NCERT Solutions for Class 11 Chemistry Chapter 7 Redox Reactions

Short Note:

• Classical Concept:

  - Oxidation: Addition of oxygen/electronegative element or removal of hydrogen/electropositive element.

  - Reduction: Addition of hydrogen/electropositive element or removal of oxygen/electronegative element.

• Electron Transfer Concept:

  - Oxidation: Loss of electron(s) by a chemical species (Increase in oxidation state).

  - Reduction: Gain of electron(s) by a chemical species (Decrease in oxidation state).

  - Oxidising Agent (Oxidant): Species that accepts electrons and gets reduced.

  - Reducing Agent (Reductant): Species that donates electrons and gets oxidised.

Short Note:

• Oxidation Number (O.N.): The residual charge an atom would carry in a compound if all surrounding bonds were completely ionic.

• Rules for Assigning Oxidation Number:

  - Free element / uncombined state = 0 (e.g., O₂, H₂, Na, S₈).

  - Monatomic ions = charge on the ion (e.g., Na⁺ = +1, Cl^- = -1, Al³⁺ = +3).

  - Fluorine = -1 in all compounds.

  - Oxygen = -2 generally (except -1 in peroxides like H₂O₂, +2 in OF₂).

  - Hydrogen = +1 with non-metals, -1 with metals (metal hydrides like NaH).

  - Sum of oxidation numbers in a neutral molecule = 0; in a polyatomic ion = total charge on the ion.

• Stock Notation: Indicating the oxidation state of a metal in Roman numerals within parentheses, e.g., Gold(III) chloride for AuCl₃, Iron(II) oxide for FeO.

Short Note:

• Combination Reactions: A + B → C (where at least one reactant is an element), e.g., C(s) + O₂(g) → CO₂(g).

• Decomposition Reactions: Breakdown of a compound into simpler substances, e.g., 2KClO₃(s) → 2KCl(s) + 3O₂(g).

• Displacement Reactions: X + YZ → XZ + Y.

  - Metal displacement: CuSO₄ + Zn → ZnSO₄ + Cu.

  - Non-metal displacement: 2Na + 2H₂O → 2NaOH + H₂.

• Disproportionation Reactions: Reactions in which the same element simultaneously undergoes oxidation and reduction, e.g., 2H₂O₂(aq) → 2H₂O(l) + O₂(g) (O changes from -1 to -2 and 0).

Short Note:

• Methods of Balancing Redox Equations:

  - Oxidation Number Method: Equating total increase in oxidation number with total decrease in oxidation number.

  - Ion-Electron (Half-Reaction) Method: Splitting the overall equation into oxidation and reduction half-reactions, balancing atoms and charges separately, and combining them.

• Redox Titrations: Using self-indicating reagents like KMnO₄ or K₂Cr₂O₇ for quantitatively determining reductants.

• Electrochemical Cells: Device converting chemical energy into electrical energy through redox reactions (e.g., Daniell Cell: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)). Anode = Oxidation (-), Cathode = Reduction (+).