Edexcel A Level Chemistry:复习笔记8.1.2 Electrochemical Cells

Core Practical 10: 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

     

5.4-Electrochemistry-Calculations-Electrochemical-Cell

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

8.1.2-Specimen-EMF-results

 

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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