Electrolytic Cells: Converting Electrical Energy into Chemical Energy
What Is an Electrolytic Cell?
Definition
Electrolytic cell
An electrolytic cell is a device that uses electrical energy to force a non-spontaneous redox reaction to occur.
It consists of three main components:
- Electrolyte: A molten or aqueous ionic compound that provides free-moving ions.
- Electrodes: A cathode (negative electrode) and an anode (positive electrode) immersed in the electrolyte.
- Power Source: A direct current (DC) power source that supplies the energy required to drive the reaction.
- External circuit:
- Provides a pathway for electrons to flow from the anode (where oxidation occurs) to the cathode (where reduction occurs).
- This flow of electrons through the external circuit generates an electric current that can be harnessed to power devices.
Note
In an electrolytic cell:
- Reduction (gain of electrons) occurs at the cathode.
- Oxidation (loss of electrons) occurs at the anode.
The electrical energy supplied by the power source forces electrons to flow from the anode to the cathode, enabling these redox reactions.
Electrolysis of Molten Sodium Chloride
- To better understand how an electrolytic cell works, let’s analyze the electrolysis of molten sodium chloride (NaCl).
- Sodium chloride is composed of sodium (Na⁺) cations and chloride (Cl⁻) anions.
- When molten, these ions are free to move, making the compound conductive.
The Setup
- Electrolyte: Molten NaCl (l).
- Cathode: Connected to the negative terminal of the power source.
- Anode: Connected to the positive terminal of the power source.
The Reactions
- At the cathode, sodium ions ($ \text{Na}^+ $) are reduced to form molten sodium metal:
$$
\text{Na}^+ + e^- \rightarrow \text{Na(l)}
$$ - At the anode, chloride ions ($ \text{Cl}^- $) are oxidized to form chlorine gas:
$$
2\text{Cl}^- \rightarrow \text{Cl}_2(g) + 2e^-
$$ - The overall reaction is:
$$
2\text{NaCl(l)} \rightarrow 2\text{Na(l)} + \text{Cl}_2(g)
$$
Hint
Observations are:
- At the cathode: Shiny molten sodium metal collects.
- At the anode: Greenish-yellow chlorine gas bubbles are released.
Key Features of Electrolysis
Energy Input
- Electrolysis requires an external power source because the reaction is non-spontaneous.
- The electrical energy provided compensates for the positive Gibbs free energy ($ \Delta G > 0 $) of the reaction.
Electrode Reactions
- Cathode (Reduction): Cations in the electrolyte gain electrons and are reduced.
- Anode (Oxidation): Anions in the electrolyte lose electrons and are oxidized.
Tip
Use the mnemonic"RED CAT"(Reduction at Cathode) and"AN OX"(Anode Oxidation) to remember the processes at each electrode.
Ionic Movement
- Cations ($ \text{Na}^+ $) move toward the cathode (negative electrode).
- Anions ($ \text{Cl}^- $) move toward the anode (positive electrode).
Note
Molten salts, as opposed to solid salts, allow ions to move freely, which is essential for electrolysis to occur.
Example
During the electrolysis of molten NaCl:
- At the cathode: $ \text{Na}^+ + e^- \rightarrow \text{Na(l)} $
- At the anode: $ 2\text{Cl}^- \rightarrow \text{Cl}_2(g) + 2e^- $
The products are molten sodium metal and chlorine gas.
Self review
- What is the role of the power source in an electrolytic cell?
- Write the half-equations for the electrolysis of molten magnesium chloride ($ \text{MgCl}_2 $).
- Why is electrolysis of molten salts often performed in an inert atmosphere?
