Electrochemistry (Class 12 NCERT) Revision notes

 short question-and-answer pairs covering the essential concepts from the NCERT Class 12 Chemistry Chapter on Electrochemistry:

Short Q&A for Electrochemistry (Class 12 NCERT)

Part 1: Electrochemical Cells and Notation

Q. No.	Question	Answer
1	What is an Electrochemical Cell?	A device that converts chemical energy into electrical energy or vice versa.
2	What are the two main types of electrochemical cells?	Galvanic (or Voltaic) and Electrolytic.
3	What is a Galvanic Cell?	A cell that converts chemical energy from a spontaneous redox reaction into electrical energy.
4	What is an Electrolytic Cell?	A cell that uses electrical energy to drive a non-spontaneous chemical reaction.
5	What is the function of the Salt Bridge?	To maintain electrical neutrality and complete the inner circuit.
6	Where does Oxidation occur in both cell types?	At the Anode.
7	Where does Reduction occur in both cell types?	At the Cathode.
8	What is the sign of the Anode in a Galvanic Cell?	Negative (source of electrons).
9	What is the sign of the Cathode in a Galvanic Cell?	Positive (where electrons flow).
10	What is the standard notation for a cell (anode	electrolyte
11	Define Electrode Potential (E).	The potential difference between an electrode and its electrolyte solution.
12	Define **Standard Electrode Potential (E∘) **.	Electrode potential when all species are at 1 M concentration and 1 bar pressure.
13	What is the potential of the Standard Hydrogen Electrode (SHE)?	0.00 V (by convention).
14	What does a positive Reduction Potential indicate?	The species is a stronger oxidizing agent than H+.
15	What does a negative Reduction Potential indicate?	The species is a stronger reducing agent than H2​.
16	Write the formula for the EMF of a cell.	Ecell​=Ecathode​−Eanode​ (Both standard reduction potentials).

Part 2: Nernst Equation and Thermodynamics

Q. No.	Question	Answer
17	What is the Nernst Equation used for?	To calculate the electrode potential or cell potential at any concentration and temperature.
18	Write the Nernst equation for a single half-cell Mn++ne−→M.	E=E∘−n0.0591​log[Mn+]1​ (at 298 K).
19	What is the concentration term for the solid (M) electrode?	Unity (1) (Activity is taken as 1).
20	Write the relationship between ΔG∘ and Ecell∘​.	ΔG∘=−nFEcell∘​.
21	What is the condition for a cell reaction to be spontaneous?	Ecell​ must be positive (>0) and ΔG must be negative (<0).
22	What is the relationship between ΔG∘ and the Equilibrium Constant (Kc​)?	ΔG∘=−RTlnKc​ (or −2.303RTlogKc​).
23	Relate Ecell∘​ to the Equilibrium Constant (Kc​).	Ecell∘​=n0.0591​logKc​ (at 298 K).
24	What happens to Ecell​ when a galvanic cell reaches equilibrium?	Ecell​ becomes zero (ΔG=0).
25	In the Nernst equation, what does 'n' represent?	The number of electrons transferred in the balanced redox reaction.
26	If a salt concentration is increased at the cathode, how does Ecell​ change?	Increases (Reaction shifts forward, favoring reduction).

Part 3: Conductance and Resistance

Q. No.	Question	Answer
27	Define Resistance (R).	The obstruction to the flow of electric current.
28	What is the SI unit of Resistance?	Ohm (Ω).
29	Define Conductance (G).	The ease with which current flows (G=1/R).
30	What is the SI unit of Conductance?	Siemens (S) or Ω−1.
31	Define Resistivity (ρ) or Specific Resistance.	The resistance offered by a conductor of unit length and unit area of cross-section.
32	What is the SI unit of Resistivity?	Ohm meter(Ω m).
33	Write the relationship between R and ρ.	R=ρ(l/A) (l/A is Cell Constant).
34	Define Conductivity (κ) or Specific Conductance.	The reciprocal of resistivity (κ=1/ρ).
35	What is the SI unit of Conductivity?	Siemens meter−1(S m−1) or Ω−1 cm−1.
36	Define the Cell Constant (G∗).	The ratio of the distance (l) between the electrodes to the area (A) of the cross-section of the electrodes (G∗=l/A).
37	Write the relationship between κ,R, and G∗.	$\kappa = G^ / R$* or κ=G⋅G∗.
38	How does the Conductivity (κ) of a solution change with dilution?	Decreases (due to fewer ions per unit volume).
39	Define Molar Conductivity (Λm​).	The conductivity of a solution containing one mole of the electrolyte placed between electrodes separated by unit distance.
40	Write the formula relating Λm​ and κ.	Λm​=Cκ×1000​ (C in mol L−1, κ in S cm−1).
41	How does Molar Conductivity (Λm​) change with dilution?	Increases (due to increased ionic mobility and greater dissociation for weak electrolytes).
42	What is the unit of Λm​?	S cm2 mol−1 or S m2 mol−1.

