Redox reaction revision notes

 

Short Q&A for Redox Reactions (Class 11 NCERT)

Part 1: Basic Definitions and Oxidation Number

Q. No.	Question	Answer
1	What is the historical definition of Oxidation?	Addition of oxygen or removal of hydrogen.
2	What is the historical definition of Reduction?	Removal of oxygen or addition of hydrogen.
3	What is the modern definition of Oxidation (electron concept)?	Loss of electrons.
4	What is the modern definition of Reduction (electron concept)?	Gain of electrons.
5	What is a Redox Reaction?	A reaction in which both oxidation and reduction occur simultaneously.
6	What is an Oxidising Agent (or Oxidant)?	The substance that accepts electrons and causes the oxidation of another substance.
7	What is a Reducing Agent (or Reductant)?	The substance that donates electrons and causes the reduction of another substance.
8	Does an oxidising agent get oxidised or reduced?	It gets Reduced.
9	Does a reducing agent get oxidised or reduced?	It gets Oxidised.
10	Define Oxidation Number (O.N.).	The charge that an atom would carry if all bonds were ionic.
11	What is the O.N. of any element in its free or uncombined state?	Zero (e.g., Na,O2​,S8​).
12	What is the O.N. of Hydrogen in most compounds?	+1.
13	What is the O.N. of Hydrogen in metal hydrides (e.g., NaH)?	-1
14	What is the O.N. of Oxygen in most compounds?	-2
15	What is the O.N. of Oxygen in peroxides (e.g., H2​O2​)?	-1
16	What is the O.N. of Oxygen in superoxides (e.g., KO2​)?	−1/2.
17	What is the fixed O.N. of alkali metals (Group 1)?	+1.
18	What is the fixed O.N. of alkaline earth metals (Group 2)?	+2.
19	What is the O.N. of Fluorine in all its compounds?	-1
20	Calculate the O.N. of Cr in K2​Cr2​O7​.	+6.
21	Calculate the O.N. of Mn in KMnO4​.	+7.
22	Calculate the O.N. of S in H2​SO4​.	+6.
23	Calculate the O.N. of P in H3​PO4​.	+5.
24	What is the O.N. of the central S atom in S2​O32−​ (Thiosulphate)?	+2 (Average O.N.).
25	What is the maximum O.N. an element in Group 17 (Halogens) can exhibit?	+7 (e.g., in HClO4​).
26	What is the maximum O.N. of an element equal to?	Its Group Number (for s and p-block elements).
27	In Fe3+→Fe2+, is this oxidation or reduction?	Reduction (gain of electron, O.N. decreases).
28	In S2−→S, is this oxidation or reduction?	Oxidation (loss of electron, O.N. increases).
29	Identify the reducing agent in Zn+Cu2+→Zn2++Cu.	Zn (Zinc, it loses electrons/gets oxidised).
30	Identify the oxidising agent in the previous reaction.	Cu2+ (Copper ion, it gains electrons/gets reduced).

Part 2: Types of Redox Reactions and Standard Potential

Q. No.	Question	Answer
31	What is a Combination Reaction?	Two or more substances combine to form a single compound (e.g., C+O2​→CO2​).
32	What is a Decomposition Reaction?	A single compound breaks down into two or more simpler substances (e.g., CaCO3​→CaO+CO2​).
33	What is a Displacement Reaction?	An atom or ion in a compound is replaced by an atom or ion of another element.
34	What are the two main types of displacement reactions?	Metal displacement and Non-metal displacement.
35	What is a Disproportionation Reaction (or Auto-oxidation)?	A single element is simultaneously oxidised and reduced (e.g., P4​ reaction with NaOH).
36	Give an example of a disproportionation reaction.	2H2​O2​→2H2​O+O2​ (O.N. of O changes from −1 to −2 and 0).
37	What is a Competitive Electron Transfer Reaction?	A reaction where one species has a greater tendency to accept electrons than another (e.g., in an electrochemical cell).
38	Define **Standard Electrode Potential (E∘) **.	The potential difference is developed between an electrode and its electrolyte when the components are in their standard states.
39	What is the potential of the Standard Hydrogen Electrode (SHE)?	Zero Volt (0.00 V), by convention.
40	What does a positive E∘ value indicate about a species?	It is a stronger oxidising agent than H+ (more easily reduced).
41	What does a negative E∘ value indicate about a species?	It is a stronger reducing agent than H (more easily oxidised).
42	How is the EMF (or Ecell∘​) of a cell calculated?	Ecell∘​=Ecathode∘​−Eanode∘​ (both are reduction potentials).
43	What is the condition for a redox reaction to be spontaneous?	Ecell∘​ must be positive (>0).
44	Which electrode in a galvanic cell has a higher reduction potential?	The Cathode (where reduction occurs).
45	What is the relationship between Ecell∘​ and Gibbs energy (ΔG∘)?	ΔG∘=−nFEcell∘​ (n is electrons, F is Faraday constant).
46	What is the value of Faraday's Constant (F)?	96487 C mol−1 (approx. 96500 C mol−1).
47	In a galvanic cell, does the electron flow from the anode to the cathode or vice versa?	From the Anode (negative) to the Cathode (positive) through the external circuit.

