Required Practical 8 (AQA A Level Chemistry): Revision Note

Required Practical 8

Required Practical 8: Measuring the EMF of an electrochemical cell

• To measure a cell’s EMF, you will need:

  • Two small beakers (approximately 75 cm³ capacity)

  • Strips of suitable 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 (a digital multimeter is usually suitable)

  • Two sets of connecting wires with crocodile clips

  • A salt bridge made from a strip of filter paper soaked in saturated potassium nitrate

Steps in the procedure

• Clean the metal strips to remove any oxide coatings

• This can be done using a piece of sandpaper

• Use long metal strips so they can be folded over the side of the beaker and held in place with crocodile clips

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

• Using tongs, dip a strip of filter paper into a beaker of saturated potassium nitrate solution

• Place the strip between the two beakers, ensuring both ends are well immersed in the solutions

• Connect the crocodile clips to the voltmeter, wait for a steady reading, and record the value

Practical tips

• If a positive reading is not obtained, swap the voltmeter terminals around

• Voltmeters have marked positive and negative terminals (usually red and black). A positive reading indicates the relative polarity of the metals in the cell

• Replace the salt bridge each time to prevent cross-contamination of ions between the half-cells

Specimen Results

• Here is a set of typical results for this experiment

Specimen EMF Results (theoretical values) Table

 Analysis

• It is unlikely that results will closely match theoretical values, as these are defined under standard conditions that are difficult to achieve in a school laboratory

• However, the relative EMFs of the cells constructed should follow the same trend as the theoretical values

• A higher EMF indicates a greater difference in reactivity between the metals, meaning electrons are transferred more readily from one metal to the other