Electrolysis - Worksheets, Questions and Revision

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GCSE · Chemistry

C4b Electrolysis

AQA 8464 · Calculator allowed · about 100 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Electrolysis is the process used to break down ionic compounds using electricity.

(a)Define the term 'electrolysis'.(1)

(b)State what is meant by an 'electrolyte'.(1)

(c)Solid sodium chloride does not conduct electricity, so it cannot be electrolysed. Explain why molten sodium chloride, or sodium chloride dissolved in water, can be electrolysed.(2)

(Total for Question 1 is 4 marks)

An electrolysis cell has two electrodes connected to a d.c. power supply.

(a)State the name given to the negative electrode.(1)

(b)State the name given to the positive electrode.(1)

(c)State whether positive ions or negative ions move towards the cathode during electrolysis, and explain why.(2)

(Total for Question 2 is 4 marks)

Zinc chloride, ZnCl2, is an ionic solid that can be melted and electrolysed using inert electrodes, as shown in Figure 1.

(a)Name the product formed at the cathode when molten zinc chloride is electrolysed, and explain, in terms of ion movement and electron transfer, why it forms there.(3)

(b)Name the product formed at the anode, and explain why it forms there.(3)

(Total for Question 3 is 6 marks)

Higher tier only. Magnesium chloride, MgCl2, is electrolysed when molten, using inert electrodes.

(a)Write the half equation for the reaction at the cathode.(2)

(b)Write the half equation for the reaction at the anode.(2)

(Total for Question 4 is 4 marks)

When an aqueous solution is electrolysed, the product formed at the cathode depends on the reactivity of the metal ion present.

(a)State the general rule used to predict whether a metal or hydrogen gas forms at the cathode during the electrolysis of an aqueous solution.(2)

(b)Dilute sulfuric acid and copper sulfate solution are separately electrolysed using inert electrodes. Predict the product formed at the cathode in each case, and justify your answer.(3)

(Total for Question 5 is 5 marks)

The product formed at the anode during the electrolysis of an aqueous solution depends on which negative ions are present.

(a)State the general rule used to predict the product formed at the anode when a solution containing a halide ion is electrolysed, compared with a solution that does not contain a halide ion.(2)

(b)Concentrated sodium chloride solution and dilute sulfuric acid are separately electrolysed using inert electrodes. Predict the product formed at the anode in each case, and justify your answer.(3)

(Total for Question 6 is 5 marks)

Required practical: a student, Priya, carries out the required practical investigation into the electrolysis of aqueous solutions, using copper sulfate solution as the electrolyte, as shown in Figure 2.

(a)Name a suitable type of electrode for this investigation, and give a reason for using this type of electrode rather than a reactive metal electrode.(2)

(b)Describe how Priya should set up and carry out this investigation.(4)

(Total for Question 7 is 6 marks)

Priya runs the electrolysis for several minutes and records her observations at each electrode.

(a)Describe what Priya observes at the cathode during the experiment.(2)

(b)Describe what Priya observes at the anode, and describe a test she could use to confirm the identity of the gas produced there, including the result of a positive test.(3)

(c)Explain why the blue colour of the copper sulfate solution gradually fades during this experiment.(1)

(Total for Question 8 is 6 marks)

Copper sulfate solution can also be electrolysed using copper electrodes instead of inert electrodes. This method is used industrially to purify impure copper.

(a)State what happens at the copper anode when copper sulfate solution is electrolysed using copper electrodes.(2)

(b)State what happens at the copper cathode.(1)

(c)Explain why the blue colour of the copper sulfate solution stays approximately constant in this experiment, unlike when inert electrodes are used.(2)

(Total for Question 9 is 5 marks)

Higher tier only. Copper sulfate solution is electrolysed using inert (graphite) electrodes.

(a)Write the half equation for the reaction at the cathode.(2)

(b)Write the half equation for the reaction at the anode.(2)

(Total for Question 10 is 4 marks)

Concentrated sodium chloride solution (brine) is electrolysed industrially using inert electrodes, producing three useful products.

(a)Name the gas produced at the anode.(1)

(b)Name the gas produced at the cathode.(1)

(c)Name the other useful substance that remains in solution, and give one industrial use for any one of the three products.(2)

(Total for Question 11 is 4 marks)

Higher tier only. Concentrated sodium chloride solution is electrolysed using inert electrodes.

