Chemistry · Class 11 Science
Ch 7Redox Reactions — Class 11 Chemistry, concept-first.
Chemistry deals with the enormous variety of matter and the many ways one kind of matter transforms into another. Redox reactions — reactions in which oxidation and reduction occur together — are one of the most important classes of these transformations.
Key concepts
Hover a concept to preview it and jump to its most relevant Q&A.
Oxidation Reduction
Let’s start with something you already know from everyday life.
Most relevant Q&A
- In the reactions given below, identify the species undergoing oxidation and reduction: (i) H2S(g) + Cl2(g) → 2HCl(g) + S(s) (ii) 3Fe3O4(s) +…Preview
- Justify that the reaction: 2Na(s) + H2(g) → 2NaH(s) is a redox change.Preview
- Justify that the reaction: 2Cu2O(s) + Cu2S(s) → 6Cu(s) + SO2(g) is a redox reaction. Identify the species oxidised/reduced, which acts as an…Preview
- Which of the following species, do not show disproportionation reaction and why ? ClO–, ClO2–, ClO3– and ClO4– Also write reaction for each…Free
- Suggest a scheme of classification of the following redox reactions (a) N2(g) + O2(g) → 2NO(g) (b) 2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g)…Preview
Chapter contents
The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.
Introduction
Chemistry deals with the enormous variety of matter and the many ways one kind of matter transforms into another.
Classical Idea of Redox Reactions – Oxidation and Reduction Reactions
The story of redox reactions begins long before electrons were discovered. Early chemists noticed that many substances changed dramatically when they reacted with the air around them.
Redox Reactions in Terms of Electron Transfer Reactions
The classical definitions of oxidation and reduction — based on addition or removal of oxygen or hydrogen — work well for many reactions, but they fail to explain what is fundamentally happening at th…
Competitive Electron Transfer Reactions
When a strip of metallic zinc is placed in an aqueous solution of copper nitrate, a striking change occurs within about an hour.
Oxidation Number
The idea of oxidation number was developed as a practical bookkeeping tool to track electron shifts in reactions, especially those involving covalent compounds. Consider the formation of water:
+−Problemsi2 questions
Types of Redox Reactions
Redox reactions are not all the same — chemists classify them into four categories based on the pattern of electron transfer: combination, decomposition, displacement and disproportionation reactions.…
+−Problemsi3 questions
- Problem 7.5Which of the following species, do not show disproportionation reaction and why ? ClO–, ClO2–, ClO3– and ClO4– Also write reaction for each…Free
- Problem 7.6Suggest a scheme of classification of the following redox reactions (a) N2(g) + O2(g) → 2NO(g) (b) 2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g)…Preview
- Problem 7.7Why do the following reactions proceed differently ? Pb3O4 + 8HCl → 3PbCl2 + Cl2 + 4H2O and Pb3O4 + 4HNO3 → 2Pb(NO3)2 + PbO2 + 2H2OPreview
Combination reactions
A combination reaction has the general form:
Decomposition reactions
Decomposition reactions are the reverse of combination reactions. A compound breaks down into two or more simpler substances, and at least one of the products must be in the elemental state (oxidation…
Displacement reactions
In a displacement reaction, an ion (or atom) in a compound is replaced by an ion (or atom) of another element. The general form is:
Metal displacement
A metal in its elemental form displaces another metal from its compound. The displacing metal must be a stronger reducing agent (more capable of losing electrons) than the metal being displaced.
Non-metal displacement
This category includes hydrogen displacement and, more rarely, oxygen displacement.
Disproportionation reactions
Disproportionation reactions are a special type of redox reaction where the same element is simultaneously oxidised and reduced. One substance acts as both the oxidising agent and the reducing agent.
The Paradox of Fractional Oxidation Number
You already know from §7.3 that oxidation numbers count whole electrons — an atom gains, loses or shares whole electrons, never a fraction of one.
Balancing of Redox Reactions
Chemical equations for redox reactions cannot be balanced by simple inspection, because both mass and charge must be conserved at the same time. Two systematic methods exist.
