4.5 Galvanic Cells (Daniel Cells)

Introduction

In the previous sections, we studied electrolytic cells, which use electrical energy to drive a non-spontaneous chemical reaction. Now we turn to the opposite type of electrochemical cell — the galvanic cell (also called a voltaic cell).

A galvanic cell uses a spontaneous redox reaction to generate electricity. This is the principle behind batteries and fuel cells.

This section explains what a galvanic cell is, describes the Daniel cell, and explains the reactions that occur in it.

Definition

Galvanic Cell (Voltaic Cell): The cell which involves a spontaneous redox reaction to generate electricity is called a galvanic or voltaic cell.

Daniel Cell: The English chemist Fredrick Daniel constructed the first voltaic cell using zinc (Zn) and copper (Cu) electrodes. Therefore, this cell is named the Daniel Cell.

In simple words: A galvanic cell is a device that converts chemical energy into electrical energy using a spontaneous redox reaction.

Main Idea

• A galvanic cell uses a spontaneous redox reaction to generate electricity.
• The name Voltaic is given to this cell because Alessandro Volta discovered the first such cell.
• The Daniel Cell was constructed by Fredrick Daniel using zinc (Zn) and copper (Cu) electrodes.
• A galvanic cell consists of:
• Each compartment of the cell is called a half-cell.
• Oxidation occurs at the anode (Zn half-cell).
• Reduction occurs at the cathode (Cu half-cell).

Core Concept

Key point:

• Electrons flow from the Zn rod (anode) through the external wire to the Cu rod (cathode).
• Oxidation occurs at the anode.
• Reduction occurs at the cathode.

Detailed Explanation

Parts of a Galvanic Cell (Daniel Cell)

A galvanic cell consists of the following parts:

(1) A zinc bar dipped into a 1M ZnSO₄ solution.

• This is the anode half-cell.
• Zinc has a greater tendency to undergo oxidation than copper.
• Zinc atoms go into the solution as Zn²⁺ ions, leaving electrons on the rod.

(2) A copper bar dipped into a 1M CuSO₄ solution.

• This is the cathode half-cell.
• Cu²⁺ ions in the copper sulphate solution capture electrons from the Cu electrode and are reduced.

(3) A salt bridge — an inverted U tube containing an inert electrolyte such as KCl.

• Its ions do not react with the electrodes or the ions in the solution.
• It makes the electrical contact between the solutions through which ions can move.

(4) A voltmeter to measure current.

- It measures the voltage generated by the cell.

How the Daniel Cell Works

When the circuit is complete:

• Electrons begin to flow from the Zn rod through the external wires to the Cu rod.
• Zn half-cell acts as the anode.
• Cu half-cell acts as the cathode.
• Oxidation half-reaction occurs at the anode compartment.
• Reduction half-reaction occurs at the cathode compartment.

Key point: A half-cell consists of a metal rod dipped in the solution of its salt.

Reactions in a Daniel Cell

In Daniel's cell, electrons flow from the Zn rod, through the external wire to the Cu rod. This is because Zn has more tendency to undergo oxidation than Cu.

At Anode (Oxidation half-reaction):

Zinc atoms from the rod go into the solution as Zn²⁺ ions, leaving electrons on the rod. These electrons flow in the external circuit.

Zn_(s) → Zn²⁺_(aq) + 2e^-

At Cathode (Reduction half-reaction):

Cu²⁺ ions in copper sulphate solution capture electrons from the Cu electrode and are reduced.

Cu²⁺_(aq) + 2e^- → Cu_(s)

Key point:

• Oxidation occurs at the anode (Zn half-cell).
• Reduction occurs at the cathode (Cu half-cell).
• These oxidation and reduction reactions are called half-cell reactions.

Summary Table

Intermediate Level

Parts of a Daniel Cell:

• Zinc bar dipped in 1M ZnSO₄ solution (anode).
• Copper bar dipped in 1M CuSO₄ solution (cathode).
• Salt bridge (inverted U tube containing KCl).
• Voltmeter to measure current.

Working:

• Zinc has more tendency to undergo oxidation than copper.
• Zinc atoms go into solution as Zn²⁺ ions, leaving electrons on the rod.
• Electrons flow through the external circuit to the copper rod.
• Cu²⁺ ions capture electrons and are reduced to copper metal.

Reactions:

• At anode: Zn_(s) → Zn²⁺_(aq) + 2e^-
• At cathode: Cu²⁺_(aq) + 2e^- → Cu_(s)

Key point:

• Oxidation occurs at the anode compartment.
• Reduction occurs at the cathode compartment.
• A half-cell consists of a metal rod dipped in the solution of its salt.

Solved Example

Example: Reactions in a Daniel Cell

Given: A Daniel cell consists of a zinc bar dipped in 1M ZnSO₄ solution and a copper bar dipped in 1M CuSO₄ solution, connected by a salt bridge.

Required: Write the oxidation and reduction half-reactions.

