Electrochemistry (LC 2027) (Leaving Cert Chemistry): Revision Notes
Electrolytic Cells
What are electrolytic cells?
An electrolytic cell is a system that uses electrical energy from an external source to drive chemical reactions that wouldn't happen naturally on their own. Unlike galvanic cells (like batteries) that produce electricity from chemical reactions, electrolytic cells do the opposite - they use electricity to force chemical changes to occur.
Think of electrolytic cells as the opposite of batteries. While a battery converts chemical energy into electrical energy, an electrolytic cell converts electrical energy into chemical energy to make reactions happen.
Understanding electrolytes
An electrolyte is a substance that can conduct electricity when it's either melted (molten) or dissolved in water. This happens because electrolytes contain ions that are free to move and carry electric current.
The term "electrolyte" has two related meanings:
- The compound itself that conducts electricity (like sodium chloride)
- The solution through which electric current passes by the movement of ions
When we dissolve salt in water or melt it, the ions become mobile and can carry electric current from one electrode to another. This ion movement is what makes electrolysis possible.
Michael Faraday's contribution
[IMAGE: Figure showing Michael Faraday (1791-1867)]
Michael Faraday was a brilliant scientist who made fundamental contributions to our understanding of electrochemistry. He introduced the word electrolysis into scientific language, creating it from two Greek words: "electro" (relating to electricity) and "lysis" (meaning to split apart).
Faraday's public demonstrations of electrolysis were so popular that they attracted huge crowds. The street where the Royal Institution is located in London even became the first one-way street because of the traffic caused by people coming to see his experiments!
How electrolytic cells work
The process that occurs in electrolytic cells is called electrolysis. This is the use of electricity to bring about a chemical reaction in an electrolyte. When we pass an electric current through an electrolyte, we force a chemical reaction to happen.
[IMAGE: Figure 22.25 showing an electrolytic cell setup with electrodes in electrolyte solution]
How Electrolytic Cells Operate: Step-by-Step Process
Step 1: External power source Unlike galvanic cells, electrolytic cells need an external source of electricity (like a battery or power supply)
Step 2: Current flow The electric current flows from the external source through the electrodes and into the electrolyte
Step 3: Ion movement In the electrolyte, ions move towards the electrodes - positive ions (cations) move to the negative electrode, and negative ions (anions) move to the positive electrode
Step 4: Chemical reactions At each electrode, chemical reactions occur that wouldn't happen without the external electrical energy
Electrodes in electrolytic cells
The electrodes used in electrolytic cells are usually made of inert materials like carbon (graphite) or platinum. These materials are chosen because they don't react with the electrolyte - they simply carry the electric current in and out of the solution.
Why Inert Electrodes Are Essential:
- They don't interfere with the electrolysis reaction
- They provide a surface where reactions can occur
- They carry current without being consumed in the process
In most cases, the electrodes simply carry the current into the electrolyte where the real chemical changes happen. The electrode where reduction occurs is still called the cathode, and the electrode where oxidation occurs is still called the anode, just like in galvanic cells.
Key differences from galvanic cells
Energy Flow Direction:
It's important to understand that the same terms (cathode and anode) apply to both galvanic cells and electrolytic cells, but the energy flow is opposite:
- Galvanic cells: Convert chemical energy → electrical energy (spontaneous)
- Electrolytic cells: Convert electrical energy → chemical energy (non-spontaneous, needs external power)
Examples of electrolysis
Electrolytic cells are used in many important industrial processes:
- Electroplating: Coating objects with thin layers of metals
- Metal extraction: Obtaining pure metals from their compounds
- Electrolysis of water: Splitting water into hydrogen and oxygen gases
- Battery charging: Reversing the discharge reactions in rechargeable batteries
Key Points to Remember:
- Electrolytic cells need external electricity to force non-spontaneous chemical reactions to occur
- Electrolytes conduct electricity through the movement of ions, not electrons
- Michael Faraday coined the term "electrolysis" from Greek words meaning "splitting with electricity"
- Inert electrodes like carbon and platinum don't react with the electrolyte but carry current
- Electrolysis is the opposite of what happens in batteries - it uses electrical energy to create chemical changes