(a)Write the half equation for the reaction at the anode.(2)

(b)Write the half equation for the reaction at the cathode.(2)

(Total for Question 12 is 4 marks)

A factory in the UK processes 61.2 tonnes of pure aluminium oxide, Al2O3, by electrolysis. (Relative atomic masses: Al = 27, O = 16.)

(a)The equation for the reaction is 2Al2O3 -> 4Al + 3O2. Calculate the maximum mass of aluminium that can be extracted from 61.2 tonnes of aluminium oxide.(3)

(b)Using conservation of mass, calculate the mass of oxygen gas released as a by-product in this process.(2)

(Total for Question 13 is 5 marks)

Aluminium is extracted from purified aluminium oxide by electrolysis, rather than by reduction with carbon.

(a)Explain why aluminium must be extracted using electrolysis, rather than by reduction with carbon (as is used for iron).(2)

(b)State the purpose of dissolving aluminium oxide in molten cryolite before electrolysis, rather than melting pure aluminium oxide alone.(2)

(Total for Question 14 is 4 marks)

Aluminium oxide, dissolved in molten cryolite, is electrolysed using carbon (graphite) electrodes.

(a)Higher tier only. Write the half equation for the reaction at the cathode.(2)

(b)Higher tier only. Write the half equation for the reaction at the anode.(2)

(c)Explain why the carbon anodes used in this process need to be replaced regularly, giving a balanced symbol equation for the reaction responsible.(2)

(Total for Question 15 is 6 marks)

Aluminium is extracted from aluminium oxide by electrolysis on a large industrial scale. Explain how this process works and why it is necessary, including reference to reactivity, the role of cryolite, the reactions occurring at each electrode, and why the process is costly.

(Total for Question 16 is 6 marks)

Required practical: Callum investigates the electrolysis of potassium bromide solution using inert electrodes, as an extension of the standard investigation.

(a)Predict the product formed at the cathode, and explain your prediction.(2)

(b)Predict the product formed at the anode, and describe how Callum could test for this product, including the result of a positive test.(3)

(Total for Question 17 is 5 marks)

Required practical: Aisha electrolyses copper sulfate solution using inert graphite electrodes. She correctly observes a reddish-brown deposit forming at the cathode and bubbles of gas at the anode, but when she tests the collected gas with a glowing splint, it does not relight clearly.

(a)Suggest one reason, other than there being no gas produced, why the glowing splint test did not give a clear positive result.(2)

(b)Suggest one improvement to Aisha's method that would address this problem.(2)

(Total for Question 18 is 4 marks)

Silver nitrate solution, AgNO3(aq), is electrolysed using inert (platinum) electrodes.

(a)Predict the product formed at the cathode, and explain your prediction.(3)

(b)Predict the product formed at the anode, and explain your prediction.(3)

(c)Higher tier only. The half equations for this electrolysis are Ag+ + e- -> Ag (cathode) and 4OH- -> O2 + 2H2O + 4e- (anode). Combine these, balancing the electrons transferred, to give the overall ionic equation for this electrolysis.(3)

(Total for Question 19 is 9 marks)

Mark scheme · C4b Electrolysis

Question 1

(a) B1 the breakdown (decomposition) of an ionic compound, molten or dissolved in water, into its elements, using electricity (an electric current) oe
(a) Answer: Electrolysis is the breakdown (decomposition) of an ionic compound, molten or dissolved in water, into its elements, using electricity.
(b) B1 a molten ionic compound, or an ionic compound dissolved in water, that conducts electricity and is decomposed during electrolysis oe
(b) Answer: An electrolyte is a molten ionic compound, or an ionic compound dissolved in water, that conducts electricity and is broken down during electrolysis.
(c) M1 in a solid, the ions are held in fixed positions in the lattice and cannot move oe
(c) A1 when molten or dissolved, the ions are free to move (are mobile) throughout the liquid, so they can carry electric charge to the electrodes oe
(c) Answer: In a solid, the ions are held in fixed positions in the lattice and cannot move, so they cannot carry charge. When molten or dissolved in water, the ions are free to move throughout the liquid, so they can carry an electric current to the electrodes.