+−Problemsi3 questions
- Problem 7.8Write the net ionic equation for the reaction of potassium dichromate(VI), K2Cr2O7 with sodium sulphite, Na2SO3, in an acid solution to give…Free
- Problem 7.9Permanganate ion reacts with bromide ion in basic medium to give manganese dioxide and bromate ion. Write the balanced ionic equation for th…Preview
- Problem 7.10Permanganate(VII) ion, MnO4–, in basic medium, oxidises iodide ion (I–) to produce molecular iodine (I2) and manganese dioxide (MnO2). Write…Preview
Redox Reactions as the Basis for Titrations
Just as acid-base titrations use a pH indicator to signal the neutralisation point, redox titrations use the transfer of electrons to determine the concentration of an oxidising or reducing agent.
Limitations of Concept of Oxidation Number
As the discussion through this chapter shows, the concept of a redox process has been evolving with time — and that evolution is still continuing.
Redox Reactions and Electrode Processes
The reaction between zinc metal and copper(II) sulphate — zinc displacing copper — is a classic redox process.
Summary
- Redox reactions form an important class of reactions in which oxidation and reduction occur simultaneously.
Exercises
The chapter closes with 30 end-of-chapter exercise questions that give you practice across every idea covered above — assigning oxidation numbers, classifying and balancing redox reactions, applying s…
+−Exercisesi30 questions
- 7.1Assign oxidation number to the underlined elements in each of the following species: (a) NaH2P̲O4 (b) NaHS̲O4 (c) H4P̲2O7 (d) K2M̲n̲O4 (e) C…Free
- 7.2What are the oxidation number of the underlined elements in each of the following and how do you rationalise your results ? (a) KI̲3 (b) H2S…Free
- 7.3Justify that the following reactions are redox reactions: (a) CuO(s) + H2(g) → Cu(s) + H2O(g) (b) Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g) (c) 4…Free
- 7.4Fluorine reacts with ice and results in the change: H2O(s) + F2(g) → HF(g) + HOF(g) Justify that this reaction is a redox reaction.Preview
- 7.5Calculate the oxidation number of sulphur, chromium and nitrogen in H2SO5, Cr2O7 2– and NO3 –. Suggest structure of these compounds. Count f…Preview
- 7.6Write formulas for the following compounds: (a) Mercury(II) chloride (b) Nickel(II) sulphate (c) Tin(IV) oxide (d) Thallium(I) sulphate (e)…Preview
- 7.7Suggest a list of the substances where carbon can exhibit oxidation states from –4 to +4 and nitrogen from –3 to +5.Preview
- 7.8While sulphur dioxide and hydrogen peroxide can act as oxidising as well as reducing agents in their reactions, ozone and nitric acid act on…Preview
- 7.9Consider the reactions: (a) 6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g) (b) O3(g) + H2O2(l) → H2O(l) + 2O2(g) Why it is more appropriate to wri…Preview
- 7.10The compound AgF2 is unstable compound. However, if formed, the compound acts as a very strong oxidising agent. Why ?Preview
- 7.11Whenever a reaction between an oxidising agent and a reducing agent is carried out, a compound of lower oxidation state is formed if the red…Preview
- 7.12How do you count for the following observations ? (a) Though alkaline potassium permanganate and acidic potassium permanganate both are used…Preview
- 7.13Identify the substance oxidised reduced, oxidising agent and reducing agent for each of the following reactions: (a) 2AgBr(s) + C6H6O2(aq) →…Preview
- 7.14Consider the reactions : 2S2O3 2–(aq) + I2(s) → S4O6 2–(aq) + 2I–(aq) S2O3 2–(aq) + 2Br2(l) + 5H2O(l) → 2SO4 2–(aq) + 4Br–(aq) + 10H+(aq) Wh…Preview
- 7.15Justify giving reactions that among halogens, fluorine is the best oxidant and among hydrohalic compounds, hydroiodic acid is the best reduc…Preview
- 7.16Why does the following reaction occur ? XeO6 4–(aq) + 2F–(aq) + 6H+(aq) → XeO3(g) + F2(g) + 3H2O(l) What conclusion about the compound Na4Xe…Preview
