Drawing Half Cell and Electrochemical Cell

An electrochemical cell is made up of 2 half cells linked together via a salt bridge and the 2 electrodes are connected via an external wire to generate flow of electrons from the anode to the cathode.

drawing half cell and electrochemical cell electrochemical cell schematic

It is also known as a voltaic cell, galvanic cell or battery. One example of an electrochemical cell is the hydrogen oxygen fuel cell.

The purpose of the salt bridge is to connect the 2 half cells together so charges can flow between them, but prevents the direct mixing of reactants between the 2 half cells.

We are required to draw a detailed set up of an electrochemical cell under standard conditions, so we need to know how to draw these 3 different types of half-cells depending on the physical states of the species in the reversible electrode.

1. Metal-ion half cell

Example: Zn2+(aq) / Zn(s) half cell

drawing half cell and electrochemical cell metal ion half cell

Concentration of aqueous ions at standard conditions has to be 1.0 mol dm-3.

2. Non metal-ion half cell or gas-ion half cell

Example: H+(aq) / H2(g) half cell

drawing half cell and electrochemical cell gas ion half cell

Gas has to be at standard temperature of 298K and standard pressure of 1 atm.

Platinum or graphite is used as an inert electrode which serves as the "electron bank".

For example, H+ gains electrons from the electrode and is reduced to H2, so electrons are withdrawn from the electrode.

Conversely H2 loses electron to the electrode and is oxidised to H+, hence electrons are deposited to the electrode.

Therefore there will not be any half cell established if the electrode is absent.

3. Ion-ion half cell

Example: Fe3+(aq) / Fe2+(aq) half cell

drawing half cell and electrochemical cell ion ion half cell

Concentration of Fe3+(aq) and Fe2+(aq) has to be 1.0 mol dm-3 each.

Platinum or graphite is chosen as the inert electrode.

4. Electrochemical Cell Example

We can now combine any 2 half cells together to form an overall electrochemical cell.

Let's use Fe3+ / Fe2+ half cell and H+ / H2 half cell as an example.

drawing half cell and electrochemical cell electrochemical cell example

We just need to draw the 2 half cells as discussed previously and use a salt bridge to link the 2 half cells together.

We also draw an external wire connecting the 2 electrodes together through a voltmeter so that the voltage or potential difference of the electrochemical cell can be measured.

For the detailed step-by-step discussion on how to draw the 3 types of half cells and overall electrochemical cell, check out this video!

Topic: Electrochemistry, Physical Chemistry, A Level Chemistry, Singapore

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