MYP Chemistry Electrochemical Cell Notes

How Does a Simple Electrochemical Cell Work?

Basic Structure

A typical cell consists of:

1. Two different metal electrodes (e.g. Mg and Cu).
2. Each metal placed in a solution containing its own ions (e.g. Mg in MgSO₄(aq), Cu in CuSO₄(aq)).
3. A salt bridge connecting the two solutions.
4. A metal wire and external circuit connecting the two electrodes (possibly with a voltmeter or bulb).

Redox in the Cell – Mg/Cu Example

1. Consider a cell made from magnesium and copper: $$\mathrm{Mg}(\mathrm{~s})+\mathrm{Cu}^{2+}(\mathrm{aq}) \rightarrow \mathrm{Mg}^{2+}(\mathrm{aq})+\mathrm{Cu}(\mathrm{~s})$$
2. This overall reaction is spontaneous (favourable), and the cell uses it to generate a current.
3. At the magnesium electrode (anode): oxidation $$\operatorname{Mg}(\mathrm{s}) \rightarrow \operatorname{Mg}^{2+}(\mathrm{aq})+2 e^{-}$$
   1. Mg loses electrons → it is oxidised.
   2. This electrode is the anode (oxidation always occurs at the anode).
   3. Released electrons enter the external wire.
4. At the copper electrode (cathode): reduction $$\mathrm{Cu}^{2+}(\mathrm{aq})+2 e^{-} \rightarrow \mathrm{Cu}(\mathrm{~s})$$
   1. Cu²⁺ gains electrons → it is reduced.
   2. This electrode is the cathode (reduction always occurs at the cathode).
   3. Solid copper is deposited on the copper electrode.
5. Electron Flow and Energy Conversion
   1. Electrons flow from anode to cathode through the external circuit (Mg → Cu).
   2. This flow of electrons provides electrical energy that can power a device (e.g. a light bulb).
   3. The driving force is the difference in tendency of the two metals to lose/gain electrons (their reduction potentials).
6. So, a voltaic cell converts the chemical energy of a spontaneous redox reaction into electrical energy (a current in the wire).

What Is the Function of the Salt Bridge?

1. The salt bridge is usually a tube or strip filled with an inert electrolyte (e.g. KNO₃ or K₂SO₄ in gel or solution).
2. It does not conduct electrons – those travel in the external wire.
3. Instead, the salt bridge:
   1. Allows ions to move between the half-cells.
   2. Maintains electrical neutrality (charge balance) in each solution.

How Do Metals and Electrolytes Affect Cell Voltage?

The Electrochemical Series and Reduction Potentials