EMF of Cell — E°cell = E°cathode - E°anode
Electrochemistry
9
JEE Qs
8%
Hard
40
min
Consistently apply the convention of using standard reduction potentials and correctly identify the cathode (reduction, higher SRP) and anode (oxidation, lower SRP) to avoid calculation errors.
🧮 Key Formulas
✅ Key Points for JEE
- 1Always use standard reduction potentials (SRP) for both cathode and anode in the formula E°cell = E°reduction(cathode) - E°reduction(anode).
- 2The species with the higher standard reduction potential (SRP) will act as the cathode (undergo reduction), and the species with the lower SRP will act as the anode (undergo oxidation).
- 3Standard electrode potentials (E°) are intensive properties; they are not multiplied by the stoichiometric coefficients of the balanced redox reaction.
- 4For a spontaneous galvanic (voltaic) cell, the calculated E°cell value must always be positive. A negative E°cell indicates a non-spontaneous process under standard conditions.
- 5Standard conditions are 1 M concentration for all ionic species, 1 atm partial pressure for gases, and 298 K (25°C).
⚠️ Common Mistakes
- ✕Incorrectly identifying cathode and anode, often leading to calculating E°anode - E°cathode instead of E°cathode - E°anode.
- ✕Using a mix of oxidation potentials and reduction potentials without converting them to a consistent form (preferably all reduction potentials).
- ✕Multiplying the E° values by stoichiometric coefficients from the balanced chemical equation, which is incorrect as E° is an intensive property.
📝 Practice Questions
See allQ54.The element that does not belong to the same period of the remaining elements (modern periodic table) is: 2025 (23 Jan Shift 1) JEE Main Previous Year Paper (1) Iridium (2) Platinum (3) Osmium (4) Palladium
Q67.For the given cell Fe2+ (aq) + Ag+ (aq) →Fe3+(aq) + Ag(s) The standard cell potential of the above reaction is Ag+ + e−→Ag Eθ = xV Given: Fe2+ + 2e−→Fe Eθ = yV Fe3+ + 3e−→Fe Eθ = zV (1) x + y −z (2) x + 2y (3) x + 2y −3z (4) y −2x
Q58.Which of the following electrolyte can be used to obtain H2 S2O8 by the process of electrolysis? (1) Dilute solution of sodium sulphate. (2) Acidified dilute solution of sodium sulphate. (3) Dilute solution of sulphuric acid (4) Concentrated solution of sulphuric acid
Q66.A solution of aluminium chloride is electrolysed for 30 minutes using a current of 2 A . The amount of the aluminium deposited at the cathode is [Given : molar mass of aluminium and chlorine are 27 g mol−1 and 35.5 g mol−1 respectively. Faraday constant = 96500Cmol−1] (1) 1.660 g (2) 0.336 g (3) 0.441 g (4) 1.007 g 2025 (22 Jan Shift 1) JEE Main Previous Year Paper
Q58.Standard electrode potentials for a few half cells are mentioned below : E∘ = 0.34 V, E∘ = −0.76 V Cu2+/Cu Zn2+/Zn Which one of the following cells gives the most negative value of E∘Ag+/Ag = 0.80 V, E∘Mg2+/Mg = −2.37 V ΔG∘ ? (1) Zn Zn2+(1M) Ag+(1M) Ag (2) Zn Zn2+(1M) Mg2+(1M) Mg (3) Ag Ag+(1M) Mg2+(1M) Mg (4) Cu Cu2+(1M)∥Ag+(1M) Ag
Q57. E∘Cr2O2−7 /Cr3+ = 1.33 V E∘Cl2/Cl(−) = 1.36 V Based on the data given below : the strongest reducing agent is : E∘ = 1.51 V E∘ = −0.74 V MnO−4 /Mn2+ Cr3+/Cr (1) Cr (2) Cl− (3) MnO−4 (4) Mn2+
NCERT Chapters
- Class 12 Chemistry Ch 3: Electrochemistry