Electrolysis is one of the most important concepts in IB Chemistry Topic 9 (Redox Processes) and Topic 19 (HL). It explains how electrical energy can be used to force chemical reactions that would not occur on their own. Electrolysis is used in metal extraction, electroplating, manufacturing chemicals, and charging batteries. Understanding how it works gives insight into redox chemistry, electron flow, and industrial processes.
What Is Electrolysis?
Electrolysis is the process of driving a non-spontaneous redox reaction using electrical energy from an external power supply.
In simpler terms:
- A power source forces electrons into or out of substances.
- This makes chemical reactions happen that normally would NOT occur.
- Electrons flow in a controlled direction to separate substances or produce new ones.
Electrolysis happens in an electrolytic cell, which is the opposite of a galvanic (spontaneous) cell.
Key Features of Electrolysis
Electrolysis always involves:
- Electrical energy input
- Non-spontaneous reactions (ΔG > 0)
- Oxidation and reduction at the electrodes
- Flow of ions through the electrolyte
- Flow of electrons through external wires
Electrolytic cells turn electrical energy → chemical energy.
The Electrolytic Cell
An electrolytic cell contains:
- A power supply (battery or DC source)
- Two electrodes: anode and cathode
- An electrolyte containing mobile ions
Important difference from galvanic cells:
- Anode is positive
- Cathode is negative
This is because the external power source forces electrons into the cathode and pulls them from the anode.
Oxidation and Reduction in Electrolysis
Even though charges are reversed compared to a battery, the redox definitions remain the same:
- Oxidation occurs at the anode (positive electrode)
- Reduction occurs at the cathode (negative electrode)
At the cathode (reduction):
Cations move toward the negative electrode and gain electrons.
Example (molten NaCl):
Na⁺ + e⁻ → Na(l)
At the anode (oxidation):
Anions move toward the positive electrode and lose electrons.
Cl⁻ → ½Cl₂ + e⁻
This produces elemental sodium and chlorine gas.
Electrolysis of Molten Ionic Compounds
Molten electrolytes contain only ions of a single compound.
This leads to simple, predictable products.
Example: molten PbBr₂
- Cathode: Pb²⁺ + 2e⁻ → Pb(l)
- Anode: 2Br⁻ → Br₂(g) + 2e⁻
Molten electrolysis is used to extract highly reactive metals like sodium, magnesium, and aluminum.
Electrolysis of Aqueous Solutions
Aqueous solutions are more complex because water can also undergo redox reactions.
At the cathode, the more easily reduced species is chosen between:
- Metal cations
- Water
At the anode, the more easily oxidized species is chosen between:
- Anions
- Water
This selection depends on electrode potentials.
Example: Electrolysis of aqueous NaCl
- Cathode: water is reduced → H₂ gas forms
- Anode: Cl⁻ is oxidized → Cl₂ gas forms
Faraday’s Laws (HL Concept)
Faraday’s laws relate:
- Amount of product formed
- Charge passed
- Moles of electrons involved
Key relationship:
Q = I × t
and
moles = Q / (F × n)
Where F is Faraday’s constant (96485 C mol⁻¹).
These laws allow you to calculate mass deposited or gas produced.
Applications of Electrolysis
1. Metal Extraction
Aluminum is extracted from alumina using molten electrolysis (Hall–Héroult process).
2. Electroplating
A thin metal layer is deposited onto objects to prevent corrosion or improve appearance.
3. Manufacture of chemicals
- Chlorine
- Hydrogen
- Sodium hydroxide
4. Rechargeable batteries
Charging a battery forces non-spontaneous reactions using electricity.
Common IB Misunderstandings
“Electrons flow through the electrolyte.”
Incorrect—only ions move through the electrolyte. Electrons move in the wire.
“Cathode is always positive.”
Only true in galvanic cells.
In electrolytic cells, the cathode is negative.
“Electrolysis is spontaneous.”
Electrolysis requires external energy—non-spontaneous by definition.
FAQs
Why does electrolysis require electricity?
Because the reaction is thermodynamically unfavorable and needs energy to proceed.
Why does the anode have a positive charge?
Electrons are pulled from the anode by the external power supply.
Can electrolysis occur without ions?
No. Mobile ions are essential for current flow inside the cell.
Conclusion
Electrolysis uses electrical energy to force non-spontaneous redox reactions. Oxidation happens at the positive anode, and reduction happens at the negative cathode. This process is vital for metal extraction, electroplating, manufacturing chemicals, and charging batteries. Understanding electrolysis is essential for mastering redox chemistry in IB Chemistry.
