
Q23. In which compound is the oxidation number of oxygen +2?
(A) OF₂
(B) H₂O₂
(C) O₂
(D) Na₂O₂
Answer: (A)
Year: 2021 | Paper: 2 | Set: 3
Q24. Identify the redox couple:
(A) Fe²⁺/Fe³⁺
(B) Na⁺/NaCl
(C) NH₄⁺/NH₃
(D) CH₄/CO₂
Answer: (A)
Year: 2020 | Paper: 2 | Set: 2
Q25. In a reaction, oxidation number of carbon changes from +2 to +4. It is:
(A) Oxidation
(B) Reduction
(C) Disproportionation
(D) Reductive coupling
Answer: (A)
Year: 2019 | Paper: 2 | Set: 3
Q26. Which of these is a redox reaction?
(A) AgNO₃ + NaCl → AgCl + NaNO₃
(B) H₂ + Cl₂ → 2HCl
(C) NH₄Cl → NH₃ + HCl
(D) CaCO₃ → CaO + CO₂
Answer: (B)
Year: 2018 | Paper: 2 | Set: 2
Q27. What is the oxidising agent in the reaction:
Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O
(A) HNO₃
(B) NO₂
(C) Cu
(D) H₂O
Answer: (A)
Year: 2018 | Paper: 2 | Set: 1
Q28. Which of the following is a redox reaction?
(A) C + O₂ → CO₂
(B) AgNO₃ + NaCl → AgCl + NaNO₃
(C) HCl + NH₃ → NH₄Cl
(D) H₂O(l) → H₂O(g)
Answer: (A)
Year: 2017 | Paper: 2 | Set: 3
Q29. In acidic medium, Cr₂O₇²⁻ is reduced to:
(A) Cr³⁺
(B) Cr⁶⁺
(C) Cr⁵⁺
(D) CrO₄²⁻
Answer: (A)
Year: 2016 | Paper: 2 | Set: 1
Q30. Which is NOT an oxidising agent?
(A) KMnO₄
(B) H₂O₂
(C) H₂S
(D) O₃
Answer: (C)
Year: 2015 | Paper: 2 | Set: 2
Q31. In the redox pair H₂O₂/H₂O, H₂O₂ acts as:
(A) Oxidising agent
(B) Reducing agent
(C) Both
(D) Neither
Answer: (C)
Year: 2014 | Paper: 2 | Set: 2
Q32. In which reaction does O.N. of sulphur increase?
(A) SO₂ → SO₃
(B) H₂SO₄ → H₂S
(C) H₂S → S
(D) SO₃ → SO₂
Answer: (A)
Year: 2013 | Paper: 2 | Set: 1
Q33. Which of the following does NOT represent a redox change?
(A) PbO₂ + HCl → PbCl₂ + Cl₂ + H₂O
(B) Fe²⁺ + Cl₂ → Fe³⁺ + Cl⁻
(C) NaCl + AgNO₃ → AgCl + NaNO₃
(D) Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag
Answer: (C)
Year: 2012 | Paper: 2 | Set: 3
Q34. Which of the following statements is correct?
(A) Oxidation involves gain of electrons
(B) Reduction involves loss of electrons
(C) Oxidising agent gets reduced
(D) Reducing agent gets oxidised
Answer: (C)
Year: 2011 | Paper: 2 | Set: 1
————————————————————————————————————————————————————————————————————————————
PRACTICE SETS FROM THIS LESSON
Q1. Which of the following species has the highest oxidation number of manganese?
(A) MnO
(B) Mn₂O₃
(C) KMnO₄
(D) MnCl₂
Answer: (C) KMnO₄
Q2. Oxidation number of Fe in Fe(CN)₆³⁻ is:
(A) +3
(B) +2
(C) 0
(D) +4
Answer: (A) +3
Q3. Which of the following is not a redox reaction?
(A) H₂ + Cl₂ → 2HCl
(B) CaCO₃ → CaO + CO₂
(C) Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag
(D) Zn + H₂SO₄ → ZnSO₄ + H₂
Answer: (B) CaCO₃ → CaO + CO₂
Q4. In the reaction:
Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O
Cl₂ acts as:
(A) Reducing agent only
(B) Oxidising agent only
(C) Both oxidising and reducing agent
(D) Neither oxidising nor reducing agent
Answer: (C) Both oxidising and reducing agent
Q5. The compound that undergoes disproportionation in alkaline medium is:
(A) HCl
(B) HNO₃
(C) Cl₂
(D) H₂SO₄
Answer: (C) Cl₂
Q6. The oxidation number of phosphorus in H₃PO₄ is:
(A) +3
(B) +5
(C) +4
(D) +1
Answer: (B) +5
Q7. What is the reducing agent in the reaction?
