---
title: "Non-standard electrode potential; Fe/Ag cell emf"
subject: "Engineering Chemistry"
chapter: 1
key: "semester_2/engineering_chemistry:nonstandard-emf-fe-ag"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/nonstandard-emf-fe-ag
site: BCT Engineering
---

# Non-standard electrode potential; Fe/Ag cell emf

Engineering Chemistry · Chapter 1: Electro-chemistry and Buffer

- Asked in: 2073 Magh, 1+4 marks
- Video: [BE 1st Year: Engineering Chemistry: Electrochemistry: Finding emf of a cell: Iron & Silver Electrode](https://www.youtube.com/watch?v=96kZ_GSl5NA) — Tausif ChemiZone
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/nonstandard-emf-fe-ag
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-1

## Question

What is non-standard electrode potential? Calculate the emf of the cell obtained from given electrode reactions.
Fe²⁺ (0.2M) + 2e⁻ → Fe  E° = (−0.44 V)
Ag⁺ (0.1M) + 2e⁻ → Ag  E° = (−0.80 V)

## Short answer

Non-standard electrode potential arises when ion concentrations are not 1 M; it is found with the Nernst equation. For Fe–Ag, Ag⁺/Ag is cathode (E°=+0.80 V) and Fe²⁺/Fe anode (−0.44 V). EMF = 1.24 − (0.0591/2) log(0.2/(0.1)² ≈ 1.20 V.

## Answer

#### Non-standard electrode potential
When the concentration of the ions is not 1 M (or the gas pressure is not 1 atm, or temperature is not 25°C), the electrode potential is called a **non-standard electrode potential**. It is calculated with the Nernst equation: for a reduction half-reaction Mⁿ⁺ + ne⁻ → M, E = E° + (0.0591/n) log [Mⁿ⁺] at 25°C.

#### Reading of the data
The Ag half reaction is written with "2e⁻" and E° = −0.80 V, which is inconsistent for Ag⁺ (one electron, reduction potential +0.80 V; −0.80 V is its oxidation potential). I read it as Ag⁺ + e⁻ → Ag, E° = +0.80 V, as in the standard Fe–Ag cell. Fe²⁺/Fe: E° = −0.44 V.

#### Cell
Ag⁺/Ag has the higher reduction potential, so it is the cathode; Fe is the anode.
```text
Anode  : Fe → Fe²⁺ + 2e⁻
Cathode: 2Ag⁺ + 2e⁻ → 2Ag
Cell   : Fe + 2Ag⁺ → Fe²⁺ + 2Ag     (n = 2)
Notation: Fe | Fe²⁺ (0.2 M) || Ag⁺ (0.1 M) | Ag

E°cell = 0.80 − (−0.44) = 1.24 V
Q = [Fe²⁺]/[Ag⁺]² = 0.2/(0.1)² = 20,  log Q = 1.301
E = 1.24 − (0.0591/2)(1.301)
  = 1.24 − 0.0384 = 1.2016 V
```
**Answer: EMF ≈ 1.20 V.**

(If the values were taken literally as reduction potentials, −0.44 V and −0.80 V, Ag would be the anode and E° = 0.36 V, which does not match the usual Fe–Ag cell.)

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. An electrode potential is non-standard when:
   A. It is measured against SHE
   B. Ion concentration is not 1 M, gas pressure not 1 atm or temperature not 25°C
   C. It uses platinum
   D. It is a metal electrode
   Answer: B. Any departure from 1 M, 1 atm and 25°C makes the potential non-standard, computed by the Nernst equation. (Key point: Non-standard potential)

2. Using E°(Ag+/Ag) = +0.80 V and E°(Fe2+/Fe) = -0.44 V, which is the cathode?
   A. Fe
   B. Ag
   C. Both
   D. Neither
   Answer: B. The electrode with the higher reduction potential (Ag) is the cathode; Fe is the anode. (Key point: Cathode choice)

3. For Fe + 2Ag+ -> Fe2+ + 2Ag with [Fe2+] = 0.2 M and [Ag+] = 0.1 M, Q is:
   A. 2
   B. 20
   C. 0.02
   D. 200
   Answer: B. Q = 0.2/(0.1)^2 = 20. (Key point: Reaction quotient)

4. E°cell = 1.24 V, n = 2, log Q = 1.301. What is E?
   A. 1.28 V
   B. 1.20 V
   C. 1.24 V
   D. 1.16 V
   Answer: B. E = 1.24 - 0.02955 x 1.301 = 1.24 - 0.0384 = 1.20 V. (Key point: Final EMF)

## Related in Engineering Chemistry

- [Standard electrode potential; Fe/Ni cell emf](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Afe-ni-cell) — Engineering Chemistry, chapter 1
- [Electrode potential; Cu/Ag cell EMF](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aelectrode-potential-cell-emf) — Engineering Chemistry, chapter 1
- [Single electrode potential; Ni/Ag cell EMF](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ani-ag-cell) — Engineering Chemistry, chapter 1
- [Standard hydrogen electrode; Mg/Ag cell emf](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ashe-mg-ag-cell) — Engineering Chemistry, chapter 1
- [Fe-Ag cell reactions and EMF](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Afe-ag-cell) — Engineering Chemistry, chapter 1
- [Standard electrode potential; Daniell cell EMF](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Adaniell-cell-emf) — Engineering Chemistry, chapter 1