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Redox reaction revision notes

Redox Reactions & Titrations (NCERT Class 11 Chemistry) - Chemca

Redox Reactions

112 Short Q&A • NCERT Class 11 Chemistry

Class 11 JEE / NEET
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112 Short Q&A for Redox Reactions

Note: Volumetric Analysis (Redox and Acid-Base Titrations) is crucial for understanding stoichiometry and solution preparation.

    Part 1: Basic Definitions and Oxidation Number
  1. Q1: What is the historical definition of Oxidation?
    A: Addition of oxygen or removal of hydrogen.
  2. Q2: What is the historical definition of Reduction?
    A: Removal of oxygen or addition of hydrogen.
  3. Q3: What is the modern definition of Oxidation (electron concept)?
    A: Loss of electrons.
  4. Q4: What is the modern definition of Reduction (electron concept)?
    A: Gain of electrons.
  5. Q5: What is a Redox Reaction?
    A: A reaction in which both oxidation and reduction occur simultaneously.
  6. Q6: What is an Oxidising Agent (or Oxidant)?
    A: The substance that accepts electrons and causes the oxidation of another substance.
  7. Q7: What is a Reducing Agent (or Reductant)?
    A: The substance that donates electrons and causes the reduction of another substance.
  8. Q8: Does an oxidising agent get oxidised or reduced?
    A: It gets Reduced.
  9. Q9: Does a reducing agent get oxidised or reduced?
    A: It gets Oxidised.
  10. Q10: Define Oxidation Number (O.N.).
    A: The charge that an atom would carry if all bonds were ionic.
  11. Q11: What is the O.N. of any element in its free or uncombined state?
    A: Zero (e.g., Na, O2, S8).
  12. Q12: What is the O.N. of Hydrogen in most compounds?
    A: +1.
  13. Q13: What is the O.N. of Hydrogen in metal hydrides (e.g., NaH)?
    A: -1.
  14. Q14: What is the O.N. of Oxygen in most compounds?
    A: -2.
  15. Q15: What is the O.N. of Oxygen in peroxides (e.g., H2O2)?
    A: -1.
  16. Q16: What is the O.N. of Oxygen in superoxides (e.g., KO2)?
    A: -1/2.
  17. Q17: What is the fixed O.N. of alkali metals (Group 1)?
    A: +1.
  18. Q18: What is the fixed O.N. of alkaline earth metals (Group 2)?
    A: +2.
  19. Q19: What is the O.N. of Fluorine in all its compounds?
    A: -1.
  20. Q20: Calculate the O.N. of Cr in K2Cr2O7.
    A: +6.
  21. Q21: Calculate the O.N. of Mn in KMnO4.
    A: +7.
  22. Q22: Calculate the O.N. of S in H2SO4.
    A: +6.
  23. Q23: Calculate the O.N. of P in H3PO4.
    A: +5.
  24. Q24: What is the O.N. of the central S atom in S2O32− (Thiosulphate)?
    A: +2 (Average O.N.).
  25. Q25: What is the maximum O.N. an element in Group 17 (Halogens) can exhibit?
    A: +7 (e.g., in HClO4).
  26. Q26: What is the maximum O.N. of an element equal to?
    A: Its Group Number (for s and p-block elements).
  27. Q27: In Fe3+ → Fe2+, is this oxidation or reduction?
    A: Reduction (gain of electron, O.N. decreases).
  28. Q28: In S2− → S, is this oxidation or reduction?
    A: Oxidation (loss of electron, O.N. increases).
  29. Q29: Identify the reducing agent in Zn + Cu2+ → Zn2+ + Cu.
    A: Zn (Zinc, it loses electrons/gets oxidised).
  30. Q30: Identify the oxidising agent in the previous reaction.
    A: Cu2+ (Copper ion, it gains electrons/gets reduced).
    31. Part 2: Types of Redox Reactions and Standard Potential
  31. Q31: What is a Combination Reaction?
    A: Two or more substances combine to form a single compound (e.g., C + O2 → CO2).
  32. Q32: What is a Decomposition Reaction?
    A: A single compound breaks down into two or more simpler substances (e.g., CaCO3 → CaO + CO2).
  33. Q33: What is a Displacement Reaction?
    A: An atom or ion in a compound is replaced by an atom or ion of another element.
  34. Q34: What are the two main types of displacement reactions?
    A: Metal displacement and Non-metal displacement.
  35. Q35: What is a Disproportionation Reaction (or Auto-oxidation)?
    A: A single element is simultaneously oxidised and reduced (e.g., P4 reaction with NaOH).
  36. Q36: Give an example of a disproportionation reaction.
    A: 2H2O2 → 2H2O + O2 (O.N. of O changes from -1 to -2 and 0).
  37. Q37: What is a Competitive Electron Transfer Reaction?
    A: A reaction where one species has a greater tendency to accept electrons than another (e.g., in an electrochemical cell).
  38. Q38: Define Standard Electrode Potential (E°).
    A: The potential difference developed between an electrode and its electrolyte when the components are in their standard states.
  39. Q39: What is the potential of the Standard Hydrogen Electrode (SHE)?
    A: Zero Volt (0.00 V), by convention.