Part 3: Balancing Redox Equations

Q. No.	Question	Answer
48	What are the two main methods for balancing redox reactions?	Oxidation Number Method and Ion-Electron Method (or Half-Reaction Method).
49	In the Oxidation Number Method, what quantity is balanced first?	The total increase and decrease in oxidation numbers.
50	In the Ion-Electron Method, what are the two separate equations called?	Oxidation Half-Reaction and Reduction Half-Reaction.
51	How are the atoms of O balanced in the Ion-Electron Method in an acidic medium?	By adding H2O to the side deficient in O.
52	How are the atoms of H balanced in the Ion-Electron Method in an acidic medium?	By adding H+ ions to the side deficient in H.
53	How are the atoms of O balanced in the Ion-Electron Method in a basic medium?	By adding H2O (and OH− to the other side).
54	How are the atoms of H balanced in the Ion-Electron Method in a basic medium?	By adding H2​O (and OH− to the opposite side).
55	After balancing atoms, what must be balanced in the final step of the Ion-Electron Method?	The Charges (by adding electrons).
56	What must be equal before adding the two half-reactions together?	The number of electrons lost in oxidation and the number of electrons gained in reduction.
57	In the reaction MnO4−​→Mn2+ (acidic), how many electrons are needed?	Five electrons (O.N. change from +7 to +2).
58	In the reaction Cr2​O72−​→2Cr3+ (acidic), how many electrons are needed?	Six electrons (Total O.N. change from 2×(+6) to 2×(+3) is −6).
59	In a balanced half-reaction, what is conserved?	Mass (atoms) and Charge.

Part 4: Electrochemical Cells and Applications

Q. No.	Question	Answer
60	What is a Galvanic (Voltaic) Cell?	A device that converts chemical energy of a spontaneous redox reaction into electrical energy.
61	What is an Electrolytic Cell?	A device that uses electrical energy to drive a non-spontaneous redox reaction.
62	What is the function of the Salt Bridge in a galvanic cell?	To maintain electrical neutrality and complete the internal circuit.
63	In a galvanic cell, which electrode is negative: anode or cathode?	The Anode (source of electrons).
64	In an electrolytic cell, which electrode is negative: anode or cathode?	The Cathode (connected to the negative terminal of the external source).
65	Where does Oxidation occur in both galvanic and electrolytic cells?	At the Anode.
66	Where does Reduction occur in both galvanic and electrolytic cells?	At the Cathode.
67	What is the common name for the cell $\text{Zn}	\text{Zn}^{2+}
68	What is Electrolysis?	The process of chemical decomposition of an electrolyte by the passage of direct current.
69	Which law relates the amount of substance deposited during electrolysis to the charge passed?	Faraday's Laws of Electrolysis.
70	How is EMF different from potential difference?	EMF is the maximum potential difference when no current is drawn; PD is when current flows.
71	What is the purpose of electroplating?	To deposit a thin layer of a more desirable metal onto a surface.