Fe₂O₃ + 3CO → 2Fe + 3CO₂
(A) Fe₂O₃
(B) CO
(C) Fe
(D) CO₂
Answer: (B) CO
Q8. Which of the following is a redox reaction?
(A) NaOH + HCl → NaCl + H₂O
(B) BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl
(C) Zn + CuSO₄ → ZnSO₄ + Cu
(D) AgNO₃ + HCl → AgCl + HNO₃
Answer: (C) Zn + CuSO₄ → ZnSO₄ + Cu
Q9. Which of the following species is both oxidised and reduced?
(A) Cl₂
(B) HCl
(C) ClO₃⁻
(D) Cl⁻
Answer: (A) Cl₂
Q10. In the reaction
2H₂O₂ → 2H₂O + O₂
the oxidising agent is:
(A) H₂O
(B) O₂
(C) H₂O₂
(D) None
Answer: (C) H₂O₂
Q11. In which compound does nitrogen have the highest oxidation state?
(A) NH₃
(B) N₂O
(C) HNO₃
(D) NO₂
Answer: (C) HNO₃
Q12. What is the oxidation number of Mn in MnO₂?
(A) +2
(B) +3
(C) +4
(D) +6
Answer: (C) +4
Q13. What is the change in oxidation number of N in the reaction:
NO₂⁻ → NO₃⁻
(A) +3 to +5
(B) +4 to +5
(C) +2 to +5
(D) +1 to +3
Answer: (B) +4 to +5
Q14. What happens to electrons in oxidation?
(A) Gain
(B) Loss
(C) Both gain and loss
(D) No change
Answer: (B) Loss
Q15. Which method is most suitable for balancing ionic redox reactions in acidic medium?
(A) Oxidation number method
(B) Ion-electron method
(C) Hit and trial method
(D) Half reaction method
Answer: (B) Ion-electron method
Q16. Which element shows disproportionation in basic medium?
(A) P
(B) Cl
(C) S
(D) N
Answer: (B) Cl
Q17. In which of the following is the oxidation number of chlorine highest?
(A) HClO
(B) HClO₄
(C) Cl₂
(D) HClO₂
Answer: (B) HClO₄
Q18. Identify the correct statement:
(A) Reducing agent is oxidised
(B) Reducing agent is reduced
(C) Oxidising agent is oxidised
(D) Reducing agent gains electrons
Answer: (A) Reducing agent is oxidised
Q19. In acidic medium, MnO₄⁻ changes to:
(A) MnO
(B) Mn²⁺
(C) MnO₂
(D) Mn(OH)₂
Answer: (B) Mn²⁺
Q20. In the reaction:
2Cu⁺ → Cu²⁺ + Cu
The reaction is:
(A) Combination
(B) Disproportionation
(C) Decomposition
(D) Redox without electron transfer
Answer: (B) Disproportionation
Q21. What is the oxidation state of Cr in Cr₂O₇²⁻?
(A) +6
(B) +3
(C) +2
(D) +4
Answer: (A) +6
Q22. A redox reaction involves:
(A) Only oxidation
(B) Only reduction
(C) Both oxidation and reduction
(D) No electron transfer
Answer: (C) Both oxidation and reduction
Q23. Oxidation state of oxygen in OF₂ is:
(A) -2
(B) -1
(C) 0
(D) +2
Answer: (D) +2
Q24. In the redox reaction, electrons are transferred from:
(A) Oxidising agent to reducing agent
(B) Reducing agent to oxidising agent
(C) Catalyst to reactant
(D) Product to reactant
Answer: (B) Reducing agent to oxidising agent
Q25. Which of the following is not an oxidising agent?
(A) KMnO₄
(B) O₃
(C) Cl₂
(D) SO₂
Answer: (D) SO₂
Q26. Which of the following has the highest reducing power in aqueous solution?
(A) Zn
(B) Fe
(C) Mg
(D) Na
Answer: (D) Na
Q27. Which reaction represents both oxidation and reduction correctly?
(A) H₂ + Cl₂ → 2HCl
(B) AgNO₃ + NaCl → AgCl + NaNO₃
(C) HCl + NaOH → NaCl + H₂O
(D) NH₃ + HCl → NH₄Cl
Answer: (A) H₂ + Cl₂ → 2HCl
Q28. The correct oxidation number of sulphur in Na₂S₂O₃ is:
(A) +2
(B) +4
(C) +6
(D) Both +2 and +6
Answer: (D) Both +2 and +6
Q29. The equivalent weight of KMnO₄ in acidic medium is:
(A) M/1
(B) M/2
(C) M/5
(D) M/3
Answer: (C) M/5
Q30. Which of the following pairs undergoes redox reaction?
