Electrochemical Cells (OCR A Level Chemistry A): Revision Note

PAG 8: Electrochemical cells

• To measure a cell EMF, you will need:

  • Two small beakers (around 75 cm³ capacity)

  • Strips of metals such as copper, zinc, iron and silver

  • 1.0 mol dm^-3 solutions of the metal ions (nitrates, chlorides or sulfates depending on solubility)

  • A high resistance voltmeter (usually a digital multimeter has this)

  • Two sets of wires with crocodile clips

  • A salt bridge made from filter paper soaked in saturated potassium nitrate solution

Method

• Clean the metal strips with sandpaper to remove any oxide coating

• Use long metal strips so you can fold them over the side of the beaker and clip them in place

• Fill each beaker about two-thirds full with the appropriate metal ion solution

• Soak a strip of filter paper in saturated potassium nitrate

  • Place it between the two beakers so both ends are submerged

• Connect the crocodile clips to the voltmeter

• Wait for a steady reading

• Record the voltage

Practical tips

• If the voltmeter shows a negative reading, swap the terminals

• Red and black terminals usually indicate positive and negative, respectively

  • A positive reading tells you which metal is acting as the positive electrode

• Use a fresh salt bridge for each setup to avoid cross-contamination between solutions

Specimen results

• Here is a set of typical results for this experiment:

Analysis

• You may not obtain results close to the theoretical values

  • This is because standard conditions are difficult to achieve in a school lab

• However, the relative EMF values between your test cells should match the theoretical trend

• A higher EMF indicates a greater difference in reactivity (or "electron pushing power") between the metals

Practical skills reminder

• This practical develops essential skills in setting up and analysing electrochemical cells, including:

  • Cleaning and preparing metal electrodes

  • Constructing electrochemical cells using a salt bridge and two ionic solutions

  • Using a voltmeter (or multimeter) to measure EMF and interpret sign conventions

  • Changing variables systematically (e.g. metals used) to explore reactivity trends

  • Understanding the impact of non-standard conditions on expected voltages