R3.2.6 Primary (voltaic) cells

Electrochemical Cells: Converting Chemical Energy into Electrical Energy

What Is an Electrochemical Cell?

These cells come in two main types:

• Voltaic (or galvanic) cells, where spontaneous redox reactions release energy, which is converted into electrical energy.
• Electrolytic cells, where electrical energy is used to drive non-spontaneous redox reactions.
• Primary cells ,which are non-rechargeable electrochemical cells designed for single use, where the chemical reactions are irreversible, commonly used in devices like flashlights and remote controls.

For now, we’ll focus on voltaic cells, which are the basis of many common batteries.

Components of a Voltaic Cell

To understand how a voltaic cell operates, let’s break it down into its key components:

1. Electrodes: Sites of Redox Reactions

• Anode: The electrode where oxidation happens. Electrons are lost here.
• Cathode: The electrode where reduction takes place. Electrons are gained here.

2. Salt Bridge: Ensuring Charge Balance

Without a salt bridge, the buildup of charges in the half-cells would prevent the flow of electrons, effectively stopping the redox reaction.

3. Electrolyte Solutions: Providing Ions for Reactions

Each half-cell contains a solution with ions of the metals involved in the redox reaction.

4.External Circuit: The Pathway for Electrons

• The electrodes are connected by a wire, creating a pathway for electrons to flow from the anode to the cathode.
• This flow of electrons is the electric current.

How Does a Voltaic Cell Work?

Let’s take a closer look at how a voltaic cell functions by examining the Daniell Cell, which uses zinc and copper electrodes:

Step 1: Oxidation at the Anode

At the zinc electrode (anode), zinc metal is oxidized to zinc ions:
$\text{Zn(s)}\to {\text{Zn}}^{2+}(\text{aq})+2{\text{e}}^{-}$
This reaction releases electrons, which travel through the external circuit to the cathode.

Step 2: Reduction at the Cathode

At the copper electrode (cathode), copper ions in the solution are reduced to form copper metal:
${\text{Cu}}^{2+}(\text{aq})+2{\text{e}}^{-}\to \text{Cu(s)}$
The copper ions gain the electrons that traveled through the wire.

Step 3: Ion Flow Through the Salt Bridge

To balance the charges in each half-cell:

• Negative ions (e.g., SO₄²⁻) flow from the cathode half-cell to the anode half-cell.
• Positive ions (e.g., K⁺) flow from the salt bridge into the cathode half-cell.

This ion flow prevents charge buildup and ensures the redox reaction can continue smoothly.

Why Does the Reaction Produce Electricity?

1. The redox reaction in a voltaic cell is spontaneous, meaning it releases energy.
2. This energy drives the movement of electrons from the anode to the cathode, generating an electric current.
3. The driving force for this electron flow is the difference in reduction potential between the two electrodes.
4. Reduction potential measures how easily a species gains electrons:
   • Zinc has a more negative reduction potential than copper, so zinc is more likely to lose electrons (oxidation).
   • Copper has a more positive reduction potential, so it is more likely to gain electrons (reduction).

The voltage of the cell, known as the cell potential ($E_{\text{cell}}^{\circ }$), is the difference between the reduction potentials of the two electrodes:
$$E_{\text{cell}}^{\circ }=E_{\text{cathode}}^{\circ }-E_{\text{anode}}^{\circ }$$

For the Daniell Cell:
$$E_{\text{cell}}^{\circ }=(+0.34\text{V})-(-0.76\text{V})=+1.10\text{V}$$

Observing the Daniell Cell in Action

As the reaction progresses:

• The zinc electrode (anode) dissolves as it forms Zn²⁺ ions.
• The copper electrode (cathode) grows as Cu²⁺ ions are reduced to copper metal.
• The blue color of the copper sulfate solution fades as Cu²⁺ ions are consumed.

Why Are Electrochemical Cells Important?

Electrochemical cells are more than just a classroom concept—they play a critical role in modern technology. Here are a few applications:

• Batteries: From alkaline batteries in flashlights to lithium-ion batteries in smartphones, electrochemical cells are essential for portable energy.
• Fuel Cells: Hydrogen fuel cells produce electricity with water as the only byproduct, offering a clean and renewable energy source.
• Corrosion Prevention: Understanding electrochemical principles helps us develop methods to prevent rusting and material degradation.

Reflection