Question 2

(a) B1 cathode
(a) Answer: Cathode.
(b) B1 anode
(b) Answer: Anode.
(c) M1 positive ions (cations) move towards the cathode oe
(c) A1 because the cathode is the negative electrode, and opposite charges attract oe
(c) Answer: Positive ions (cations) move towards the cathode, because the cathode is negatively charged and opposite charges attract.

Question 3

(a) B1 zinc (metal) forms at the cathode
(a) M1 positive Zn2+ ions are attracted to (move towards) the negative cathode oe
(a) A1 the Zn2+ ions gain electrons (are reduced) at the cathode, forming zinc atoms oe
(a) Answer: Zinc metal forms at the cathode: positive Zn2+ ions are attracted to the negative cathode, where they gain electrons (are reduced) to form zinc atoms.
(b) B1 chlorine (gas) forms at the anode
(b) M1 negative Cl- ions are attracted to (move towards) the positive anode oe
(b) A1 the Cl- ions lose electrons (are oxidised) at the anode, forming chlorine molecules (gas) oe
(b) Answer: Chlorine gas forms at the anode: negative Cl- ions are attracted to the positive anode, where they lose electrons (are oxidised) to form chlorine gas.

Question 4

(a) M1 correct species and direction: Mg2+ + 2e- -> Mg
(a) A1 fully balanced, including the correct number of electrons cao
(a) Answer: Mg2+ + 2e- -> Mg
(b) M1 correct species and direction: 2Cl- -> Cl2 + 2e-
(b) A1 fully balanced, including the correct number of electrons cao
(b) Answer: 2Cl- -> Cl2 + 2e-

Question 5

(a) M1 if the metal is less reactive than hydrogen, the metal is produced (deposited) at the cathode oe
(a) A1 if the metal is more reactive than hydrogen, hydrogen gas is produced instead oe
(a) Answer: If the metal is less reactive than hydrogen, the metal is deposited at the cathode. If the metal is more reactive than hydrogen, hydrogen gas is produced at the cathode instead.
(b) B1 dilute sulfuric acid: hydrogen gas forms at the cathode (the only positive ion available is H+)
(b) B1 copper sulfate solution: copper metal forms at the cathode
(b) B1 justification: copper is less reactive than hydrogen, so Cu2+ ions are discharged (deposited) at the cathode in preference to hydrogen oe
(b) Answer: Dilute sulfuric acid gives hydrogen gas at the cathode, since H+ is the only cation present. Copper sulfate solution gives copper metal at the cathode, because copper is less reactive than hydrogen, so Cu2+ ions are discharged in preference to H+.

Question 6

(a) M1 if a halide ion (e.g. chloride, bromide or iodide) is present, the corresponding halogen is produced at the anode oe
(a) A1 if no halide ion is present (e.g. a sulfate or nitrate solution), oxygen gas is produced instead, from hydroxide ions/water oe
(a) Answer: If a halide ion is present, the corresponding halogen is produced at the anode. If no halide ion is present, oxygen gas is produced instead, formed from hydroxide ions in the water.
(b) B1 concentrated sodium chloride solution: chlorine gas forms at the anode
(b) B1 dilute sulfuric acid: oxygen gas forms at the anode
(b) B1 justification: chloride is a halide ion so chlorine is discharged; sulfate ions are never discharged, so oxygen forms from hydroxide ions/water instead oe
(b) Answer: Concentrated sodium chloride solution gives chlorine gas at the anode, because chloride is a halide ion. Dilute sulfuric acid gives oxygen gas at the anode, because sulfate ions are never discharged, so hydroxide ions (from the water) are oxidised instead.