Part 5: Advanced Oxidation Number and Periodic Trends

Q. No.	Question	Answer
72	What is the O.N. of Fe in Fe3​O4​?	+8/3 (Average O.N., it's a mixed oxide FeO⋅Fe2​O3​).
73	What is the O.N. of Cl in ClO3−​?	+5.
74	What is the lowest O.N. that nitrogen (Group 15) can exhibit?	−3 (e.g., in NH3​).
75	What is the highest O.N. that nitrogen (Group 15) can exhibit?	+5 (e.g., in HNO3​).
76	Can N in HNO3​ act as a reducing agent? Why?	No, it is already in its highest O.N. (+5) and can only be reduced.
77	Can N in HNO2​ act as both an oxidising and reducing agent?	Yes, its O.N. is +3 (intermediate state).
78	Which element is the strongest oxidising agent?	Fluorine (F2​).
79	Which element is the strongest reducing agent?	Lithium (Li) (in aqueous solution).
80	How does the oxidising power of halogens change down the group (F2​→I2​)?	It decreases (F2​ is the strongest).
81	How does the reducing power of alkali metals change down the group (Li→Cs)?	It increases (excluding Li, based on standard reduction potentials).
82	Which species is generally a stronger reducing agent: Fe2+ or Fe?	Fe (Neutral iron).
83	Which species is generally a stronger oxidising agent: Cl2​ or I2​?	Cl2​ (Higher positive reduction potential).
84	What kind of reaction occurs when a metal (M) reacts with H2​SO4​ to form MSO4​ and H2​?	Displacement Redox Reaction.
85	Why is NO an exception to the rules of O.N.?	It has a fractional O.N. of +2 (N is +2, O is −2).
86	What is the O.N. of C in CH4​?	-4
87	What is the O.N. of C in CCl4​?	+4.
88	What is the O.N. of C in CO?	+2.

Part 6: Stoichiometry and Redox Titrations

Q. No.	Question	Answer
89	What is the term for a titration based on a redox reaction?	Redox Titration.
90	What is the key requirement for using the mole concept in redox titrations?	The balanced redox equation (stoichiometry).
91	Define the Equivalent Weight of an oxidizing or reducing agent.	Molar Mass divided by the change in oxidation number or n-factor.
92	What is the n-factor (change in O.N.) for KMnO4​ in acidic medium (→Mn2+)?	5 (+7 to +2).
93	What is the n-factor for KMnO4​ in neutral/basic medium (→MnO2​)?	3 (+7 to +4).
94	What is the n-factor for Fe2+→Fe3+?	1
95	What is the n-factor for C2​O42−​ (oxalate) →2CO2​?	2 (Change of +3→+4 for each C, total 2×1).
96	Define Normality (N) of a solution.	Number of gram equivalents of solute per litre of solution.
97	What is the relationship between Normality and Molarity?	N=M×n (where n is the n-factor/valency).
98	In a titration, what is the key relationship used between the two reactants?	N1​V1​=N2​V2​ (Equivalents are equal at the endpoint).
99	Name a common indicator used in KMnO4​ titrations.	KMnO4​ itself (permanganate is self-indicating).
100	What is the equivalence point in a titration?	The point where the equivalents of oxidant equal the equivalents of reductant.
101	What is the primary difference between a redox titration and an acid-base titration?	Redox involves change in O.N./electron transfer; acid-base involves proton transfer.
102	What is the standard concentration for SHE's H+ ion?	1 M.
103	What is the O.N. of Cl in Ca(OCl)2​?	+1 (Average O.N., it is a mixed salt).
104	What is the O.N. of C in C6​H12​O6​ (glucose)?	0 (Average O.N.).
105	Which compound acts as an oxidant in the formation of H2​O from H2​+O2​?	O2​ (Oxygen).
106	Why is F2​ the strongest oxidant?	It has the highest tendency to gain electrons (highest EN and highly negative Δeg​H).
107	When a metal wire is placed in a solution of its own ions, what equilibrium is established?	Mn+(aq)+ne−⇌M(s) (Redox half-reaction).
108	What process occurs at the electrode with the more negative standard reduction potential?	Oxidation (it acts as the anode).
109	What is the O.N. of a monoatomic ion equal to?	The charge of the ion (e.g., Na+ is +1).
110	Is the reaction NaCl+AgNO3​→AgCl+NaNO3​ a redox reaction?	No, it is a double displacement reaction; no change in O.N. occurs.
111	What is the term for a compound that can act as both an oxidising agent and a reducing agent?	A compound in an intermediate oxidation state.
112	What is the relationship between the spontaneity of a redox reaction and ΔG∘?	Spontaneous if ΔG∘<0 (Negative).