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C6 RP Required Practical: Electrolysis

GCSE Combined Science · AQA · Chapter C6: Electrolysis

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: required practical 9: investigating what happens when aqueous solutions are electrolysed 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 the tests for hydrogen, oxygen and chlorine and by observation
  • Write a hypothesis linking the products to the reactivity of the metal and the type of negative ion
  • 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 an observation, 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 so the test is fair, e.g. voltage, concentration.
  • Resolution: The smallest change an instrument can show, e.g. a balance that reads to 0.01 g.

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 dish with two graphite rods fitted through the base), low-voltage d.c. power pack, leads and crocodile clips, two small test tubes, wooden splints, damp blue litmus paper, tweezers, eye protection, and solutions of copper(II) chloride, copper(II) sulfate, sodium chloride and sodium sulfate (all dilute).

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 and note which is the anode (+) and which is the cathode (−).
  • 3. Fill two small test tubes with the solution and turn them upside down over the electrodes 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: a lighted splint for hydrogen, a glowing splint for oxygen, damp blue litmus paper held near the anode for chlorine.
  • 6. Switch off, rinse and clean the electrodes, then repeat with the other solutions using the same volume, voltage and time.
  • Independent; In this practical the solution (which metal ion and which negative ion 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 the lungs and eyes; Control measure run for a short time only; well-ventilated room; take care if asthmatic
  • copper(II) salts; Risk harmful; irritant to eyes and skin; Control measure wear eye protection; wash hands after use
  • electricity; Risk short circuit; hot wires; Control measure low voltage; electrodes must not touch

Typical results and conclusion

  • copper(II) chloride; Cathode (−) pink-brown 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 a lighted splint: hydrogen; Anode (+) bubbles; damp litmus bleached: 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 copper forms at the cathode; sodium is more reactive, so hydrogen forms. Chloride solutions give chlorine at the anode; sulfate solutions give oxygen.

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). In your conclusion, compare what you saw with your predictions.

Going further: measuring the mass of copper

You can make the practical quantitative. Weigh a clean, dry copper cathode on a balance with a resolution of 0.01 g (or better). Electrolyse copper(II) sulfate solution at a constant current for a measured time. Wash the cathode gently with distilled water, dry it and reweigh it. Repeat for different times.

Worked example: rate of deposition from the graph

The points lie close to a straight line through the origin, so the mass of copper deposited is directly proportional to time.
Gradient = change in mass ÷ change in time = 0.30 g ÷ 20 min = 0.015 g/min.

Answer: copper is deposited at 0.015 g per minute

Evaluating the method

  • Chlorine is slightly soluble, so less gas is collected than expected; Improvement compare which products form rather than volumes; use a concentrated solution
  • Copper can flake off the cathode when it is lifted out; Improvement lift it out gently; use a smaller current so the coating sticks well
  • Cathode still wet when weighed, so the mass is too high; Improvement dry it in a warm oven and reweigh until the mass stays constant
  • Small amounts of gas are hard to test; Improvement run for longer, but not with chlorine, to keep the gas to a safe amount

Repeating each timing three times and calculating a mean (ignoring any anomalous result) makes the results more repeatable. A balance with a better resolution, e.g. 0.001 g, gives more precise masses.

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, several salt solutions are electrolysed to see which products form. What is the independent variable? (Choose from: The solution being electrolysed, The product at each electrode, The voltage, The type of electrode.)

Answer: The solution being electrolysed

Working: The student changes the solution, so that is the independent variable. The products are the dependent variable; voltage and electrode type 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 make the solution conduct, They react with chlorine to remove it, They speed up the reaction as a catalyst.)

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

Working: Graphite conducts and is inert: it does not react with the electrolyte or products, so it does not affect what forms.

Question 3. A gas from the anode relights a glowing splint. Name the gas.

Answer: oxygen

Working: Oxygen relights a glowing splint.

Related lessons

Continue in the full course

This lesson is section C6 RP (AGCSC2305) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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