Electrochemical Cells: Converting Chemical Energy into Electrical Energy
What Is an Electrochemical Cell?
Electrochemical cell
An electrochemical cell is a device that uses redox (reduction-oxidation) reactions to either produce or consume electrical energy.
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:
Electrodes: Sites of Redox Reactions
Electrodes
Electrodes are solid surfaces where the redox reactions occur
- Anode: The electrode where oxidation happens. Electrons are lost here.
- Cathode: The electrode where reduction takes place. Electrons are gained here.
Salt Bridge: Ensuring Charge Balance
Salt bridge
The salt bridge is a tube or porous material filled with an ionic solution (e.g., KNO₃ or Na₂SO₄). It allows ions to flow between the two half-cells, maintaining electrical neutrality and completing the circuit.
Without a salt bridge, the buildup of charges in the half-cells would prevent the flow of electrons, effectively stopping the redox reaction.
Electrolyte Solutions: Providing Ions for Reactions
Each half-cell contains a solution with ions of the metals involved in the redox reaction.
ExampleIn a zinc-copper cell, the anode is immersed in a solution of Zn²⁺ ions (e.g., ZnSO₄), and the cathode is immersed in a solution of Cu²⁺ ions (e.g., CuSO₄).
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:
$$
\text{Zn(s)} \rightarrow \text{Zn}^{2+}(\text{aq}) + 2\text{e}^-
$$ - 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:
$$
\text{Cu}^{2+}(\text{aq}) + 2\text{e}^- \rightarrow \text{Cu(s)}
$$
