EMF of Electrochemical Cells (Oxford AQA International A Level Chemistry)

Required Practical 8: Measuring EMF

Objective

Measure the electromotive force (EMF) of an electrochemical cell.

Apparatus

• Copper and zinc foil pieces

• Propanone

• 2.0 mol dm^–3 NaCl solution

• 1.0 mol dm^–3 CuSO₄ solution

• 1.0 mol dm^–3 ZnSO₄ solution

• Sandpaper

• Cotton wool

• 100 cm³ beakers

• Plastic / glass U-tube

• High resistance voltmeter

• Two wires and crocodile clips

• Metal samples

Method - Setting up a standard cell

1. Take one piece of copper and one piece of zinc:

   • Clean them using sandpaper

   • Degrease the metal using cotton wool and propanone.

2. Place the copper into a 100 cm³ beaker with 50 cm³ of 1 mol dm^–3 CuSO₄ solution

3. Place the zinc into a 100 cm³ beaker with 50 cm³ of 1 mol dm^–3 ZnSO₄ solution

4. Prepare the salt bridge:

   • Plug one end of the plastic / glass U-tube with cotton wool

   • Fill the plastic / glass U-tube with 2 mol dm^–3 NaCl solution

   • Plug the free end of the tube with cotton wool which has been soaked in 2 mol dm^–3 NaCl solution

5. Connect the two beakers with the inverted U-tube so that the plugged ends are in the separate beakers.

6. Connect the Cu (s) | Cu²⁺ (aq) and Zn (s) | Zn²⁺ (aq) half-cells:

   • Using a crocodile clip, connect a wire to the piece of copper

   • Plug the other end of this wire into the high resistance voltmeter

   • Using a crocodile clip, connect a wire to the piece of zinc

   • Plug the other end of this wire into the high resistance voltmeter

7. Read off the voltage

Diagram

Practical Tip

• If you set up another cell, replace the salt bridge to prevent cross contamination of ions between half cells

Evaluation

• Write the conventional representation for the cell that you have constructed

• You may be asked to explain why the value you recorded is different to the theoretical value

Method - Measuring comparative electrode potentials of different metals

1. Clean a piece of copper using sandpaper

2. Using a crocodile clip, connect a wire to the piece of copper

3. Plug the other end of this wire into the high resistance voltmeter

4. Prepare the salt bridge:

   • Cut a piece of filter paper to about the same area as the copper

   • Wet the filter paper with the sodium chloride solution

   • Place this on top of the copper

5. Preparing the metal sample:

   • Connect a second wire lead to the voltmeter

   • Connect a crocodile clip to the other end of the wire

   • Place a piece of sample metal in the crocodile clip

6. Testing the metal sample

   • Hold the sample metal against the filter paper

   • Record the voltage and sign in a suitable table

   • Repeat this process with different metal samples

Diagram

Practical Tip

• 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

Results - Measuring comparative electrode potentials of different metals

• Record your results for each test carefully in a suitable table like the one below: 

Evaluation

• The half-equation and E^ꝋ value for copper are:

Cu²⁺ (aq) + 2e^- → Cu (s)              E^ꝋ = +0.34 V

• Use this information and your results to determine the E^ꝋ values for the sample metals

• Suggest how you could construct the cell with the largest electromotive force from the sample metals