- 7.17Consider the reactions: (a) H3PO2(aq) + 4AgNO3(aq) + 2H2O(l) → H3PO4(aq) + 4Ag(s) + 4HNO3(aq) (b) H3PO2(aq) + 2CuSO4(aq) + 2H2O(l) → H3PO4(a…Preview
- 7.18Balance the following redox reactions by ion-electron method: (a) MnO4– (aq) + I– (aq) → MnO2 (s) + I2 (s) (in basic medium) (b) MnO4– (aq)…Preview
- 7.19Balance the following equations in basic medium by ion-electron method and oxidation number methods and identify the oxidising agent and the…Preview
- 7.20What sorts of informations can you draw from the following reaction ? (CN)2(g) + 2OH–(aq) → CN–(aq) + CNO–(aq) + H2O(l)Preview
- 7.21The Mn3+ ion is unstable in solution and undergoes disproportionation to give Mn2+, MnO2, and H+ ion. Write a balanced ionic equation for th…Preview
- 7.22Consider the elements : Cs, Ne, I and F (a) Identify the element that exhibits only negative oxidation state. (b) Identify the element that…Preview
- 7.23Chlorine is used to purify drinking water. Excess of chlorine is harmful. The excess of chlorine is removed by treating with sulphur dioxide…Preview
- 7.24Refer to the periodic table given in your book and now answer the following questions: (a) Select the possible non metals that can show disp…Preview
- 7.25In Ostwald’s process for the manufacture of nitric acid, the first step involves the oxidation of ammonia gas by oxygen gas to give nitric o…Preview
- 7.26Using the standard electrode potentials given in the Table 8.1, predict if the reaction between the following is feasible: (a) Fe3+(aq) and…Preview
- 7.27Predict the products of electrolysis in each of the following: (i) An aqueous solution of AgNO3 with silver electrodes (ii) An aqueous solut…Preview
- 7.28Arrange the following metals in the order in which they displace each other from the solution of their salts. Al, Cu, Fe, Mg and Zn.Preview
- 7.29Given the standard electrode potentials, K+/K = –2.93 V, Ag+/Ag = 0.80 V, Hg2+/Hg = 0.79 V, Mg2+/Mg = –2.37 V, Cr3+/Cr = –0.74 V, arrange th…Preview
- 7.30Depict the galvanic cell in which the reaction Zn(s) + 2Ag +(aq) → Zn2+(aq) +2Ag(s) takes place, Further show: (i) which of the electrode is…Preview
NCERT Exemplar
Higher-order thinking problems from the NCERT Exemplar.
+−Show 38 questionsHide questions38 questions
- Q1Which of the following is not an example of redox reaction? (i) CuO + H2 → Cu + H2O (ii) Fe2O3 + 3CO → 2Fe + 3CO2 (iii) 2K + F2 → 2KF (iv) B…Free
- Q2The more positive the value of E⊖, the greater is the tendency of the species to get reduced. Using the standard electrode potential of redo…Free
- Q3E⊖ values of some redox couples are given below. On the basis of these values choose the correct option. E⊖ values: Br2/Br^- = +1.90; Ag^+/A…Free
- Q4Using the standard electrode potential, find out the pair between which redox reaction is not feasible. E⊖ values: Fe^3+/Fe^2+ = +0.77; I2/I…Preview
- Q5Thiosulphate reacts differently with iodine and bromine in the reactions given below: 2S2O3^2- + I2 → S4O6^2- + 2I^- S2O3^2- + 2Br2 + 5H2O →…Preview
- Q6The oxidation number of an element in a compound is evaluated on the basis of certain rules. Which of the following rules is not correct in…Preview
- Q7In which of the following compounds, an element exhibits two different oxidation states. (i) NH2OH (ii) NH4NO3 (iii) N2H4 (iv) N3HPreview
- Q8Which of the following arrangements represent increasing oxidation number of the central atom? (i) CrO2^- , ClO3^- , CrO4^2- , MnO4^- (ii) C…Preview
- Q9The largest oxidation number exhibited by an element depends on its outer electronic configuration. With which of the following outer electr…Preview
- Q10Identify disproportionation reaction (i) CH4 + 2O2 → CO2 + 2H2O (ii) CH4 + 4Cl2 → CCl4 + 4HCl (iii) 2F2 + 2OH^- → 2F^- + OF2 + H2O (iv) 2NO2…Preview