(A) NaCl + AgNO₃
(B) BaCl₂ + Na₂SO₄
(C) HCl + NaOH
(D) Cu + AgNO₃
Answer: (D) Cu + AgNO₃
Q31. Which method is best to balance redox equations in basic medium?
(A) Ion-electron method
(B) Oxidation number method
(C) Hit and trial method
(D) Valency method
Answer: (A) Ion-electron method
Q32. Which of the following is not correct for oxidising agents?
(A) They gain electrons
(B) They are reduced
(C) They oxidise others
(D) They lose electrons
Answer: (D) They lose electrons
Q33. Which element can show disproportionation?
(A) Na
(B) Mg
(C) Cl
(D) Ca
Answer: (C) Cl
Q34. In the reaction:
MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
What is the change in oxidation number of Mn?
(A) +7 to +4
(B) +7 to +3
(C) +7 to +2
(D) +7 to 0
Answer: (C) +7 to +2
Q35. In a redox reaction, oxidising agent:
(A) Gains electrons
(B) Loses electrons
(C) Remains unchanged
(D) Supplies protons
Answer: (A) Gains electrons
Q36. What is the oxidation number of N in NO₂⁺?
(A) +3
(B) +4
(C) +5
(D) +1
Answer: (C) +5
Q37. Identify the correct redox reaction among the following:
(A) CaCO₃ → CaO + CO₂
(B) AgNO₃ + NaCl → AgCl + NaNO₃
(C) Fe + CuSO₄ → FeSO₄ + Cu
(D) HCl + NaOH → NaCl + H₂O
Answer: (C) Fe + CuSO₄ → FeSO₄ + Cu
Q38. Which reaction is an example of disproportionation?
(A) 2Na + Cl₂ → 2NaCl
(B) Cl₂ + H₂O → HCl + HClO
(C) H₂ + O₂ → H₂O
(D) 2H₂ + O₂ → 2H₂O
Answer: (B) Cl₂ + H₂O → HCl + HClO
Q39. Which of the following is not a redox couple?
(A) Fe³⁺/Fe²⁺
(B) Cu²⁺/Cu
(C) Zn²⁺/Zn
(D) Na⁺/NaOH
Answer: (D) Na⁺/NaOH
Q40. Which is the best definition of oxidation?
(A) Gain of electrons
(B) Loss of electrons
(C) Gain of hydrogen
(D) Gain of protons
Answer: (B) Loss of electrons
Q41. Which pair will not produce redox reaction?
(A) Zn + CuSO₄
(B) Fe + AgNO₃
(C) H₂ + Cl₂
(D) NaOH + HCl
Answer: (D) NaOH + HCl
Q42. In which of the following oxidation number of nitrogen is maximum?
(A) NH₃
(B) N₂O
(C) NO
(D) HNO₃
Answer: (D) HNO₃
Q43. Which of the following is amphoteric oxide?
(A) Al₂O₃
(B) MgO
(C) Na₂O
(D) SO₂
Answer: (A) Al₂O₃
Q44. What is the n-factor of Fe²⁺ → Fe³⁺?
(A) 1
(B) 2
(C) 3
(D) 0
Answer: (A) 1
Q45. In a redox reaction, the species that gets reduced:
(A) Loses electrons
(B) Gains electrons
(C) Remains unchanged
(D) Becomes basic
Answer: (B) Gains electrons
Q46. Identify the redox reaction:
(A) NH₄Cl → NH₃ + HCl
(B) H₂ + I₂ → 2HI
(C) AgNO₃ + KCl → AgCl + KNO₃
(D) CaO + CO₂ → CaCO₃
Answer: (B) H₂ + I₂ → 2HI
Q47. Which of the following is used in volumetric analysis of reducing agents?
(A) KMnO₄
(B) HCl
(C) NaOH
(D) NH₄OH
Answer: (A) KMnO₄
Q48. Reducing nature of halogen acids increases in order:
(A) HF < HCl < HBr < HI
(B) HI < HBr < HCl < HF
(C) HCl < HF < HBr < HI
(D) HF < HI < HCl < HBr
Answer: (A) HF < HCl < HBr < HI
Q49. Disproportionation is shown by:
(A) HNO₃
(B) NH₄Cl
(C) HClO
(D) H₂S
Answer: (C) HClO
Q50. What is the oxidation number of chlorine in ClO₂⁻?
(A) +3
(B) +5
(C) +1
(D) +4
Answer: (D) +3
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MISCONCEPTIONS “ALERTS”
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KNOWLEDGE WITH FUN
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MNEMONICS
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