Specifications

Core Practicals

Core Practical 4

Electrolysis of copper sulfate solution

Introducing the Practical

This involves setting up an electrolysis to investigate the effect of changing the current on the mass of the copper electrodes used in the electrolysis of copper sulfate solution.

  

The second part of this investigation covers the products formed during the electrolysis of copper sulfate solution using inert (graphite) electrodes.

  

Quantitative analysis when using copper electrodes will be expected.

Risk Assessment

As a general rule, eye protection (goggles) must be worn for all practicals.

  

This risk assessment is provided as an example only, and you must perform your own risk assessment before doing this experiment.

Apparatus

Each group will need:

  • low voltage supply (0‐12 V)

  • ammeter (0‐1 A)

  • variable resistor

  • connecting leads

  • crocodile clips

  • 100 ml beaker

  • stop watch

  • 2 graphite rods

  • strips of copper foil

  • copper sulfate solution

  • access to a balance (at least 2 d.p.)

Experiment Set-up

Method

Part 1 - Investigation using inert electrodes

  1. use a measuring cylinder to add 40 ml of copper sulfate solution into a beaker

  2. place two graphite rods into the copper sulfate solution - attaching one electrode to the negative terminal of a dc supply, and the other electrode to the positive terminal

  3. fill a small test tube with copper sulfate solution and position over the positive electrode (anode) - as shown in the diagram above

  4. turn on the power supply and observe what happens at each electrode

  5. test any gas produced with a glowing splint

  6. record your observations and the results of your tests

Part 2 - Investigation using copper electrodes

  1. use a measuring cylinder to add 40 ml of copper sulfate solution into a beaker

  2. measure and record the masses of two pieces of copper foil, labelling one the anode, and one the cathode

  3. attaching the cathode to the negative terminal of a dc supply, and the anode to the positive terminal

  4. turn on the power supply, adjust the power supply to achieve a constant current as directed by your teacher, and observe what happens at each electrode

  5. after 10 minutes, turn off the power

  6. carefully remove the electrodes and allow all the liquid to evaporate - do not wipe the electrodes clean

  7. measure and record the mass of the electrode

  8. repeat the experiment again with new electrodes, and different currents

Results and Analysis

Part 1 - Investigation using inert electrodes

What did you observe happening at each electrode?

Where there any gases produced? How could you test to see which gas was produced?

Part 2 - Investigation using copper electrodes

Calculate the change in mass of each electrode, and then plot a graph to show how the current affected the change in mass

  • change in mass plotted on the Y axis (vertical)

  • current plotted on the X axis (horizontal)

Why did one increase in mass, but the other decrease? Was it by the same amount?

Exam Question and Model Answer

A student conducts an investigation to find out what is produced during the electrolysis of sodium sulfate.
Describe how the student could carry out an investigation, and show (using ions) what is given off at each electrode.

[6 marks}

Level 1 (1-2 marks)

Add sodium sulfate solution to a beaker, and connect two electrodes to a power supply.
Completely fill two small test tubes with sodium sulfate solution and position a test tube over each electrode, then turn on the power supply and observe what happens at each electrode. 

hydrogen gas will form at the negative electrode
oxygen gas will form at the positive electrode

Level 2 (3-4 marks)

Wearing safety glasses, add sodium sulfate solution to a beaker, and connect two inert electrodes (make sure they do not touch) to a power supply.
Completely fill two small test tubes with sodium sulfate solution and position a test tube over each electrode, then turn on the power supply and observe what happens at each electrode.
Any gases produced can be collected in the test tubes and tested (squeaky pop test - hydrogen, relight a glowing splint - oxygen). 

hydrogen gas will form at the negative electrode
oxygen gas will form at the positive electrode

Level 3 (5-6 marks)

Wearing safety glasses, add sodium sulfate solution to a beaker, and connect two inert electrodes (make sure they do not touch) to a power supply.
Completely fill two small test tubes with sodium sulfate solution and position a test tube over each electrode, then turn on the power supply and observe what happens at each electrode.
Any gases produced can be collected in the test tubes and tested (squeaky pop test - hydrogen, relight a glowing splint - oxygen). 

hydrogen gas will form at the negative electrode:   2H(aq) + 2e → H(g)
oxygen gas will form at the positive electrode:   4OH(aq) → 2HO(l) + O(g) + 4e

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