- Q11Which of the following elements does not show disproportionation tendency? (i) Cl (ii) Br (iii) F (iv) IPreview
- Q12Which of the following statement(s) is/are not true about the following decomposition reaction. 2KClO3 → 2KCl + 3O2 (Note: more than one of…Preview
- Q13Identify the correct statement (s) in relation to the following reaction: Zn + 2HCl → ZnCl2 + H2 (Note: more than one of the given options m…Preview
- Q14The exhibition of various oxidation states by an element is also related to the outer orbital electronic configuration of its atom. Atom(s)…Preview
- Q15Identify the correct statements with reference to the given reaction P4 + 3OH^- + 3H2O → PH3 + 3H2PO2^- (Note: more than one of the given op…Preview
- Q16Which of the following electrodes will act as anodes, when connected to Standard Hydrogen Electrode? (Note: more than one of the given optio…Preview
- Q17The reaction Cl2(g) + 2OH^-(aq) → ClO^-(aq) + Cl^-(aq) + H2O(l) represents the process of bleaching. Identify and name the species that blea…Preview
- Q18MnO4^2- undergoes disproportionation reaction in acidic medium but MnO4^- does not. Give reason.Preview
- Q19PbO and PbO2 react with HCl according to following chemical equations : 2PbO + 4HCl → 2PbCl2 + 2H2O PbO2 + 4HCl → PbCl2 + Cl2 + 2H2O Why do…Preview
- Q20Nitric acid is an oxidising agent and reacts with PbO but it does not react with PbO2. Explain why?Preview
- Q21Write balanced chemical equation for the following reactions: (i) Permanganate ion (MnO4^-) reacts with sulphur dioxide gas in acidic medium…Preview
- Q22Calculate the oxidation number of phosphorus in the following species. (a) HPO3^2- and (b) PO4^3-Preview
- Q23Calculate the oxidation number of each sulphur atom in the following compounds: (a) Na2S2O3 (b) Na2S4O6 (c) Na2SO3 (d) Na2SO4Preview
- Q24Balance the following equations by the oxidation number method. (i) Fe^2+ + H^+ + Cr2O7^2- → Cr^3+ + Fe^3+ + H2O (ii) I2 + NO3^- → NO2 + IO3…Preview
- Q25Identify the redox reactions out of the following reactions and identify the oxidising and reducing agents in them. (i) 3HCl(aq) + HNO3(aq)…Preview
- Q26Balance the following ionic equations (i) Cr2O7^2- + H^+ + I^- → Cr^3+ + I2 + H2O (ii) Cr2O7^2- + Fe^2+ + H^+ → Cr^3+ + Fe^3+ + H2O (iii) Mn…Preview
- Q27Match Column I with Column II for the oxidation states of the central atoms. Column I (i) Cr2O7^2- (ii) MnO4^- (iii) VO3^- (iv) FeF6^3- Colu…Preview
- Q28Match the items in Column I with relevant items in Column II. Column I (i) Ions having positive charge (ii) The sum of oxidation number of a…Preview
- Q29Assertion (A): Among halogens fluorine is the best oxidant. Reason (R): Fluorine is the most electronegative atom. (i) Both A and R are true…Preview
- Q30Assertion (A): In the reaction between potassium permanganate and potassium iodide, permanganate ions act as oxidising agent. Reason (R): Ox…Preview
- Q31Assertion (A): The decomposition of hydrogen peroxide to form water and oxygen is an example of disproportionation reaction. Reason (R): The…Preview
- Q32Assertion (A): Redox couple is the combination of oxidised and reduced form of a substance involved in an oxidation or reduction half cell.…Preview
- Q33Explain redox reactions on the basis of electron transfer. Give suitable examples.Preview
- Q34On the basis of standard electrode potential values, suggest which of the following reactions would take place? (Consult the book for E⊖ val…Preview
- Q35Why does fluorine not show disporportionation reaction?Preview
- Q36Write redox couples involved in the reactions (i) to (iv) given in question 34.Preview
- Q37Find out the oxidation number of chlorine in the following compounds and arrange them in increasing order of oxidation number of chlorine. N…Preview
- Q38Which method can be used to find out strength of reductant/oxidant in a solution? Explain with an example.Preview