R3.2.5 Electrochemical cells: Oxidation and reduction

Why Do Electrons Matter in Electrochemistry?

Imagine your flashlight dying during a camping trip—what's happening inside the battery?
At its core, a redox reaction drives the battery, where electrons transfer between substances.
Beyond batteries, redox chemistry powers electroplating, metal extraction, and water purification.

Oxidation and Reduction: The Basics

In any redox reaction, oxidation and reduction happen simultaneously:

Oxidation is the loss of electrons.
Reduction is the gain of electrons.

Tip

To remember this, use the mnemonic OIL RIG:

Oxidation Is Loss (of electrons).
Reduction Is Gain (of electrons).

In electrochemical cells, these processes are separated into two locations: the anode and the cathode. Let’s explore how this works.

Oxidation at the Anode

The anode is where oxidation occurs. At this electrode:

A species loses electrons, which are then transferred through an external circuit.
The electrons flow away from the anode toward the cathode.

Example

In a zinc–copper voltaic cell:

At the zinc anode: $Z n (s) \to Z n^{2 +} (a q) + 2 e^{-}$ Zinc is oxidized, releasing electrons.

Example

Consider the electrolysis of molten sodium chloride $N a C l$ . At the anode, chloride ions are oxidized to chlorine gas: $2 C l^{-} (l) \to C l_{2} (g) + 2 e^{-}$

Common Mistake

Students often confuse the anode as always being "negative." This is true for voltaic cells but not for electrolytic cells. Always consider the type of cell before assigning polarity.

Reduction at the Cathode

The cathode is where reduction occurs. At this electrode:

A species gains electrons from the external circuit.

Example

In the same zinc–copper voltaic cell:

At the copper cathode: $C u^{2 +} (a q) + 2 e^{-} \to C u (s)$ Copper ions are reduced to solid copper.

Analogy

Think of the cathode as a "magnet" for electrons, pulling them in to reduce positively charged ions into neutral atoms. Just like a magnet attracts certain materials, the cathode attracts electrons to complete the reduction process.

Electrode Polarity: Voltaic vs. Electrolytic Cells

The polarity of the electrodes depends on the type of electrochemical cell. Let’s compare the two:

1. Voltaic (Primary) Cells

Voltaic cells generate electrical energy from spontaneous redox reactions.

Example

In a Daniell cell (zinc–copper primary cell):

Anode (oxidation): $Z n (s) \to Z n^{2 +} (a q) + 2 e^{-}$ (negative).
Cathode (reduction): $C u^{2 +} (a q) + 2 e^{-} \to C u (s)$ (positive).

2. Electrolytic Cells

Electrolytic cells use electrical energy to drive non-spontaneous redox reactions.

Example

In the electrolysis of water:

Anode (oxidation): $2 H_{2} O (l) \to O_{2} (g) + 4 H^{+} (a q) + 4 e^{-}$ (positive).
Cathode (reduction): $4 H^{+} (a q) + 4 e^{-} \to 2 H_{2} (g)$ (negative).

Note

In electrolytic cells, the anode is positive because it is connected to the positive terminal of the power supply, which attracts anions.

Schematic drawing of voltaic and electrolytic cells.

Reflection and Practice

Self review

In which type of cell is the cathode positive?
What happens at the anode of an electrolytic cell?
Why does the polarity of electrodes differ between voltaic and electrolytic cells?

Theory of Knowledge

How does the distinction between voltaic and electrolytic cells reflect the interplay between science and technology?
Consider how understanding spontaneous versus non-spontaneous processes has influenced the development of modern energy storage and manufacturing systems.

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Electrochemical Cells: Oxidation and Reduction

An electrochemical cell is a device that converts chemical energy into electrical energy (or vice versa) through redox reactions. These cells are fundamental to understanding how batteries, electrolysis, and many industrial processes work.

Redox reactions involve the transfer of electrons between substances.
The two key processes are:
- Oxidation: Loss of electrons
- Reduction: Gain of electrons

Analogy

Think of a redox reaction like a bank transaction: oxidation is like withdrawing money (losing electrons), while reduction is like depositing money (gaining electrons).

Definition

Electrochemical Cell

A system where chemical energy is converted to electrical energy through redox reactions.

Example

A simple example of an electrochemical cell is a lemon battery, where a copper coin and a zinc nail create a small voltage.