Electrochemical Cells (OCR A Level Chemistry)

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PAG 8: Electrochemical Cells

  • 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

Electrochemistry Calculations - Electrochemical Cell, downloadable AS & A Level Chemistry revision notes

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

Steps in 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

Specimen Results

  • Here is a set of typical results for this experiment

Specimen EMF Results (theoretical values) Table

Specimen EMF results, downloadable AS & A Level Chemistry revision notes

 Analysis

  • It is unlikely you will get very close to the theoretical results as these would be obtained under standard conditions which 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

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Richard

Author: Richard

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Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.