Part 4: Kohlrausch's Law and Weak Electrolytes

Q. No.	Question	Answer
43	Define Limiting Molar Conductivity (Λm∘​)	Molar conductivity of an electrolyte when the concentration approaches zero (infinite dilution).
44	What is the Debye-Hückel-Onsager Equation used for?	To describe the variation of Λm​ with C​ for strong electrolytes.
45	State Kohlrausch's Law of Independent Migration of Ions.	Limiting molar conductivity of an electrolyte is the sum of the limiting ionic conductivities of the cation and the anion.
46	Write the mathematical form of Kohlrausch's Law for Ax​By​.	Λm∘​=xλAy+∘​+yλBx−∘​ (λ∘ is limiting ionic conductivity).
47	How is Kohlrausch's Law useful for weak electrolytes?	It allows the calculation of Λm∘​ for weak electrolytes using Λm∘​ values of strong electrolytes.
48	How is the Degree of Dissociation (α) for a weak electrolyte calculated?	α=Λm∘​Λm​​.
49	How is the Dissociation Constant (Ka​) related to α (Ostwald's Law)?	Ka​=(1−α)Cα2​ (where C is concentration).
50	What is the value of α for a strong electrolyte at all concentrations?	≈1 (Assumed to be 100% dissociated).
51	Why does the Λm​ vs C​ plot for a weak electrolyte not allow extrapolation?	Due to the sharp increase in dissociation at low concentrations.

Part 5: Electrolysis and Faraday's Laws

Q. No.	Question	Answer
52	What is Electrolysis?	The process of chemical decomposition by the passage of direct electric current through an electrolyte.
53	Where does the reduction product appear in an electrolytic cell?	At the Cathode.
54	Where does the oxidation product appear in an electrolytic cell?	At the Anode.
55	State Faraday's First Law of Electrolysis.	The amount of chemical reaction is proportional to the quantity of electricity passed.
56	State Faraday's Second Law of Electrolysis.	When the same quantity of electricity is passed through different electrolytes, the masses of substances liberated are proportional to their equivalent weights.
57	Define **One Faraday (1 F) **.	The charge carried by one mole of electrons (96487 C mol−1).
58	How much charge is required to deposit one mole of Ag from Ag+?	1 F (Ag++1e−→Ag).
59	How much charge is required to deposit one mole of Al from Al3+?	3 F (Al3++3e−→Al).
60	What is the formula to calculate the quantity of charge (Q)?	Q=I×t (Current in Amperes, time in seconds).
61	What is the mass (m) deposited in electrolysis related to Q (mathematically)?	m=ZQ (Z is the Electrochemical Equivalent).
62	What is the relationship between Z (Electrochemical Equivalent) and Equivalent Mass (E)?	Z=E/F.
63	In the electrolysis of molten NaCl, what product is formed at the anode?	Cl2​ gas.
64	In the electrolysis of aqueous NaCl, what product is formed at the cathode?	H2​ gas (due to lower reduction potential of H2​O vs Na+).
65	In the electrolysis of aqueous H2​SO4​, what product is formed at the anode?	O2​ gas.

Part 6: Batteries and Corrosion

Q. No.	Question	Answer
66	What is a Primary Battery?	A battery that cannot be recharged (reaction occurs only once).
67	Give an example of a Primary Battery.	Leclanché Cell (Dry cell) or Mercury Cell.
68	What is a Secondary Battery?	A battery that can be recharged (cell reaction can be reversed).
69	Give an example of a Secondary Battery.	Lead Storage Battery or Nickel-Cadmium Cell.
70	What is the reaction at the cathode during the discharge of a Lead Storage Battery?	PbO2​+4H++SO42−​+2e−→PbSO4​+2H2​O.
71	What is formed at both electrodes of the Lead Storage Battery during discharge?	PbSO4​ (Lead Sulphate).
72	What is a Fuel Cell?	A galvanic cell that converts the energy from combustion of fuels (like H2​,CH4​) directly into electrical energy.
73	Give an example of a Fuel Cell.	H2​−O2​ Fuel Cell.
74	What is the main advantage of a Fuel Cell?	High efficiency and non-polluting (by-product is H2​O).
75	Define Corrosion.	The process of slow decay of metal due to its reaction with air or water in the environment (an electrochemical process).
76	Give the chemical formula for rust (iron corrosion).	Fe2​O3​⋅xH2​O (Hydrated ferric oxide).
77	In the corrosion of iron, what acts as the anode?	Pure iron (where oxidation occurs).
78	In the corrosion of iron, what acts as the cathode?	Area with high O2​ concentration or an impurity (where O2​ is reduced).
79	Name a method to prevent corrosion.	Galvanization (coating with Zn), Electroplating, or Sacrificial Protection.
80	What is the most effective method of corrosion prevention?	Sacrificial protection (using a more reactive metal like Zn).

