---
title: "Standard electrode potential; Daniell cell EMF"
subject: "Engineering Chemistry"
chapter: 1
key: "semester_2/engineering_chemistry:daniell-cell-emf"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/daniell-cell-emf
site: BCT Engineering
---

# Standard electrode potential; Daniell cell EMF

Engineering Chemistry · Chapter 1: Electro-chemistry and Buffer

- Asked in: 2070 Magh, 1+4 marks
- Video: [Lesson 13: EMF of Daniel Cell | Topic: Electrochemical Cell | Electrochemistry](https://www.youtube.com/watch?v=RFiTVXPxF1M) — BdS 71
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/daniell-cell-emf
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-1

## Question

What is meant by standard electrode potential? Calculate the EMF of a Daniel cell at 25°C, when the concentration of ZnSO₄ and CuSO₄ are 0.001 M and 0.1 M respectively. The standard potential of the cell is 1.2 V.

## Short answer

Standard electrode potential is the potential of an electrode in 1 M ion solution at 25°C measured against the standard hydrogen electrode. The Daniell cell EMF is calculated using the Nernst equation: E = 1.2 − (0.0591/2) log(0.001/0.1) ≈ 1.26 V.

## Answer

#### Standard electrode potential
The electrode potential measured when the electrode is in contact with a 1 M solution of its ions (1 atm for gases) at 25°C, relative to the standard hydrogen electrode (E° = 0). It is written as E°.

#### EMF of Daniell cell
Cell: Zn | Zn²⁺ (0.001 M) || Cu²⁺ (0.1 M) | Cu
```text
Anode   : Zn → Zn²⁺ + 2e⁻
Cathode : Cu²⁺ + 2e⁻ → Cu
Cell    : Zn + Cu²⁺ → Zn²⁺ + Cu      (n = 2)

E = E° − (0.0591/n) log ([Zn²⁺]/[Cu²⁺])
  = 1.2 − 0.02955 × log (0.001/0.1)
  = 1.2 − 0.02955 × (−2)
  = 1.2 + 0.0591 = 1.259 V
```
**Answer: EMF ≈ 1.26 V.** (The given standard potential 1.2 V is used as stated; the usual textbook value is 1.10 V.)

## Check yourself

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

1. Standard electrode potential is measured with ions at:
   A. 1 M and 25°C against SHE
   B. 0.1 M and 0°C
   C. 1 M against Zn
   D. any concentration
   Answer: A. Defined at 1 M (1 atm for gases), 25°C, relative to SHE = 0. (Key point: Standard electrode potential)

2. Daniell cell reaction:
   A. Zn + Cu2+ -> Zn2+ + Cu
   B. Cu + Zn2+ -> Cu2+ + Zn
   C. Zn + Cu -> Zn2+ + Cu2+
   D. Zn2+ + Cu2+ -> Zn + Cu
   Answer: A. Zn is the anode and is oxidised; Cu2+ is reduced at the cathode. (Key point: Cell reaction)

3. Q = [Zn2+]/[Cu2+] with 0.001 M and 0.1 M:
   A. 0.01 (log = -2)
   B. 100
   C. 0.1
   D. 1
   Answer: A. 0.001/0.1 = 0.01, log Q = -2. (Key point: Q)

4. E = 1.2 - 0.02955 x (-2) =
   A. 1.26 V
   B. 1.14 V
   C. 1.20 V
   D. 1.31 V
   Answer: A. Negative log Q increases E: 1.2 + 0.0591 = 1.259 V. (Key point: EMF)

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