Electrochemical Cells (Edexcel International A Level (IAL) Chemistry): Revision Note

Core Practical 12: Investigating Electrochemical Cells

Measuring the EMF of a cell

• To measure a cell EMF you will need

  • Two small beakers, around 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 their solubility)

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

  • Two sets of wires with crocodile clips

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

The experimental set up for measuring the EMF of a cell made of two metal / metal ion half cells

The procedure

• The strips of metals need to be freshly cleaned to remove any oxide coatings

  • This can be done with a piece of sandpaper

• To support the metals, it is easiest to have long strips that can be folded over the side of the beaker and held in place with the crocodile clips

• Fill up the beakers to about two thirds of the way with the metal ion solutions

• Using tongs, dip a strip of filter paper into a beaker of saturated potassium nitrate solution and then place it between the two beakers making sure the ends of the strip are well immersed in the solutions

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

Practical tips

• If you don't get a positive reading on the voltmeter swap the terminals around

• Voltmeters will have marked positive and negative terminals (usually in red and black, respectively), so when you get a positive reading this tells you the relative polarity of the metals in the cell

• Change the salt bridge each time, to prevent cross contamination of ions between half cells

Calculating the Theoretical EMF of a Cell

• The voltage you measure in your experiment can be compared to a theoretical value calculated using standard electrode potentials (E°)

  • These values are found in the electrochemical series in your data book.

• The EMF of a cell is the difference between the E° values of the two half-cells

• The formula is:

E°(cell) = E°(positive electrode) - E°(negative electrode)

• A simple way to remember this is "most positive E° value minus most negative E° value

This calculation is how the theoretical values in the specimen results table are determined.

Specimen Results

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

Analysis

• It is unlikely you will get very close to the theoretical results as these would be obtained under standard conditions (1.0 mol dm^-3 concentration, 298 K temperature)

  • This is because these conditions are hard to achieve in a school laboratory

• However, the relative EMF of cells you construct should match the theoretical values

• The higher the EMF, the larger the difference in reactivity ('electron pushing power') between the metals