Electrolysis

1 Plan and risk assessment

In a solution the ions move, not electrons: positive ions to the negative electrode, negative ions to the positive. At the negative electrode the less reactive of the metal and hydrogen forms. At the positive electrode a halide gives the halogen; otherwise hydroxide ions from the water give oxygen.

Cu²⁺ + 2e⁻ → Cu 2Cl⁻ → Cl₂ + 2e⁻

Prediction

2 Apparatus

• ✓Safety goggles Protect your eyes from splashes of the solutions.
• ✓Low-voltage power pack (d.c.) Set to 4 V: pushes the current through the solution.
• ✓Ammeter (digital multimeter, 10 A range) (resolution 0.01 A) Shows that a current is flowing, and how big it is.
• ✓Leads with crocodile clips Connect the rods to the power pack, red to positive.
• ✓100 cm³ beaker, Petri-dish lid and two graphite rods The cell: the lid holds the rods apart.
• ✓Copper(II) chloride, copper(II) sulfate, sodium chloride and sodium sulfate solutions, 0.5 mol/dm³ The four electrolytes, 50 cm³ each.
• ✓Blue litmus paper, tweezers and a wash bottle Tests for chlorine; the water dampens the paper and rinses the rods.
• ✓Gas-collection cell with graphite rods through its base Part B: the gases collect over each rod.
• ✓Two 10 cm³ measuring cylinders, clamped upside down (resolution 0.2 cm³) Measure the volume of each gas.
• ✓Two test tubes The other way to collect the gases (no scale).
• ✓Stopclock (resolution 1 s) Times the readings every minute.
• ✓Wooden splints and a Bunsen burner A lit splint tests for hydrogen; a glowing one for oxygen.
• ✓Copper residues bottle Copper solutions go here, not down the sink.

3 Method: your choices

• Which version are you doing?: AQA required practical 3: electrolysis of aqueous solutionsFour salt solutions with graphite rods: the products at each electrode, then the gases.
• Test the leads: DoneA lead that is broken inside its plug looks fine but carries no current. Testing every lead first saves you hunting for the fault later.
• Keep the chlorine downSafety: DoneThe chloride solutions give off chlorine, which is toxic. A low potential difference and a short time keep the amount small.
• Clean the rods between solutions: DoneAnything left on a rod from one solution is still there in the next, and looks like a product.
• Choose how to collect the gases: Two 10 cm³ measuring cylinders (0.2 cm³ divisions)Each has a scale, so you can measure each volume to 0.2 cm³. You can plot the volumes against time and work out the ratio.
• Test the gas at the positive electrode: Damp litmus, held in the gas just above the solutionChlorine dissolves in the water on the paper and bleaches it. Chloride solutions bleach it; with the sulfates it stays blue, because oxygen does not change litmus.
• Read the upturned scale at eye level: DoneLooking from above, the surface seems to line up with a mark higher up the cylinder. On an upturned scale that is a smaller number, so you read too low.
• Keep the hydrogen tube upside down: DoneHydrogen is much less dense than air, so it rises out of a tube that is turned the right way up.

Anomaly: No anomalous reading.

6 Analysis

1. The product at each electrode

   Cu / Cl₂, Cu / O₂, H₂ / Cl₂, H₂ / O₂ Right first time

2. Explaining the negative electrode

   Copper formed from the copper salts, but hydrogen from the sodium salts. Use the reactivity series to explain why.

   Copper is less reactive than hydrogen, so copper ions gain electrons instead of hydrogen ions. Sodium is more reactive, so hydrogen ions from the water gain them. Right first time

3. Explaining the positive electrode

   Chlorine (it bleached the litmus) formed from the chlorides; oxygen from the sulfates. Which rule fits?

   Halide ions lose electrons to form the halogen. Sulfate ions are not discharged, so hydroxide ions from the water lose electrons and oxygen forms. Right first time

4. Going further: the concentration of chloride

   Another class electrolysed sodium chloride solutions of different concentrations and measured what share of the gas from the positive electrode was chlorine. What does their data show?

   In very dilute solutions, oxygen forms as well as chlorine: there are too few chloride ions near the electrode. Right first time

5. Half equations

   Cu²⁺ + 2e⁻ → Cu; 2Cl⁻ → Cl₂ + 2e⁻; 2H⁺ + 2e⁻ → H₂ Right first time

6. The two gases

   Look at your table of gas volumes. How does the volume of hydrogen compare with the volume of oxygen?