Part 7: Conceptual and Mixed Problems

Q. No.	Question	Answer
81	Why do Λm​ values for strong electrolytes decrease steeply at high concentrations?	Due to strong inter-ionic attraction between ions.
82	How does increasing the temperature affect the conductivity (κ) of a metallic conductor?	Decreases (due to increased resistance from thermal motion).
83	How does increasing the temperature affect the conductivity (κ) of an electrolytic solution?	Increases (due to increased ionic mobility).
84	What is the standard EMF of a cell where the reaction quotient Q=1?	Ecell​=Ecell∘​ (Nernst equation simplifies).
85	What is the effect of changing the size of the electrodes in a cell on Ecell​?	No effect (potential is an intensive property).
86	If ΔG∘=0, what is the value of Kc​?	Kc​=1 (Ecell∘​=0).
87	Which factor determines the order of discharge of ions at an electrode?	The Standard Reduction Potentials of the competing ions.
88	Why are Li ions the strongest reducing agent in aqueous solution despite high IE?	Due to their highest hydration enthalpy (small size).
89	In a concentration cell, what drives the cell potential?	The difference in the concentration of the electrolyte at the two half-cells.
90	What is the maximum number of decimal places allowed in the final EMF calculated using 0.0591?	Three (limited by the constant 0.0591).
91	What is the role of the porous plate in a Lead Storage Battery?	To separate the anode and cathode compartments and allow ion migration.
92	What are the typical fuels used in commercial Fuel Cells?	H2​,CH4​,CO, and Alcohols (like methanol).
93	What happens at the anode when Cu is purified by electrolysis?	Impure Cu dissolves (Cu→Cu2++2e−).
94	What happens at the cathode when Cu is purified by electrolysis?	Pure Cu is deposited (Cu2++2e−→Cu).
95	What is the condition for Ecell​ to increase with increasing temperature for a spontaneous reaction?	ΔS must be positive (ΔH<0).
96	What are the dimensions of the Cell Constant (G∗)?	Length−1 (m−1 or cm−1).
97	What is the reciprocal of the Cell Constant?	A/l (Ratio of area to length).
98	Why is the SHE difficult to use in practice?	Requires constant supply of H2​ gas and 1 M H+ solution.
99	What is the value of the slope of the Λm​ vs C​ plot for a strong electrolyte?	Negative (according to the Debye-Hückel-Onsager equation).
100	What is the general name for the non-rechargeable cell?	Primary Cell.
101	What is the general name for the rechargeable cell?	Secondary Cell.
102	What substance is reduced at the cathode of a dry cell?	MnO2​ (Manganese dioxide).
103	What prevents the continuous rusting of iron?	The rust itself is porous and does not form a protective layer.
104	How many Faradays are required to deposit 10 g of Ca (MM=40 g/mol)?	0.5 F (Ca2++2e−, so Eq Mass=20; 10/20=0.5).
105	What is the EMF of a cell represented by Ered​(A)=−0.76 V and Ered​(B)=−0.44 V?	0.32 V (B is cathode: −0.44−(−0.76)=0.32 V).
106	Which metal is oxidized in the previous cell?	A (The one with the less positive/more negative reduction potential).
107	Does Ecell​ increase or decrease with dilution of the electrolyte in a concentration cell?	Depends on the concentrations; it increases if dilution exaggerates the concentration difference.
108	Is Cu2+ a stronger oxidising agent than Ag+ if EAg+∘​>ECu2+∘​?	No, Ag+ is the stronger oxidising agent.
109	What is the final product in the decomposition of water by electrolysis?	H2​ (at cathode) and O2​ (at anode) in a 2:1 volume ratio.
110	Why are strong electrolytes effective in maintaining electrical neutrality in the salt bridge?	They provide highly mobile ions that don't react with the electrolytes.
111	Name the specific type of corrosion that occurs on the surface of copper.	Verdigris (Green layer, a basic carbonate Cu(OH)2​⋅CuCO3​).
112	What is the ΔG of a spontaneous cell reaction?	Negative (ΔG<0).

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