  40. Q40: What does a positive E° value indicate about a species?
    A: It is a stronger oxidising agent than H+ (more easily reduced).
  41. Q41: What does a negative E° value indicate about a species?
    A: It is a stronger reducing agent than H2 (more easily oxidised).
  42. Q42: How is the EMF (or E°cell) of a cell calculated?
    A: E°cell = E°cathode - E°anode (both are reduction potentials).
  43. Q43: What is the condition for a redox reaction to be spontaneous?
    A: E°cell must be positive (>0).
  44. Q44: Which electrode in a galvanic cell has a higher reduction potential?
    A: The Cathode (where reduction occurs).
  45. Q45: What is the relationship between E°cell and Gibbs energy (ΔG°)?
    A: ΔG° = -nFE°cell (n is electrons, F is Faraday constant).
  46. Q46: What is the value of Faraday's Constant (F)?
    A: 96487 C mol−1 (approx. 96500 C mol−1).
  47. Q47: In a galvanic cell, does the electron flow from the anode to the cathode or vice versa?
    A: From the Anode (negative) to the Cathode (positive) through the external circuit.
    48. Part 3: Balancing Redox Equations
  48. Q48: What are the two main methods for balancing redox reactions?
    A: Oxidation Number Method and Ion-Electron Method (or Half-Reaction Method).
  49. Q49: In the Oxidation Number Method, what quantity is balanced first?
    A: The total increase and decrease in oxidation numbers.
  50. Q50: In the Ion-Electron Method, what are the two separate equations called?
    A: Oxidation Half-Reaction and Reduction Half-Reaction.
  51. Q51: How are the atoms of O balanced in the Ion-Electron Method in an acidic medium?
    A: By adding H2O to the side deficient in O.
  52. Q52: How are the atoms of H balanced in the Ion-Electron Method in an acidic medium?
    A: By adding H+ ions to the side deficient in H.
  53. Q53: How are the atoms of O balanced in the Ion-Electron Method in a basic medium?
    A: By adding H2O (and OH− to the other side).
  54. Q54: How are the atoms of H balanced in the Ion-Electron Method in a basic medium?
    A: By adding H2O (and OH− to the opposite side).
  55. Q55: After balancing atoms, what must be balanced in the final step of the Ion-Electron Method?
    A: The Charges (by adding electrons).
  56. Q56: What must be equal before adding the two half-reactions together?
    A: The number of electrons lost in oxidation and the number of electrons gained in reduction.
  57. Q57: In the reaction MnO4− → Mn2+ (acidic), how many electrons are needed?
    A: Five electrons (O.N. change from +7 to +2).
  58. Q58: In the reaction Cr2O72− → 2Cr3+ (acidic), how many electrons are needed?
    A: Six electrons (Total O.N. change from 2 × (+6) to 2 × (+3) is -6).
  59. Q59: In a balanced half-reaction, what is conserved?
    A: Mass (atoms) and Charge.
    60. Part 4: Electrochemical Cells and Applications
  60. Q60: What is a Galvanic (Voltaic) Cell?
    A: A device that converts chemical energy of a spontaneous redox reaction into electrical energy.
  61. Q61: What is an Electrolytic Cell?
    A: A device that uses electrical energy to drive a non-spontaneous redox reaction.
  62. Q62: What is the function of the Salt Bridge in a galvanic cell?
    A: To maintain electrical neutrality and complete the internal circuit.
  63. Q63: In a galvanic cell, which electrode is negative: anode or cathode?
    A: The Anode (source of electrons).
  64. Q64: In an electrolytic cell, which electrode is negative: anode or cathode?
    A: The Cathode (connected to the negative terminal of the external source).
  65. Q65: Where does Oxidation occur in both galvanic and electrolytic cells?
    A: At the Anode.
  66. Q66: Where does Reduction occur in both galvanic and electrolytic cells?
    A: At the Cathode.
  67. Q67: What is the common name for the cell Zn | Zn2+ || Cu2+ | Cu ?
    A: The Daniell Cell.
  68. Q68: What is Electrolysis?
    A: The process of chemical decomposition of an electrolyte by the passage of direct current.
  69. Q69: Which law relates the amount of substance deposited during electrolysis to the charge passed?
    A: Faraday's Laws of Electrolysis.
  70. Q70: How is EMF different from potential difference?
    A: EMF is the maximum potential difference when no current is drawn; PD is when current flows.
  71. Q71: What is the purpose of electroplating?
    A: To deposit a thin layer of a more desirable metal onto a surface.
    72. Part 5: Advanced Oxidation Number and Periodic Trends
  72. Q72: What is the O.N. of Fe in Fe3O4?
    A: +8/3 (Average O.N., it's a mixed oxide FeO · Fe2O3).
  73. Q73: What is the O.N. of Cl in ClO3−?