   There is about twice as much hydrogen as oxygen. Right first time

7. Shape of the graph

   What does the hydrogen line show?

   A straight line (almost through the origin): hydrogen forms at a steady rate, because the current is steady. Right first time

8. Gradient of the hydrogen line (rate of hydrogen production)

   gradient = change in y ÷ change in x

   1.26 cm³/min Right first time

9. The ratio of the gases

   ratio = hydrogen collected from 1 to 5 min ÷ oxygen collected from 1 to 5 min

   Use the volumes at 1 minute and at 5 minutes to work out the ratio of the hydrogen to the oxygen collected between them. Leaving out the first minute leaves out the gas that dissolved at the start.

   1.92 Right first time

10. Why two to one?

    Your ratio is 1.92. Why is about twice as much hydrogen as oxygen made?

    Each H₂ needs 2 electrons but each O₂ releases 4: the same current makes twice as many H₂ molecules. (In effect water, H₂O, is split.) Right first time

11. Moles of hydrogen (Chemistry only, Higher)

    amount in mol = volume in dm³ ÷ 24

    One mole of any gas takes up 24 dm³ at room temperature and pressure. How many moles of hydrogen are in 6.4 cm³?

    0.00027 mol Right first time

7 Conclusion and evaluation

Conclusion

Sources of error

Improvement

Repeatable or reproducible

9 Exam questions on your data

On your own readings, self-marked against the mark scheme, with error carried forward where it applies. Total 30/30.

1. 01.1[1 mark]

   In part A you electrolysed four different salt solutions and looked at what formed at each electrode.

   Which of these is a control variable?

   Tick (✓) one box.

   • The salt in the solution
   • The product at each electrode
   • ✓ The potential difference of the power pack (your choice)
   • The number of bubbles at the positive electrode

   Your answer

   The potential difference of the power pack

   1/1

   • The potential difference of the power pack

   Full marks. Well done.

   Model answer: The potential difference of the power pack (kept at 4 V for every solution).

2. 01.2[2 marks]

   Describe the test for chlorine. Give the result of the test.

   Your answer

   Hold damp blue litmus paper in the gas. The litmus paper is bleached and turns white.

   2/2 self-marked

   • Damp (blue) litmus paper
   • Is bleached (turns white)

   Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

   Model answer: Hold damp blue litmus paper in the gas. Chlorine bleaches it: it turns white.

3. 01.3[2 marks]

   A gas forms at the negative electrode when sodium sulfate solution is electrolysed.

   Describe the test to show that the gas is hydrogen. Give the result of the test.

   Your answer

   Hold a burning splint at the mouth of the test tube. The gas burns with a squeaky pop.

   2/2 self-marked

   • A lit (burning) splint
   • (Squeaky) pop

   Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

   Model answer: Hold a lit (burning) splint at the mouth of the tube. Hydrogen burns with a squeaky pop.

4. 01.4[1 mark]

   Give one reason why the room must be well ventilated when the chloride solutions are electrolysed.

   Your answer

   Chlorine gas is toxic and irritates the lungs if you breathe it in.

   1/1 self-marked

   • Chlorine is toxic (irritates the lungs or eyes; can cause breathing problems)

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: Chlorine is toxic: breathing it in irritates the lungs.

5. 02.1[1 mark]

   Table 1 shows your observations from part A.

   Which solution produced copper at the negative electrode and oxygen at the positive electrode?

   Select one row.

   Table 1

   Solution	Positive electrode	Damp blue litmus at the positive electrode	Negative electrode
   Copper(II) chloride	Bubbles of gas	Turns red, then white (bleached)	A pink-brown coating
   Copper(II) sulfate	Bubbles of gas	Stays blue: no change	A pink-brown coating
   Sodium chloride	Bubbles of gas	Turns red, then white (bleached)	Bubbles of gas
   Sodium sulfate	Bubbles of gas	Stays blue: no change	Bubbles of gas

   Your answer

   copper(II) sulfate

   1/1

   • Copper(II) sulfate: a pink-brown coating at the negative electrode, and a gas that does not bleach litmus

   Full marks. Clear and precise. Copper(II) sulfate: copper coats the negative electrode, and the gas at the positive electrode does not bleach the litmus, so it is oxygen.

   Model answer: Copper(II) sulfate: pink-brown copper on the negative electrode; the gas at the positive electrode does not bleach damp litmus, so it is oxygen.

6. 02.2[2 marks]

   A student electrolyses copper(II) nitrate solution, Cu(NO₃)₂(aq), using inert electrodes.

