Electrolytic Cells and Electrolysis of Molten Salts
Quick Notes
- Electrolytic cells use electrical energy to drive non-spontaneous redox reactions.
- Current is conducted by:
- Electrons in external wires
- Ions in the molten or aqueous electrolyte
- Anode = electrode where oxidation occurs (positive).
- Cathode = electrode where reduction occurs (negative).
- A molten salt is decomposed into its elements at the electrodes.
Full Notes
What Is an Electrolytic Cell?
An electrolytic cell is made up of two electrodes placed into an electrolyte and connected to a DC power source.
Electrical energy is used to force a non-spontaneous redox reaction to occur.
How Does Electrolysis Work?
An electric current is passed through a liquid or molten ionic substance (the electrolyte), which contains free ions that can move and carry charge.
Electrons flow from the DC power supply to the cathode. Positive ions migrate to the cathode to gain electrons (reduction). Negative ions move to the anode to lose electrons (oxidation), which return to the positive terminal of the power supply.
Key point:
- The cathode is the negative electrode (reduction happens).
- The anode is the positive electrode (oxidation happens).
You should always remember the cathode is where reduction takes place and anode where oxidation takes place to avoid any confusion. This is because in voltaic cells the cathode is positively charged and the anode is negatively charged (the opposite way round to electrolysis) however still the cathode is where reduction takes place and the anode where oxidation takes place. If you remember cathode = reduction and anode = oxidation, you will always be correct, regardless of whether the question is about electrolysis or voltaic cells.
Predicting Products of Molten Electrolytes
We can predict the products formed at each electrode based on the type of electrolyte being used.
The ionic compound is melted to a liquid state, and only the cations and anions of the compound are present:
- Cation is reduced at the cathode.
- Anion is oxidised at the anode.
Example Electrolysis of molten NaCl
- Cathode: Na⁺ + e⁻ → Na
- Anode: 2Cl⁻ → Cl₂ + 2e⁻
Summary
- Electrolytic cells use external energy to drive non-spontaneous redox reactions.
- Electrons flow in wires and ions move in the electrolyte.
- Cathode = reduction (negative electrode).
- Anode = oxidation (positive electrode).
- Molten salts break into elements at the electrodes.
Linked Course Question
Under what conditions can ionic compounds act as electrolytes?
Ionic compounds act as electrolytes only when their ions are free to move. This happens under the following conditions:
- Molten state: The ionic lattice is broken due to heat, allowing ions to move freely. Example: molten NaCl conducts electricity as Na⁺ and Cl⁻ ions are mobile.
- Aqueous solution: The compound dissolves in water, dissociating into free ions. Example: NaCl(aq) → Na⁺(aq) + Cl⁻(aq).
- Solid state: Ionic compounds do not conduct electricity, because the ions are fixed in a rigid lattice and cannot move.