Question 7

(a) B1 an inert electrode, e.g. graphite (carbon) or platinum
(a) B1 inert electrodes do not react with the electrolyte or dissolve during electrolysis, so the results show only the reactions of the ions in solution oe
(a) Answer: Inert electrodes, such as graphite or platinum, should be used, because they do not react with the electrolyte or dissolve during electrolysis, so the results show only the reactions of the ions in solution.
(b) B1 pour copper sulfate solution into the electrolysis cell (beaker) so that both graphite electrodes are covered by the solution oe
(b) B1 connect the electrodes to a d.c. power supply, with wires/crocodile clips attached correctly oe
(b) B1 switch on the power supply for a measured/fixed period of time and observe both electrodes throughout oe
(b) B1 wear eye protection throughout the investigation
(b) Answer: Pour copper sulfate solution into the electrolysis cell so both graphite electrodes are covered, connect the electrodes to a d.c. power supply, then switch on for a measured period of time, observing both electrodes throughout and wearing eye protection.

Question 8

(a) B1 a reddish-brown (pink-brown) solid coating gradually builds up on the electrode oe
(a) B1 this deposit is copper metal
(a) Answer: A reddish-brown solid coating (copper metal) gradually builds up on the cathode.
(b) B1 colourless gas bubbles (effervescence) are produced at the anode
(b) B1 test: collect a sample of the gas and insert a glowing splint into it oe
(b) B1 positive result: the glowing splint relights, confirming the gas is oxygen
(b) Answer: Colourless gas bubbles are produced at the anode. Collecting a sample and inserting a glowing splint tests for the gas: if it relights, this confirms the gas is oxygen.
(c) B1 Cu2+ ions are removed from solution (deposited as copper at the cathode) and are not replaced, since the graphite electrodes are inert and do not dissolve, so the concentration of Cu2+ ions (and the blue colour) decreases oe
(c) Answer: Cu2+ ions are steadily removed from solution as they are deposited as copper at the cathode, and are not replaced (since the inert electrodes do not dissolve), so the concentration of Cu2+ ions and the blue colour both decrease.

Question 9

(a) B1 copper atoms at the anode lose electrons (are oxidised)
(a) B1 they dissolve into the solution as Cu2+ ions, so the anode loses mass oe
(a) Answer: Copper atoms at the anode lose electrons and dissolve into the solution as Cu2+ ions, so the anode gradually loses mass.
(b) B1 Cu2+ ions from solution gain electrons (are reduced) and are deposited as pure copper, so the cathode gains mass
(b) Answer: Cu2+ ions from solution gain electrons and are deposited as pure copper, so the cathode gains mass.
(c) M1 copper dissolves from the anode at approximately the same rate as copper is deposited at the cathode oe
(c) A1 so the concentration of Cu2+ ions in solution stays roughly constant, unlike with inert electrodes, where Cu2+ ions are removed but not replaced oe
(c) Answer: Copper dissolves from the anode at approximately the same rate as it is deposited at the cathode, so the concentration of Cu2+ ions in solution (and the blue colour) stays roughly constant, unlike with inert electrodes, where Cu2+ ions are removed but not replaced.

Question 10

(a) M1 correct species and direction: Cu2+ + 2e- -> Cu
(a) A1 fully balanced, including the correct number of electrons cao
(a) Answer: Cu2+ + 2e- -> Cu
(b) M1 correct species and direction: 4OH- -> O2 + 2H2O + 4e-
(b) A1 fully balanced (4 O and 4 H on each side; charge -4 on each side) cao
(b) Answer: 4OH- -> O2 + 2H2O + 4e-

Question 11

(a) B1 chlorine
(a) Answer: Chlorine.
(b) B1 hydrogen
(b) Answer: Hydrogen.
(c) B1 sodium hydroxide solution remains in solution
(c) B1 one valid industrial use, e.g. chlorine to sterilise water/make bleach or plastics (PVC); hydrogen to make margarine or as a fuel; sodium hydroxide to make soap, paper or bleach oe
(c) Answer: Sodium hydroxide solution remains in solution. For example, chlorine is used to sterilise water, hydrogen is used to make margarine, and sodium hydroxide is used to make soap.