   Which two statements are correct?

   Tick (✓) two boxes.

   • ✓ Copper forms at the negative electrode. (your choice)
   • Hydrogen forms at the negative electrode.
   • ✓ Oxygen forms at the positive electrode. (your choice)
   • Nitrogen forms at the positive electrode.
   • Copper forms at the positive electrode.

   Your answer

   Copper forms at the negative electrode.; Oxygen forms at the positive electrode.

   2/2

   • Copper forms at the negative electrode.
   • Oxygen forms at the positive electrode.

   Full marks. Well done.

   Model answer: Copper forms at the negative electrode (copper is less reactive than hydrogen). Oxygen forms at the positive electrode (nitrate is not a halide).

7. 02.3[1 mark]

   Copper forms at the negative electrode. (Higher.)

   Complete the half equation for the reaction at the negative electrode.

   Cu²⁺ + ______ → Cu

   Write e- for an electron.

   Your answer

   2e-

   1/1 self-marked

   • 2e⁻ (two electrons)

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: Cu²⁺ + 2e⁻ → Cu

8. 02.4[2 marks]

   Chlorine forms at the positive electrode when copper(II) chloride solution is electrolysed. (Higher.)

   Write a half equation for the formation of chlorine.

   Type it as, for example, Na+ + e- -> Na.

   Your answer

   2Cl- -> Cl2 + 2e-

   2/2 self-marked

   • Cl⁻ on the left; Cl₂ and e⁻ on the right
   • Balanced: 2Cl⁻ → Cl₂ + 2e⁻

   Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

   Model answer: 2Cl⁻ → Cl₂ + 2e⁻ (allow 1 mark for Cl₂ and e⁻ on the right with the wrong numbers)

9. 02.5[1 mark]

   Which particles move through the solution to carry the current?

   Tick (✓) one box.

   • Electrons
   • ✓ Ions (your choice)
   • Atoms
   • Water molecules

   Your answer

   Ions

   1/1

   • Ions

   Full marks. Clear and precise.

   Model answer: Ions: positive ions move to the negative electrode and negative ions to the positive electrode. Electrons flow only in the wires and rods.

10. 02.6[2 marks]

    A student put a crystal of copper(II) chromate on damp filter paper between two electrodes and switched on.

    After a few minutes a blue colour spread towards the negative electrode and a yellow colour spread towards the positive electrode.

    Explain why the blue colour moved towards the negative electrode.

    Your answer

    The blue colour is due to copper ions, Cu2+, which are positive. Opposite charges attract, so the copper ions are attracted to the negative electrode.

    2/2 self-marked

    • The blue colour is copper ions (Cu²⁺), which are positive
    • Opposite charges attract, so they are attracted to the negative electrode

    Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

    Model answer: The blue colour is copper ions, Cu²⁺, which are positive. Opposite charges attract, so they are attracted to the negative electrode.

11. 02.7[3 marks]

    Oxygen forms at the positive electrode when sodium sulfate solution is electrolysed. (Higher.)

    Explain how oxygen is produced at the positive electrode.

    Your answer

    Water breaks down to give hydroxide ions, OH-. The hydroxide ions are attracted to the positive electrode. There the hydroxide ions lose electrons, they are discharged, and form oxygen.

    3/3 self-marked

    • Water (molecules) break down / ionise to give hydroxide ions (OH⁻)
    • Hydroxide ions are attracted to / move to the positive electrode
    • Hydroxide ions lose electrons (are discharged / oxidised) to form oxygen
    • Sulfate ions are not discharged (they stay in solution)

    Self-marked: 3 of 3 marks. This is your own judgement against the mark scheme.

    Model answer: Water molecules ionise to give hydroxide ions (OH⁻). The hydroxide ions are attracted to the positive electrode, where they lose electrons (are discharged, or oxidised) to form oxygen. The sulfate ions are not discharged.

12. 03.1[1 mark]

    Table 2 shows the currents another student measured in part A, at 4 V.

    Each solution was 0.5 mol/dm³ and was tested with the same electrodes and power supply. Different solutions need not give the same current.

    Which current should be checked first by repeating it?

    Select one row.

    Table 2

    Solution	Current in A
    Copper(II) chloride	0.44
    Copper(II) sulfate	0.21
    Sodium chloride	0.02
    Sodium sulfate	0.25

    Your answer

    the current through sodium chloride

    1/1

    • Sodium chloride: several times smaller than every other current, so repeat it to check

    Full marks. Clear and precise. Sodium chloride (0.02 A) is several times smaller than every other current. Different solutions can give different currents, so this does not prove a mistake on its own, but it is the reading to repeat first. If the repeat comes out close to the others, the first reading was anomalous; a loose clip is one possible cause.

