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6.5 Required Practicals: Electrolysis

GCSE Chemistry · AQA · Chapter 6: Electrolysis

A free GCSE Chemistry lesson for AQA (8462) from OBA School: required practical 3: electrolysis of aqueous solutions using inert electrodes. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe how to electrolyse aqueous solutions safely using inert electrodes
  • Identify the products using tests for hydrogen, oxygen and chlorine and by observation
  • Develop a hypothesis that links the products to the reactivity of the metal and the type of anion
  • Name the variables and write a risk assessment
  • Process and evaluate results, including mass changes at the cathode

Key words

  • Hypothesis: A testable idea that explains something, e.g. “if the metal is less reactive than hydrogen, the metal forms at the cathode”.
  • Independent variable: The variable you change: here, the solution being electrolysed.
  • Dependent variable: What you observe or measure: the product at each electrode.
  • Control variable: Kept the same for a fair test, e.g. voltage.

Aim and equipment

Aim: to find out what is produced at each electrode when different aqueous solutions are electrolysed with inert electrodes, and to test the hypothesis that the products depend on the reactivity of the metal and on whether halide ions are present.

You need: a small electrolysis cell (a plastic container with two graphite electrodes fitted through its base), low-voltage d.c. power pack, leads and crocodile clips, two small test tubes, wooden splints, damp blue litmus paper, tweezers, eye protection; solutions (about 0.5 mol/dm³) of copper(II) chloride, copper(II) sulfate, sodium chloride and sodium sulfate.

Method

  • 1. Pour about 50 cm³ of copper(II) chloride solution into the cell so the electrodes are covered.
  • 2. Connect the electrodes to the d.c. supply. Note which electrode is the anode (+) and which is the cathode (−).
  • 3. Fill two test tubes with the solution and turn them upside down over each electrode to collect any gas.
  • 4. Switch on at about 4 V. Watch both electrodes for a few minutes and record what you see.
  • 5. Test any gas: lighted splint (hydrogen), glowing splint (oxygen); hold damp blue litmus paper near the anode (chlorine).
  • 6. Switch off, clean the electrodes and repeat with the other solutions, keeping the volume, voltage and time the same.
  • Independent; In this practical the solution (which metal ion and which anion it contains)
  • Dependent; In this practical the product at each electrode (identified by observation and gas tests)
  • Control; In this practical voltage, concentration and volume of solution, type and size of electrodes, distance between electrodes, time
  • chlorine gas; Risk toxic; irritates lungs and eyes; Control measure short run times; well-ventilated room; take care if asthmatic
  • copper(II) salts; Risk harmful; irritant; Control measure eye protection; wash hands after use
  • electricity; Risk short circuit; hot wires; Control measure low voltage; electrodes not touching

Typical results and what they show

  • copper(II) chloride; Cathode (−) pink-brown solid coating: copper; Anode (+) bubbles; damp blue litmus bleached: chlorine
  • copper(II) sulfate; Cathode (−) pink-brown coating: copper; Anode (+) bubbles; relight a glowing splint: oxygen
  • sodium chloride; Cathode (−) bubbles; squeaky pop with lighted splint: hydrogen; Anode (+) bubbles; bleaches damp litmus: chlorine
  • sodium sulfate; Cathode (−) bubbles; squeaky pop: hydrogen; Anode (+) bubbles; relight a glowing splint: oxygen

Conclusion: the results support the hypothesis. Copper is less reactive than hydrogen, so it forms at the cathode; sodium is more reactive, so hydrogen forms. Chloride solutions give chlorine at the anode; sulfate solutions give oxygen.

Going further: measuring the mass of copper

You can make the practical quantitative. Weigh a dry copper cathode, electrolyse copper(II) sulfate solution at a constant current, then wash the cathode with distilled water, dry it and reweigh it. Repeat for different times.

Worked example: rate of deposition from the graph

The graph below shows the results. The points lie close to a straight line through the origin, so mass of copper is directly proportional to time.
Gradient = 0.40 g ÷ 20 min = 0.020 g/min.

Answer: Copper is deposited at 0.020 g per minute

Evaluating the method

  • Chlorine is slightly soluble, so less gas is collected than expected; Improvement Use a saturated salt solution, or compare products rather than volumes
  • Copper can flake off the cathode when it is removed; Improvement Lift it out gently; use a lower current so the coating sticks well
  • Cathode still wet when weighed, so mass is too high; Improvement Rinse with propanone or dry in a warm oven, then reweigh until the mass stays constant
  • Hard to see small amounts of gas; Improvement Run for longer or use a higher (but still safe) voltage

Extending the investigation: you can also test whether the concentration changes the products. For example, electrolyse sodium chloride solutions of 0.1, 0.5 and 2.0 mol/dm³ and test the gas at the anode each time. With concentrated solutions the anode gas bleaches damp litmus (chlorine). With very dilute solutions, oxygen is also given off, because there are not enough chloride ions near the anode – the glowing splint test becomes positive. Here the independent variable is the concentration, and the solution type is a control variable.

Exam tip: Before you start, predict the product at each electrode and give a reason (reactivity of the metal compared with hydrogen; halide or not). Then compare your observations with your predictions in the conclusion. Your plan must be checked by your teacher before you begin.

Common mistakes

  • Testing for chlorine with a splint. Use damp blue litmus paper; it is bleached.
  • Saying the dependent variable is “the solution”. The solution is what you change (independent variable).
  • Forgetting to clean the electrodes between solutions, so copper from one run affects the next.
  • Letting the electrodes touch, which causes a short circuit and no electrolysis.

Revision questions

Try these, then check your answers below each one.

Question 1. In the required practical, different salt solutions are electrolysed to see what forms. What is the independent variable? (Choose from: The solution being electrolysed, The products at each electrode, The voltage, The size of the electrodes.)

Answer: The solution being electrolysed

Working: You change the solution. The products are what you observe (dependent). Voltage and electrode size are control variables.

Question 2. Why are graphite electrodes used in this practical? (Choose from: They are inert, so they do not react with the products, They dissolve to provide ions, They are magnetic, They react with chlorine to show it is there.)

Answer: They are inert, so they do not react with the products

Working: Graphite is inert. The products then come only from the ions in the solution.

Question 3. Which gas bleaches damp blue litmus paper?

Answer: chlorine

Working: Chlorine bleaches damp litmus paper (it may turn red first).

Related lessons

Continue in the full course

This lesson is section 6.5 (ARPCH0605) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

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