Summary notes for AQA A-Level Chemistry Unit 3.1.11 - Electrode potentials and electrochemical cells (A-level only) by an Imperial College London MSci Chemistry graduate. Notes divided into the following sections: Electrode Potentials and Cells, Commercial Applications of Electrochemical Cells
Electrode Potentials and Electrochemical Cells
Electrode Potentials and Cells
Electrochemical Cells
Electrochemical cells can be made from 2 half cells - 2 different metals dipped in salt solutions of
their own ions + connected by a wire (the external circuit).
• A half cell is an electrode in an electrochemical cell.
- An electrode has to be a solid that conducts electricity. In electrochemical cells w/…
• 2 solid pieces of metal involved, the 2 metals = the electrodes.
• a half cell which involves gas w/ ions in solution or 2 ions in solution, in that half cell platinum =
the electrode.
- Platinum is used because it is inert + conducts electricity.
• An electrode where oxidation occurs is called the negative electrode (anode).
• An electrode where reduction occurs is called the positive electrode (cathode).
The 2 solutions in the half cells are connected using a salt bridge.
• An example of a salt bridge is a piece of lter paper soaked in a solution of KNO3.
- The salt bridge has mobile ions that complete the circuit. KNO3 is used as its ions are relatively
unreactive + to ensure there is no ppt. as nitrates are soluble.
There are always 2 reactions w/in an electrochemical cell - one is an oxidation reaction + the other a
reduction reaction - ∴ it’s a redox process.
E.g. electrochemical cell made using copper + zinc…
The two metals in the half cells (copper + zinc) are connected externally using a conducting wire + the
solutions are connected using a salt bridge.
• The half-equations for the electrode reactions occurring are: Zn → Zn2+ + 2e-
Cu2+ + 2e- → Cu
• ∴ the overall equation for the electrode reactions occurring is: Zn + Cu2+ → Zn2+ + Cu
• In this cell…
- Zinc = the anode
- Copper = the cathode
- In the half-cell on the LHS, Zn (from the Zn
electrode) is oxidised to form Zn2+. This releases
electrons into the external circuit.
• Zn forms Zn2+ + this ↓ the mass of the Zn
electrode.
- In the half-cell on the RHS, Cu2+ is reduced to form
Cu using the electrons transferred via the external
circuit from the Zn electrode.
• Cu forms from Cu2+ on the Cu electrode + this ↑
the mass of the electrode.
The Conventional Representation of Cells (Cell Notation)
Rather than drawing complex diagrams of electrochemical cells, the conventional representation of
cells is used. The rules for writing this cell notation are:
1. The oxidation electrode (i.e. the electrode w/ the more -ve electrode potential) is written on the
LHS w/ the components in order of being oxidised.
2. The reduction electrode (i.e. the electrode w/ the less -ve electrode potential) is written on the RHS
w/ the components in order of being reduced.
3. ‘||’ is used between the 2 electrodes to represent the salt bridge.
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