Electrochemistry / Class 12 Chemistry / CBSE
Dr E. Ramanathan PhD
Lecture Notes:
Electrochemical Cells (Daniel Cell)
Key Concepts
- Electrochemical Cell Definition:
- A device that converts chemical energy from a redox reaction into electrical energy.
- Daniell Cell Reaction:
- Redox Reaction:
Zinc is oxidized: loses electrons. Copper ion is reduced: gains electrons.
Construction of Daniell Cell
- Two beakers with:
- Left: Zinc rod in Zn²⁺ solution.
- Right: Copper rod in Cu²⁺ solution.
- Salt bridge connects the two solutions.
- Electrons move from Zn to Cu externally; current flows from Cu to Zn internally.
- Components:
- Zinc Electrode in ZnSO₄ solution (anode).
- Copper Electrode in CuSO₄ solution (cathode).
- Salt Bridge: Maintains electrical neutrality.
- External Wire: Connects Zn and Cu electrodes; allows electron flow.
- Electron Flow:
- From Zinc (Anode) to Copper (Cathode) through the wire.
- Ion Movement:
- Anions from salt bridge move toward zinc half-cell.
- Cations move toward copper half-cell.
Key Features
- Anode (−): Zinc electrode (oxidation occurs).
- Cathode (+): Copper electrode (reduction occurs).
- Standard Electrode Potential: ~1.1 V when ion concentrations are 1 mol dm⁻³.
Galvanic vs Electrolytic Cells
- Galvanic Cell:
- Spontaneous redox reaction.
- Produces electrical energy.
- Electrolytic Cell:
- Requires external voltage.
- Drives non-spontaneous reaction.
- When an opposing potential >1.1 V is applied, Daniell cell stops functioning as a galvanic cell and behaves as an electrolytic cell.
Applications
- Galvanic cells are used in batteries.
- Electrolytic cells are used in electroplating, metal refining, and chemical synthesis.
Reversal of Daniell Cell Reaction under External Potential
Effect of External Voltage on Daniell Cell (Fig. 3.2)
Key Concept
The Daniell cell has a standard cell potential of 1.1 V. Applying an external voltage (E_ext) opposite to the cell potential can alter the direction and occurrence of redox reactions.
Three Conditions of External Voltage
(a) When Eext < 1.1 V
- Electron flow: From Zn (anode) to Cu (cathode)
- Current: From Cu to Zn in the external circuit
- Reactions:
- Zinc is oxidized at the anode (Zn → Zn²⁺ + 2e⁻)
- Copper is reduced at the cathode (Cu²⁺ + 2e⁻ → Cu)
- Result:
- Zn dissolves into solution
- Cu is deposited on the copper electrode
(b) When Eext = 1.1 V
- Electron flow: None
- Current: Zero (I = 0)
- Reaction: No net redox reaction occurs
- Explanation: The external voltage exactly cancels the cell’s potential, so the system is at equilibrium
(c) When Eext > 1.1 V
- Electron flow: Reversed; from Cu to Zn
- Current: From Zn to Cu in external circuit
- Reactions:
- Zinc is reduced at cathode (Zn²⁺ + 2e⁻ → Zn)
- Copper is oxidized at anode (Cu → Cu²⁺ + 2e⁻)
- Result:
- Zn is deposited on the zinc electrode
- Cu dissolves into the solution
Conclusion
- Eext < 1.1 V: Daniell cell works as galvanic cell (spontaneous)
- Eext = 1.1 V: No reaction (equilibrium)
- Eext > 1.1 V: Daniell cell acts as an electrolytic cell (non-spontaneous, driven by external power)
Note:
- Concentration in real applications is replaced by activity, but in dilute solutions, they are effectively the same.
Summary Table: Functioning of Daniell Cell under External Voltage
| Condition | Electron Flow | Current Direction | Redox Reaction | Cell Type | Electrode Changes |
|---|---|---|---|---|---|
| Eext < 1.1 V | Zn → Cu | Cu → Zn (external circuit) | Zn oxidized (anode), Cu²⁺ reduced (cathode) | Galvanic Cell | Zn dissolves, Cu deposits |
| Eext = 1.1 V | No flow | No current (I = 0) | No net redox reaction (equilibrium) | No Reaction | No change |
| Eext > 1.1 V | Cu → Zn | Zn → Cu (external circuit) | Cu oxidized (now anode), Zn²⁺ reduced (now cathode) | Electrolytic Cell | Cu dissolves, Zn deposits |
