IB DP Chemistry -R3.2.6 Primary (voltaic) cells- Study Notes - New Syllabus - 2026, 2027 & 2028
IB DP Chemistry – R3.2.6 Primary (voltaic) cells – Study Notes – New Syllabus
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Reactivity 3.2.6 — Primary (Voltaic) Cells
Reactivity 3.2.6 — Primary (Voltaic) Cells
A primary cell, also called a voltaic cell or galvanic cell, is an electrochemical device that converts the chemical energy from a spontaneous redox reaction into electrical energy. These reactions are exothermic and occur naturally when two different metals or metal ions are connected through a conductive path.
Key Features of a Primary Cell
- Two separate half-cells, each containing a metal electrode immersed in a solution of its own ions.
- An external circuit that allows the flow of electrons from the anode (oxidation) to the cathode (reduction).
- A salt bridge that allows the movement of ions to maintain electrical neutrality.
Half-Cell Construction
| Component | Description |
|---|---|
| Anode | Site of oxidation — electrons are lost here. Electrons flow away from the anode via external wire. |
| Cathode | Site of reduction — electrons are gained here. Electrons flow into the cathode. |
| Salt Bridge | Allows the movement of ions (e.g. Na+, NO3−) to balance charge. Prevents solution build-up of charge. |
| External Circuit | Connects electrodes with a wire, allowing electrons to flow from the anode to the cathode. |
Example: Zn/Cu Primary Cell
This is a classic example of a voltaic cell composed of zinc and copper half-cells.
Anode: Zinc metal is oxidized
\( \text{Zn (s)} \rightarrow \text{Zn}^{2+} \text{(aq)} + 2e^- \)
Cathode: Copper(II) ions are reduced
\( \text{Cu}^{2+} \text{(aq)} + 2e^- \rightarrow \text{Cu (s)} \)
Overall cell reaction:
\( \text{Zn (s)} + \text{Cu}^{2+} \text{(aq)} \rightarrow \text{Zn}^{2+} \text{(aq)} + \text{Cu (s)} \)
Electron Flow and Ion Movement
- Electrons flow from the anode (Zn) to the cathode (Cu) through the external circuit.
- The salt bridge allows anions to move toward the anode compartment and cations to move toward the cathode compartment.
Direction of Electron Flow
ALWAYS:
Electrons flow from anode to cathode externally.
Ions flow in the salt bridge to maintain charge balance.
Important Concepts to Remember
- Oxidation occurs at the anode.
- Reduction occurs at the cathode.
- The cell potential is positive for a spontaneous redox reaction.
- The salt bridge prevents charge buildup in either compartment.
Example
A voltaic cell is constructed using a magnesium half-cell and a silver half-cell. Write the half-equations and overall redox reaction, and indicate the direction of electron flow.
▶️Answer/Explanation
Half-equations:
Oxidation (anode): \( \text{Mg (s)} \rightarrow \text{Mg}^{2+} \text{(aq)} + 2e^- \)
Reduction (cathode): \( \text{Ag}^{+} \text{(aq)} + e^- \rightarrow \text{Ag (s)} \)
Balanced overall equation:
\( \text{Mg (s)} + 2\text{Ag}^{+} \text{(aq)} \rightarrow \text{Mg}^{2+} \text{(aq)} + 2\text{Ag (s)} \)
Electrons flow from magnesium to silver.
Example
In a voltaic cell, zinc and iron half-cells are used. The concentration of ions in both solutions is 1.0 mol dm−3. Which metal acts as the anode? Justify your answer using standard electrode potentials.
▶️Answer/Explanation
The standard electrode potential for:
- Zn2+/Zn = −0.76 V
- Fe2+/Fe = −0.44 V
Since zinc has a more negative standard electrode potential, it is more easily oxidized. Therefore:
- Zinc is the anode (oxidation occurs).
- Iron is the cathode (reduction occurs).
Electrons flow from zinc to iron.