Representing Cells (AQA A-Level Chemistry): Revision Notes

5.4.1 Representing Cells

Understanding Electrochemical Cells

Electrochemical cells consist of two half-cells, each containing an electrode (solid conducting material) immersed in an electrolyte solution of its ions. The half-cells are connected by:

• A wire (allowing electron flow between electrodes).
• A salt bridge, usually a philtre paper soaked in $KNO_3$, which completes the circuit by allowing ion movement while preventing direct mixing of solutions.

Anode vs. Cathode

In an electrochemical cell:

• Anode (Negative Electrode): Where oxidation occurs, releasing electrons.
• Cathode (Positive Electrode): Where reduction occurs, gaining electrons.

The Conventional Representation of Cells (Cell Notation)

The IUPAC developed a standardised notation to represent cells concisely without needing complex diagrams. Here's how to write this notation:

1. Oxidation Half-Cell (Anode):

• Write the oxidation half-cell (more negative electrode potential) on the left.

2. Reduction Half-Cell (Cathode):

• Write the reduction half-cell (less negative electrode potential) on the right.

3. Phase Boundaries (|):

• Use a single vertical line | between components within the same half-cell, representing a phase boundary.

4. Salt Bridge (||):

• Place a double vertical line || between the two half-cells, representing the salt bridge.

5. Inert Electrodes:

• If Pt (platinum) is used as an inert electrode (for gases or ions in solution), place it at the extreme left or right, depending on the half-cell it represents.

Measuring Cell Voltage (EMF)

To measure the potential difference (EMF) of a cell:

• Connect a high-resistance voltmeter in the external circuit to measure EMF without significant electron flow.
• This value, $E_{\text{cell}}$, indicates the voltage between the two half-cells.