IB DP Chemistry - R3.2.5 Electrochemical cells: Oxidation and reduction- Study Notes - New Syllabus - 2026, 2027 & 2028
IB DP Chemistry – R3.2.5 Electrochemical cells: Oxidation and reduction – 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 3.2.5 — Oxidation and Reduction in Electrochemical Cells
Reactivity 3.2.5 — Oxidation and Reduction in Electrochemical Cells
In all electrochemical cells, oxidation occurs at the anode and reduction occurs at the cathode. The direction of electron flow, electrode polarity, and chemical changes depend on whether the cell is voltaic (galvanic) or electrolytic.
Definition of Terms:
- Oxidation: Loss of electrons.
- Reduction: Gain of electrons.
- Anode: Electrode where oxidation occurs.
- Cathode: Electrode where reduction occurs.
- Electrolyte: Ionic solution or molten ionic compound that conducts electricity.
I. Voltaic (Galvanic) Cells
Purpose: Convert chemical energy into electrical energy (spontaneous redox reaction).
- Anode: Site of oxidation. The negative electrode.
- Cathode: Site of reduction. The positive electrode.
- Electron flow: From anode to cathode through the external circuit.
- Salt bridge: Maintains electrical neutrality by allowing ion migration between half-cells.
| Electrode | Half-Equation | Process | Charge |
|---|---|---|---|
| Anode (Zn) | \( \text{Zn}(s) \rightarrow \text{Zn}^{2+}(aq) + 2e^- \) | Oxidation | Negative |
| Cathode (Cu) | \( \text{Cu}^{2+}(aq) + 2e^- \rightarrow \text{Cu}(s) \) | Reduction | Positive |
II. Electrolytic Cells
Purpose: Use electrical energy to drive a non-spontaneous chemical reaction.
- Anode: Site of oxidation. The positive electrode.
- Cathode: Site of reduction. The negative electrode.
- Electron flow: From power supply (positive terminal to anode, negative to cathode).
- No salt bridge: Since electrolyte is in a single compartment.
| Electrode | Half-Equation | Process | Charge |
|---|---|---|---|
| Anode | \( 2\text{Cl}^- \rightarrow \text{Cl}_2(g) + 2e^- \) | Oxidation | Positive |
| Cathode | \( \text{Na}^+ + e^- \rightarrow \text{Na}(l) \) | Reduction | Negative |
Example
A student constructs a voltaic cell using magnesium and copper electrodes. Write the overall redox equation and identify the anode and cathode.
▶️Answer/Explanation
Half-equations:
At anode: \( \text{Mg}(s) \rightarrow \text{Mg}^{2+}(aq) + 2e^- \)
At cathode: \( \text{Cu}^{2+}(aq) + 2e^- \rightarrow \text{Cu}(s) \)
Overall equation:
\( \text{Mg}(s) + \text{Cu}^{2+}(aq) \rightarrow \text{Mg}^{2+}(aq) + \text{Cu}(s) \)
Anode: Mg (oxidation, negative electrode)
Cathode: Cu (reduction, positive electrode)
Example
During the electrolysis of molten lead(II) bromide (\( \text{PbBr}_2 \)), identify the products at each electrode and write the half-equations.
▶️Answer/Explanation
At the cathode (−):
\( \text{Pb}^{2+} + 2e^- \rightarrow \text{Pb}(l) \)
At the anode (+):
\( 2\text{Br}^- \rightarrow \text{Br}_2(g) + 2e^- \)
Products: Lead at the cathode, bromine gas at the anode.