Solution:

At Anode (Oxidation half-reaction): Zn_(s) → Zn²⁺_(aq) + 2e^-

At Cathode (Reduction half-reaction): Cu²⁺_(aq) + 2e^- → Cu_(s)

Final Answer:

• Anode: Zinc (Zn) — oxidation occurs
• Cathode: Copper (Cu) — reduction occurs
• Electron flow: From Zn (anode) to Cu (cathode) through the external circuit

Concept Assessment Exercise 4.2

Sketch a voltaic cell labeling the cathode, the anode and the direction of flow of the electrons. Use the following chemicals:

Silver, Zinc, Silver Nitrate, and zinc sulphate (ZnSO₄).

(Hint: Zn is more active than Ag)

Answer:

• Anode: Zinc (Zn) — oxidation occurs here
• Cathode: Silver (Ag) — reduction occurs here
• Direction of electron flow: From Zn (anode) to Ag (cathode) through the external circuit
• Reactions:
• At anode: Zn → Zn²⁺ + 2e^-
• At cathode: Ag⁺ + 1e^- → Ag

Important Points

• - Galvanic cell = cell that uses a spontaneous redox reaction to generate electricity.
• - Also called a voltaic cell.
• - Daniel Cell was constructed by Fredrick Daniel using Zn and Cu electrodes.
• - Parts:
• 1. Zinc bar in 1M ZnSO₄ (anode)
• 2. Copper bar in 1M CuSO₄ (cathode)
• 3. Salt bridge (KCl)
• 4. Voltmeter
• - Anode: Zinc — oxidation occurs.
• - Cathode: Copper — reduction occurs.
• - Electron flow: From Zn to Cu through external circuit.
• - Anode reaction: Zn_(s) → Zn²⁺_(aq) + 2e^-
• - Cathode reaction: Cu²⁺_(aq) + 2e^- → Cu_(s)
• - Salt bridge: Makes electrical contact between the solutions.
• - Half-cell: Metal rod dipped in the solution of its salt.

Remember

• - Galvanic cell = generates electricity from a spontaneous redox reaction.
• - Also called voltaic cell.
• - Daniel Cell uses Zn and Cu electrodes.
• - Anode = Zn — oxidation.
• - Cathode = Cu — reduction.
• - Electrons flow from Zn to Cu.
• - Salt bridge connects the two half-cells.
• - Anode reaction: Zn → Zn²⁺ + 2e^-.
• - Cathode reaction: Cu²⁺ + 2e^- → Cu.
• - Half-cell = metal rod in its salt solution.

Common Mistakes

Quick Revision

Galvanic Cell (Voltaic Cell): The cell which involves a spontaneous redox reaction to generate electricity is called a galvanic or voltaic cell. The name Voltaic is given to this cell because Alessandro Volta discovered the first such cell. The English chemist Fredrick Daniel constructed the first voltaic cell using zinc (Zn) and copper (Cu) electrodes. Therefore this cell is named Daniel Cell.

A galvanic cell consists of the following parts:

• A zinc bar dipped into a 1M ZnSO₄ solution.
• A copper bar dipped into a 1M CuSO₄ solution.
• A salt bridge is an inverted U tube containing an inert electrolyte such as KCl. Its ions do not react with the electrodes or the ions in the solution. It makes the electrical contact between the solutions through which ions can move.
• A voltmeter to measure current.

Each compartment of the cell is called a half-cell. Thus a Daniel cell consists of two half-cells joined in series. When the circuit is complete, electrons begin to flow from the Zn rod through the external wires to the Cu rod. Thus Zn half-cell acts as an anode and the Cu half-cell as a cathode. Note that a half cell consists of a metal rod dipped in the solution of its salt.

Reactions in a Daniel Cell:

In Daniel's cell, electrons flow from the Zn rod, through the external wire to the Cu rod. This is because Zn has more tendency to undergo oxidation than Cu. Zn atoms from the rod go into the solution as Zn²⁺ ions leaving electrons on the rod. These electrons flow in the external circuit. Thus oxidation half reaction occurs at the anode compartment. Cu²⁺ ions in copper sulphate solution capture electrons from the Cu electrode and are reduced. Reduction half reaction occurs at the cathode compartment. Such oxidation and reduction reactions are called half-cell reactions.

• At Anode (Oxidation half reaction):
• Zn(s) → Zn²⁺(aq) + 2e^-

• At Cathode (Reduction half-reaction):
• Cu²⁺(aq) + 2e^- → Cu(s)

Concept Assessment Exercise 4.2: Sketch a voltaic cell labeling the cathode, the anode and the direction of flow of the electrons. Use the following chemicals: Silver, Zinc, Silver Nitrate and zinc sulphate (ZnSO₄). (Hint: Zn is more active than Ag)

60-Second Revision

• Galvanic cell = generates electricity from spontaneous redox reaction.
• Also called voltaic cell.
• Daniel Cell = Zn and Cu electrodes.
• **Anode** = Zn — oxidation.
• **Cathode** = Cu — reduction.
• Electrons flow from Zn to Cu.
• Salt bridge = KCl in inverted U tube.
• Anode reaction: Zn → Zn²⁺ + 2e^-.
• Cathode reaction: Cu²⁺ + 2e^- → Cu.
• Half-cell = metal rod in its salt solution.
• Voltmeter measures current.
• ⚠️ Warning: in a galvanic cell, oxidation occurs at the anode, and reduction occurs at the cathode — do not mix them up.