IB DP Chemistry -R3.2.8 Electrolytic cells - Study Notes - New Syllabus - 2026, 2027 & 2028
IB DP Chemistry – R3.2.8 Electrolytic cells – Study Notes – New Syllabus
IITian Academy excellent Introduction to the Proton transfer reactions – Study Notes and effective strategies will help you prepare for your IB DP Chemistry exam.
- IB DP Chemistry 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
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Reactivity 3.2.8 — Electrolytic Cells
Reactivity 3.2.8 — Electrolytic Cells
An electrolytic cell is an electrochemical cell in which electrical energy is converted into chemical energy. These cells are used to drive non-spontaneous redox reactions by applying a direct current (DC) voltage.
Features of Electrolytic Cells
- Require a DC power source to provide electrical energy.
- Oxidation occurs at the anode, and reduction occurs at the cathode.
- Electrons flow from the power supply’s negative terminal to the cathode, and from the anode to the power supply’s positive terminal.
- The electrolyte is usually a molten ionic compound or an aqueous ionic solution that conducts electricity by the movement of ions.
Current Conduction in Electrolytic Cells
The movement of charge in an electrolytic cell occurs via:
- External circuit: Electrons flow from the DC source to the cathode, and away from the anode to the positive terminal.
- Electrolyte: Ions migrate through the electrolyte to balance charge:
- Cations move toward the cathode to gain electrons (reduction).
- Anions move toward the anode to lose electrons (oxidation).
Construction of an Electrolytic Cell
- Power supply: Provides external energy (DC).
- Anode (positive electrode): Site of oxidation. Connected to the + terminal.
- Cathode (negative electrode): Site of reduction. Connected to the – terminal.
- Electrolyte: Molten or aqueous ionic compound allowing ion movement.
Electrolysis of Molten Salts
When an ionic compound is molten, it contains only its constituent ions – no water molecules – which simplifies the prediction of electrolysis products.
- Ions present: \( \text{Na}^+ \), \( \text{Cl}^- \)
- At cathode (reduction): \( \text{Na}^+ + e^- \rightarrow \text{Na} \)
- At anode (oxidation): \( 2\text{Cl}^- \rightarrow \text{Cl}_2 + 2e^- \)
- Overall reaction: \( 2\text{NaCl} (l) \rightarrow 2\text{Na} (l) + \text{Cl}_2 (g) \)
Rules for Predicting Products (Molten Electrolytes)
- Cations are reduced to the metal at the cathode.
- Anions are oxidized to non-metals at the anode.
Example
The electrolysis of molten magnesium chloride is carried out using inert graphite electrodes. Identify the species oxidized and reduced and write the half-equations at each electrode.
▶️Answer/Explanation
- At cathode (reduction): \( \text{Mg}^{2+} + 2e^- \rightarrow \text{Mg} \)
- At anode (oxidation): \( 2\text{Cl}^- \rightarrow \text{Cl}_2 + 2e^- \)
- Oxidized species: \( \text{Cl}^- \)
- Reduced species: \( \text{Mg}^{2+} \)
Example
Sketch and label a simple electrolytic cell used for the electrolysis of molten lead(II) bromide, including the direction of electron and ion flow.
Then write the half-equations for the reactions at each electrode.
▶️Answer/Explanation
- At cathode: \( \text{Pb}^{2+} + 2e^- \rightarrow \text{Pb (l)} \)
- At anode: \( 2\text{Br}^- \rightarrow \text{Br}_2 (g) + 2e^- \)
- Electron flow: From power supply to cathode.
- Ion movement: \( \text{Pb}^{2+} \) migrates to cathode, \( \text{Br}^- \) to anode.