    A: +5.
  74. Q74: What is the lowest O.N. that nitrogen (Group 15) can exhibit?
    A: -3 (e.g., in NH3).
  75. Q75: What is the highest O.N. that nitrogen (Group 15) can exhibit?
    A: +5 (e.g., in HNO3).
  76. Q76: Can N in HNO3 act as a reducing agent? Why?
    A: No, it is already in its highest O.N. (+5) and can only be reduced.
  77. Q77: Can N in HNO2 act as both an oxidising and reducing agent?
    A: Yes, its O.N. is +3 (intermediate state).
  78. Q78: Which element is the strongest oxidising agent?
    A: Fluorine (F2).
  79. Q79: Which element is the strongest reducing agent?
    A: Lithium (Li) (in aqueous solution).
  80. Q80: How does the oxidising power of halogens change down the group (F2 → I2)?
    A: It decreases (F2 is the strongest).
  81. Q81: How does the reducing power of alkali metals change down the group (Li → Cs)?
    A: It increases (excluding Li, based on standard reduction potentials).
  82. Q82: Which species is generally a stronger reducing agent: Fe2+ or Fe?
    A: Fe (Neutral iron).
  83. Q83: Which species is generally a stronger oxidising agent: Cl2 or I2?
    A: Cl2 (Higher positive reduction potential).
  84. Q84: What kind of reaction occurs when a metal (M) reacts with H2SO4 to form MSO4 and H2?
    A: Displacement Redox Reaction.
  85. Q85: Why is NO an exception to the rules of O.N.?
    A: It has a fractional average formal oxidation state, but actually N is +2, O is -2.
  86. Q86: What is the O.N. of C in CH4?
    A: -4.
  87. Q87: What is the O.N. of C in CCl4?
    A: +4.
  88. Q88: What is the O.N. of C in CO?
    A: +2.
    89. Part 6: Stoichiometry and Redox Titrations
  89. Q89: What is the term for a titration based on a redox reaction?
    A: Redox Titration.
  90. Q90: What is the key requirement for using the mole concept in redox titrations?
    A: The balanced redox equation (stoichiometry).
  91. Q91: Define the Equivalent Weight of an oxidizing or reducing agent.
    A: Molar Mass divided by the change in oxidation number or n-factor.
  92. Q92: What is the n-factor (change in O.N.) for KMnO4 in acidic medium (→ Mn2+)?
    A: 5 (+7 to +2).
  93. Q93: What is the n-factor for KMnO4 in neutral/basic medium (→ MnO2)?
    A: 3 (+7 to +4).
  94. Q94: What is the n-factor for Fe2+ → Fe3+?
    A: 1.
  95. Q95: What is the n-factor for C2O42− (oxalate) → 2CO2?
    A: 2 (Change of +3 → +4 for each C, total 2 × 1).
  96. Q96: Define Normality (N) of a solution.
    A: Number of gram equivalents of solute per litre of solution.
  97. Q97: What is the relationship between Normality and Molarity?
    A: N = M × n-factor (where n is the valency factor).
  98. Q98: In a titration, what is the key relationship used between the two reactants?
    A: N1V1 = N2V2 (Equivalents are equal at the endpoint).
  99. Q99: Name a common indicator used in KMnO4 titrations.
    A: KMnO4 itself (permanganate is self-indicating).
  100. Q100: What is the equivalence point in a titration?
    A: The point where the equivalents of oxidant equal the equivalents of reductant.
  101. Q101: What is the primary difference between a redox titration and an acid-base titration?
    A: Redox involves change in O.N./electron transfer; acid-base involves proton transfer.
  102. Q102: What is the standard concentration for SHE's H+ ion?
    A: 1 M.
  103. Q103: What is the O.N. of Cl in Ca(OCl)2?
    A: +1 (Average O.N., it is a mixed salt).
  104. Q104: What is the O.N. of C in C6H12O6 (glucose)?
    A: 0 (Average O.N.).
  105. Q105: Which compound acts as an oxidant in the formation of H2O from H2 + O2?
    A: O2 (Oxygen).
  106. Q106: Why is F2 the strongest oxidant?
    A: It has the highest tendency to gain electrons (highest EN and highly negative ΔegH).
  107. Q107: When a metal wire is placed in a solution of its own ions, what equilibrium is established?
    A: Mn+(aq) + ne− ⇌ M(s) (Redox half-reaction).
  108. Q108: What process occurs at the electrode with the more negative standard reduction potential?
    A: Oxidation (it acts as the anode).
  109. Q109: What is the O.N. of a monoatomic ion equal to?
    A: The charge of the ion (e.g., Na+ is +1).
  110. Q110: Is the reaction NaCl + AgNO3 → AgCl + NaNO3 a redox reaction?
    A: No, it is a double displacement reaction; no change in O.N. occurs.
  111. Q111: What is the term for a compound that can act as both an oxidising agent and a reducing agent?
    A: A compound in an intermediate oxidation state.
  112. Q112: What is the relationship between the spontaneity of a redox reaction and ΔG°?
    A: Spontaneous if ΔG° < 0 (Negative).

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