Core Practical 10 – Electrochemical Cells and Electrode Potentials

Key Concepts

Electrochemical cells consist of two half-cells connected by a salt bridge and a voltmeter. The potential difference between the two electrodes can be measured and compared to standard electrode potentials.

Use the formula:

E_cell = E_right – E_left

Where:

• E_cell is the measured potential difference (V).
• E_right is the potential of the cathode (reduction occurs).
• E_left is the potential of the anode (oxidation occurs).

Safety

• Wear eye protection.
• Zinc sulfate and iron(II) sulfate are harmful.
• Potassium nitrate is oxidising.
• Handle silver nitrate carefully – corrosive in concentrated form.
• Consult CLEAPSS Hazcards before running the practical.

Equipment List

• Zinc, copper, iron, and silver metal strips
• Solutions:
  • 0.4 mol dm⁻³ zinc sulfate
  • 0.4 mol dm⁻³ copper(II) sulfate
  • 1.0 mol dm⁻³ iron(II) sulfate
  • 0.1 mol dm⁻³ silver nitrate
• Saturated potassium nitrate (salt bridge)
• 4 × 100 cm³ beakers
• Filter paper strips (~12 cm long)
• Sandpaper
• Voltmeter (2 d.p.)
• Wires and crocodile clips

Procedure

1. Clean metal strips with sandpaper.
2. Set up half-cells by placing each metal into its matching ion solution in a beaker.
3. Connect the half-cells with a salt bridge (filter paper soaked in saturated potassium nitrate).

4. Connect the metals to a voltmeter using wires and crocodile clips.
5. Record the voltage. If negative, reverse the connections.
6. Repeat for these pairs:
   • Zn | Zn²⁺ and Cu²⁺ | Cu
   • Zn | Zn²⁺ and Fe²⁺ | Fe
   • Fe | Fe²⁺ and Cu²⁺ | Cu
   • Zn | Zn²⁺ and Ag⁺ | Ag
   • Cu | Cu²⁺ and Ag⁺ | Ag

Results Table

Example Calculation

Given: E_cell = 1.10 V, E_Cu²⁺/Cu = +0.34 V
Find E_Zn²⁺/Zn:
E_cell = E_right – E_left
1.10 = 0.34 – E_left ⇒ E_left = 0.34 – 1.10 = –0.76 V
So: E_Zn²⁺/Zn = –0.76 V

Matt’s Exam Tip

• When solving for unknown E values, always identify which is the right (positive terminal, cathode) and left (negative terminal, anode).
• Use the standard equation: E_cell = E_right – E_left (i.e. E_cathode – E_anode).
• Rearrange algebraically to solve for the unknown.
• Why is Mg problematic? It reacts slowly with water, increasing [Mg²⁺] and shifting equilibrium during measurement, making values unreliable.