---
title: "Electrode potential origin; Zn-Cu cell notation"
subject: "Engineering Chemistry"
chapter: 1
key: "semester_2/engineering_chemistry:electrode-potential-zn-cu"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/electrode-potential-zn-cu
site: BCT Engineering
---

# Electrode potential origin; Zn-Cu cell notation

Engineering Chemistry · Chapter 1: Electro-chemistry and Buffer

- Asked in: 2069 Bhadra, 2+1+2 marks
- Video: [Electrode Potentials: How They Work and Why They Matter!](https://www.youtube.com/watch?v=aJG_yAe6hLA) — Chemistry Student
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/electrode-potential-zn-cu
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-1

## Question

How does electrode potential originate? Define standard electrode potential. Write the cell notation and cell reaction of Zn-Cu cell.

## Short answer

Electrode potential arises from charge separation at a metal-solution interface. Standard electrode potential is measured at 1 M, 25°C relative to SHE. The Zn-Cu cell is written Zn | Zn²⁺ || Cu²⁺ | Cu; cell reaction: Zn + Cu²⁺ → Zn²⁺ + Cu with E°cell = 1.10 V.

## Answer

#### Origin of electrode potential
A metal rod dipped in a solution of its ions has two tendencies: the metal atoms may pass into solution as ions leaving electrons on the rod (M → Mⁿ⁺ + ne⁻), or ions may take electrons from the rod and deposit (Mⁿ⁺ + ne⁻ → M). An equilibrium is set up and a double layer of charge forms at the interface. The potential difference across it is the electrode potential. For Zn the rod becomes negative; for Cu it becomes positive.

#### Standard electrode potential
The potential of an electrode in contact with a 1 M solution of its ions at 25°C (1 atm for gases), measured against the standard hydrogen electrode taken as 0 V. Symbol E°.

#### Zn–Cu cell (Daniell cell)
Cell notation: Zn | Zn²⁺ (1 M) || Cu²⁺ (1 M) | Cu

```text
Anode (oxidation)  : Zn → Zn²⁺ + 2e⁻
Cathode (reduction): Cu²⁺ + 2e⁻ → Cu
Cell reaction      : Zn + Cu²⁺ → Zn²⁺ + Cu
```
E°cell = E°(Cu²⁺/Cu) − E°(Zn²⁺/Zn) = 0.34 − (−0.76) = 1.10 V.

## Check yourself

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

1. In a Zn rod dipped in Zn2+ solution, the rod becomes negative because:
   A. Zn atoms pass into solution as ions leaving electrons on the rod
   B. Zn2+ deposits on rod
   C. Electrons come from solution
   D. The rod loses protons
   Answer: A. Tendency M -> Mn+ + ne- leaves electrons on the rod. (Key point: Origin)

2. The Zn-Cu cell notation is:
   A. Zn | Zn2+ (1 M) || Cu2+ (1 M) | Cu
   B. Cu | Cu2+ || Zn2+ | Zn
   C. Zn2+ | Zn || Cu | Cu2+
   D. Zn | Cu2+ || Zn2+ | Cu
   Answer: A. Anode (Zn, oxidation) on the left and cathode (Cu) on the right. (Key point: Cell notation)

3. E°cell = E°(Cu2+/Cu) - E°(Zn2+/Zn) =
   A. 1.10 V
   B. 0.42 V
   C. -1.10 V
   D. 0.34 V
   Answer: A. 0.34 - (-0.76) = 1.10 V. (Key point: Standard emf)

4. At the cathode of the Zn-Cu cell:
   A. Cu2+ + 2e- -> Cu
   B. Zn -> Zn2+ + 2e-
   C. Cu -> Cu2+ + 2e-
   D. Zn2+ + 2e- -> Zn
   Answer: A. Cu2+ is reduced to Cu at the cathode. (Key point: Cathode reaction)

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