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Chemistry · Class 11 Science

Ch 10Redox Reactions — Class 11 Chemistry, concept-first.

The oldest working definition of these reactions comes directly from the two elements chemists met most often in the laboratory: oxygen and hydrogen.

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Key concepts

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Classical Concept of Oxidation and Reduction

Before oxidation numbers or electrons were used to define redox reactions, chemists used substance-based definitions.

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Chapter contents

The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.

9.1

Classical Concept of Oxidation and Reduction

The oldest working definition of these reactions comes directly from the two elements chemists met most often in the laboratory: oxygen and hydrogen.

9.2

Redox Reactions in Terms of Electron Transfer

The classical oxygen/hydrogen definition works well for the reactions historically used to discover redox chemistry, but it cannot describe a reaction like , where neither oxygen nor hydrogen appears…

9.3

Oxidation Number: Definition and Rules for Assignment

Directly counting electrons transferred works cleanly for simple ionic reactions like , but many redox reactions involve covalently bonded species — , , — where no whole electron is ever fully transfe…

9.4

Types of Redox Reactions

Once a reaction is confirmed to be redox — some element's oxidation number changes — it usually falls into one of a small number of recognizable structural patterns, based on how many reactants and pr…

9.5

Balancing Redox Reactions by the Oxidation-Number Method

The oxidation-number method balances a redox equation directly, by tracking the total change in oxidation number for every atom that changes state, without ever splitting the equation into separate ha…

9.6

Balancing Redox Reactions by the Ion-Electron (Half-Reaction) Method

The ion-electron method (also called the half-reaction method) is the more systematic and more widely applicable of the two balancing techniques, and it is the one that generalizes cleanly to both aci…

Worked Example: Balancing MnO4⁻ + C2O4²⁻ by the Ion-Electron Method

9.7

Redox Reactions as the Basis of Titrations: Equivalence and Normality

A redox titration uses a balanced redox equation the same way an acid-base titration uses a neutralization equation: as an exact stoichiometric bridge between a known volume and concentration of one r…

9.8

Permanganometry: Redox Titrations Using Potassium Permanganate

Permanganometry is redox titrimetry using standardized potassium permanganate, , as the oxidizing titrant.

9.9

Dichromatometry: Redox Titrations Using Potassium Dichromate

Dichromatometry is redox titrimetry using standardized potassium dichromate, , as the oxidizing titrant, following the half-reaction:

Summary

- Classical definition: oxidation = addition of oxygen / removal of hydrogen; reduction = the reverse. Both always occur together in the same reaction.

Sample & Board Papers

Sample papers and previous-year board questions for this subject.

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  1. Q1(i) What is the oxidation number of Mn in K₂MnO₄? (ii) Balance the following chemical equation by ion-electron method: Cr₂O₇²⁻ + Fe²⁺ + H⁺ →…Preview

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  1. Example 1Using the classical (pre-electronic) idea of oxidation and reduction, explain why the reaction $2\text{Mg}(s) + \text{O}_2(g) \to 2\text{MgO…Free
  2. Example 3Write the two electron-transfer half-reactions for $\text{Zn}(s) + \text{Cu}^{2+}(aq) \to \text{Zn}^{2+}(aq) + \text{Cu}(s)$, label each as…Free
  3. Example 5Calculate (a) the average oxidation number of $\text{Fe}$ in $\text{Fe}_3\text{O}_4$, and (b) the oxidation number of $\text{Cl}$ in each of…Free
  4. Example 7Balance the following ionic equation in acidic medium by the oxidation-number-change method: $\text{Fe}^{2+} + \text{Cr}_2\text{O}_7^{2-} \t…Preview
  5. Example 9Balance the following ionic equation in acidic medium by the ion-electron (half-reaction) method, showing both half-reactions separately: $\…Preview
  6. Example 11Balance the following ionic equation in basic (alkaline) medium by the ion-electron method: $\text{MnO}_4^{-} + \text{I}^{-} \to \text{MnO}_…Preview
  7. Example 13The reaction of cold, dilute $\text{NaOH}$ with $\text{Cl}_2$ is $\text{Cl}_2 + 2\text{NaOH} \to \text{NaCl} + \text{NaOCl} + \text{H}_2\tex…Preview
  8. Example 15In an acidified titration, $25.0\ \text{mL}$ of an $\text{FeSO}_4$ solution exactly reacted with $20.0\ \text{mL}$ of $0.0200\ \text{M}\ \te…Preview
  9. Example 17A $1.00\ \text{g}$ sample of an iron ore, containing iron entirely as $\text{Fe}^{2+}$, is dissolved and titrated against $0.0167\ \text{M}\…Preview
  10. Example 19Calculate the equivalent weight of $\text{KMnO}_4$ (molar mass $158\ \text{g mol}^{-1}$) when it acts as an oxidant in (a) strongly acidic m…Preview
+Show 10 questions10 questions
  1. Q2In the reaction $\text{Zn}(s) + \text{CuSO}_4(aq) \to \text{ZnSO}_4(aq) + \text{Cu}(s)$, identify the oxidizing agent and the reducing agent…Free
  2. Q4Calculate the oxidation number of the underlined atom in each of the following: (a) $\underline{\text{Mn}}$ in $\text{KMnO}_4$, (b) $\underl…Free
  3. Q6State, with reasons based on oxidation-number change, whether each of the following is a redox reaction: (a) $\text{NaCl}(aq) + \text{AgNO}_…Free
  4. Q8Balance the following ionic equation in acidic medium by the oxidation-number-change method: $\text{MnO}_4^{-} + \text{Fe}^{2+} \to \text{Mn…Preview
  5. Q10Balance the following ionic equation in acidic medium by the ion-electron method, showing both half-reactions separately: $\text{Cr}_2\text{…Preview
  6. Q12Balance the following ionic equation in acidic medium by the ion-electron method: $\text{Zn} + \text{NO}_3^{-} \to \text{Zn}^{2+} + \text{NH…Preview
  7. Q14Show that $2\text{Cu}^{+} \to \text{Cu} + \text{Cu}^{2+}$ is a disproportionation reaction by tracking the oxidation number of copper on bot…Preview
  8. Q16A $0.250\ \text{g}$ sample of pure oxalic acid ($\text{H}_2\text{C}_2\text{O}_4\cdot 2\text{H}_2\text{O}$, molar mass $126\ \text{g mol}^{-1…Preview
  9. Q18Calculate the equivalent weight of potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$, molar mass $294\ \text{g mol}^{-1}$) when it act…Preview
  10. Q20Explain why $\text{KMnO}_4$ can act as its own (self) indicator in a redox titration performed in acidic medium. Write the ionic half-equati…Preview