    Model answer: Sodium chloride, 0.02 A: it is several times smaller than every other current, so it is the one to repeat. If the repeat is close to the others, the first reading was anomalous (a loose connection is one possible cause).

13. 03.2[2 marks]

    Another student collected the gases in two test tubes upturned over the electrodes, instead of measuring cylinders.

    Suggest one improvement to the apparatus so that the volumes of the two gases can be compared. Give a reason for your answer.

    Your answer

    Collect the gases in measuring cylinders instead of test tubes, because test tubes have no scale, so the volume of gas can be measured rather than estimated.

    2/2 self-marked

    • Use measuring cylinders (allow burettes, gas syringes or a Hofmann voltameter) instead of test tubes
    • Because test tubes have no scale, so the volume can be measured rather than estimated

    Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

    Model answer: Use measuring cylinders instead of test tubes, because test tubes have no scale: the volume of each gas can then be measured, not estimated.

14. 03.3[3 marks]

    You collected 6.4 cm³ of hydrogen in 5 minutes.

    The volume of one mole of any gas at room temperature and pressure is 24 dm³. (Chemistry only, Higher.)

    Calculate the amount of hydrogen, in moles, collected.

    Give your answer in standard form to 2 significant figures.

    Your answer

    2.7 x 10^-4

    Working: 6.4 cm3 = 0.0064 dm3 moles = 0.0064 / 24 = 2.7 x 10^-4 mol

    3/3

    • 6.4 cm³ = 0.0064 dm³ (or ÷ 24 000 cm³)
    • 2.7 × 10⁻⁴ mol
    • Given to 2 significant figures

    Full marks. Your answer meets all the points for this question.

    Model answer: 6.4 cm³ = 0.0064 dm³ moles = 0.0064 ÷ 24 = 2.7 × 10⁻⁴ mol

15. 04[6 marks]

    A student wants to find out what is produced at each electrode when sodium chloride solution is electrolysed.

    Describe a method the student could use. Include how the student could identify the products.

    Your answer

    1. Pour about 50 cm³ of sodium chloride solution into a beaker. 2. Put two graphite (inert) electrodes into the solution, held apart so they do not touch. 3. Connect them to a low-voltage d.c. power supply (4 V) and switch on. 4. Look for bubbles at each electrode. 5. Collect the gas from each electrode in an upturned test tube full of the solution. 6. Test the gas from the positive electrode with damp blue litmus paper: it is bleached, so it is chlorine. 7. Test the gas from the negative electrode with a lit splint: a squeaky pop shows it is hydrogen. 8. Do it in a well-ventilated room, wear goggles, and switch off after a few minutes so little chlorine forms.

    6/6 self-marked

    • Put sodium chloride solution in a beaker (or electrolysis cell)
    • Two inert (graphite / carbon) electrodes in the solution, not touching
    • Connect to a low-voltage d.c. power supply and switch on
    • Look for bubbles (gas) at each electrode
    • Collect the gas from each electrode in an upturned test tube (or measuring cylinder)
    • Damp (blue) litmus paper is bleached by the gas from the positive electrode: chlorine
    • A lit splint gives a squeaky pop with the gas from the negative electrode: hydrogen
    • Safety: well-ventilated room or fume cupboard, goggles, low voltage, short time

    Self-marked: Level 3, 6 of 6 marks. You ticked 8 of 8 indicative points. This is your own judgement against the mark scheme.

    Model answer: 1. Pour about 50 cm³ of sodium chloride solution into a beaker. 2. Put two graphite (inert) electrodes into the solution, held apart so they do not touch. 3. Connect them to a low-voltage d.c. power supply (4 V) and switch on. 4. Look for bubbles at each electrode. 5. Collect the gas from each electrode in an upturned test tube full of the solution. 6. Test the gas from the positive electrode with damp blue litmus paper: it is bleached, so it is chlorine. 7. Test the gas from the negative electrode with a lit splint: a squeaky pop shows it is hydrogen. 8. Do it in a well-ventilated room, wear goggles, and switch off after a few minutes so little chlorine forms.

Unseen data: the same skills, new apparatus

1. 01[3 marks]

   A student electrolyses dilute sulfuric acid with graphite electrodes and a steady current. The hydrogen is collected in an upturned burette over the negative electrode and read every 2 minutes.

   Hydrogen collected

   Time in min	Volume of hydrogen in cm³
   0	0.0
   2	2.8
   4	5.9
   6	9.1
   8	12.2
   10	15.4

   Calculate the mean rate of hydrogen production between 2 minutes and 10 minutes. Give your answer to 3 significant figures.

   Your answer

   1.57

   Working: 15.4 - 2.8 = 12.6 12.6 / 8

   3/3

   • Change in volume = 15.4 − 2.8 = 12.6 cm³.
   • ÷ (10 − 2) = 8 minutes.
   • The mean rate, in cm³/min, to 3 significant figures.

   Full marks. Well done.

   Model answer: Change in volume = 15.4 − 2.8 = 12.6 cm³ in 8 minutes. Mean rate = 12.6 ÷ 8 = 1.57 cm³/min.

2. 02[2 marks]

   A student electrolyses potassium bromide solution and then copper(II) bromide solution, using graphite electrodes.

   Part of the reactivity series (most reactive first)

   Metal
   Potassium
   Sodium
   Lithium
   Magnesium
   (Hydrogen)
   Copper
   Silver

   Which TWO statements are correct? Tick (✓) two boxes.

   • Potassium bromide solution: potassium at the negative electrode, bromine at the positive electrode.
   • ✓ Potassium bromide solution: hydrogen at the negative electrode, bromine at the positive electrode. (your choice)
   • Copper(II) bromide solution: hydrogen at the negative electrode, oxygen at the positive electrode.
   • ✓ Copper(II) bromide solution: copper at the negative electrode, bromine at the positive electrode. (your choice)
   • Both solutions: hydrogen at the negative electrode and oxygen at the positive electrode, because both contain water.

   Your answer

   Potassium bromide solution: hydrogen at the negative electrode, bromine at the positive electrode.; Copper(II) bromide solution: copper at the negative electrode, bromine at the positive electrode.

   2/2

   • Potassium bromide solution: hydrogen at the negative electrode, bromine at the positive electrode.
   • Copper(II) bromide solution: copper at the negative electrode, bromine at the positive electrode.

   Full marks. Well done.

   Model answer: Potassium bromide solution: hydrogen at the negative electrode, bromine at the positive electrode. Copper(II) bromide solution: copper at the negative electrode, bromine at the positive electrode. At the negative electrode the metal forms only if it is less reactive than hydrogen; otherwise hydrogen. At the positive electrode a halide gives the halogen; otherwise oxygen (from the water).

3. 03[2 marks]

   A student electrolyses concentrated sodium chloride solution (brine). Each gas is collected in an upturned measuring cylinder full of the solution, and the volumes are read every 2 minutes.

   Gas collected

   Time in min	Volume of hydrogen in cm³	Volume of chlorine in cm³
   2	1.0	0.0
   4	2.0	0.5
   6	3.0	1.5
   8	4.0	2.5
   10	5.0	3.0
   12	6.0	4.0

   Which TWO statements describe and explain these results? Tick (✓) two boxes.

   • ✓ The volume of hydrogen rises steadily: it is roughly proportional to the time (the readings, to the nearest 0.5 cm³, scatter a little about a straight line through the origin). (your choice)
   • ✓ Less chlorine is collected at first because some of the chlorine dissolves in the solution. (your choice)
   • Chlorine forms at half the rate of hydrogen all the way through, as oxygen does from sodium sulfate.
   • There is less chlorine because chlorine is denser than hydrogen.
   • Chlorine stops forming once the solution has dissolved all the chlorine it can.

   Your answer

   The volume of hydrogen rises steadily: it is roughly proportional to the time (the readings, to the nearest 0.5 cm³, scatter a little about a straight line through the origin).; Less chlorine is collected at first because some of the chlorine dissolves in the solution.

   2/2

   • The volume of hydrogen rises steadily: it is roughly proportional to the time (the readings, to the nearest 0.5 cm³, scatter a little about a straight line through the origin).
   • Less chlorine is collected at first because some of the chlorine dissolves in the solution.

   Full marks. Clear and precise.

   Model answer: The hydrogen goes up by about the same amount every 2 minutes: roughly proportional to time, with the small differences in each step coming from reading the cylinder to the nearest 0.5 cm³. Chlorine lags behind at first because some dissolves in the solution; once the solution has dissolved all the chlorine it can, chlorine is collected at the same rate as hydrogen.