Question 12

(a) M1 correct species and direction: 2Cl- -> Cl2 + 2e-
(a) A1 fully balanced cao
(a) Answer: 2Cl- -> Cl2 + 2e-
(b) M1 correct species and direction: 2H+ + 2e- -> H2
(b) A1 fully balanced cao
(b) Answer: 2H+ + 2e- -> H2

Question 13

(a) M1 Mr(Al2O3) = (2 x 27) + (3 x 16) = 102 stated or used
(a) M1 mass of Al = 61.2 x (4 x 27) / (2 x 102), or equivalent moles method shown (e.g. moles Al2O3 = 61.2/102 = 0.6; moles Al = 0.6 x 2 = 1.2)
(a) A1 32.4 tonnes cao
(a) Answer: 32.4 tonnes
(b) M1 mass of Al2O3 = mass of Al + mass of O2, i.e. 61.2 = 32.4 + mass of O2 (ft from their part a)
(b) A1 28.8 tonnes cao (ft consistent with their part a answer)
(b) Answer: 28.8 tonnes

Question 14

(a) M1 aluminium is more reactive than carbon oe
(a) A1 so carbon cannot displace/reduce aluminium from its oxide (aluminium oxide is too stable to be reduced by carbon) oe
(a) Answer: Aluminium is more reactive than carbon, so carbon cannot reduce (displace) aluminium from its oxide; electrolysis must be used instead.
(b) M1 pure aluminium oxide has a very high melting point (over 2000 degrees C) oe
(b) A1 dissolving it in molten cryolite lowers the operating temperature needed (to around 900-1000 degrees C), reducing the energy required and so the cost of the process oe
(b) Answer: Pure aluminium oxide has a very high melting point (over 2000 degrees C). Dissolving it in molten cryolite lowers the temperature needed to around 900-1000 degrees C, reducing the energy required and the cost of the process.

Question 15

(a) M1 correct species and direction: Al3+ + 3e- -> Al
(a) A1 fully balanced cao
(a) Answer: Al3+ + 3e- -> Al
(b) M1 correct species and direction: 2O2- -> O2 + 4e-
(b) A1 fully balanced cao
(b) Answer: 2O2- -> O2 + 4e-
(c) M1 the oxygen produced at the hot anode reacts with the hot carbon of the anode itself, so the anode gradually burns away (erodes) and must be replaced oe
(c) A1 balanced equation: C + O2 -> CO2
(c) Answer: The oxygen gas produced at the hot anode reacts with the hot carbon anode itself, forming carbon dioxide gas (C + O2 -> CO2), so the anode gradually burns away and has to be replaced regularly.

Question 16

Level 1 (1-2): Simple, isolated statements are made, such as naming aluminium oxide or electrolysis, with little or no explanation of why electrolysis is necessary, or of what happens at the electrodes.
Level 2 (3-4): A logical explanation is given covering some of the required points (e.g. why electrolysis rather than carbon reduction is used, and/or the products formed at the electrodes), with some detail, but coverage of the role of cryolite, the electrode reactions and the cost factor is incomplete.
Level 3 (5-6): A complete, logically structured explanation is given, correctly linking the reactivity of aluminium (compared with carbon) to the need for electrolysis, the role of cryolite in lowering the melting point, the products formed at the cathode and anode, and the reasons the process is energy-intensive and costly (including the need to replace carbon anodes).
Indicative content:
- Aluminium is more reactive than carbon, so carbon cannot reduce (displace) aluminium from aluminium oxide; electrolysis must be used instead.
- Pure aluminium oxide has a very high melting point (over 2000 degrees C), so it is dissolved in molten cryolite, which lowers the operating temperature to around 900-1000 degrees C and reduces the energy needed.
- At the cathode (negative electrode), Al3+ ions gain electrons (are reduced) and are deposited as molten aluminium, which is tapped off from the bottom of the cell.
- At the anode (positive electrode, made of carbon), O2- ions lose electrons (are oxidised) to form oxygen gas.
- The hot oxygen gas produced reacts with the hot carbon anodes, forming carbon dioxide gas, so the anodes gradually burn away and must be replaced regularly, adding to the running cost.
- The process requires a very large amount of electrical energy (to melt the cryolite mixture and to drive the electrolysis reaction), which makes extracting aluminium considerably more expensive than extracting a metal such as iron by reduction with carbon.

Question 17

(a) B1 hydrogen gas forms at the cathode
(a) B1 potassium is more reactive than hydrogen, so hydrogen is produced in preference to potassium metal oe
(a) Answer: Hydrogen gas forms at the cathode, because potassium is more reactive than hydrogen, so hydrogen is discharged in preference to potassium metal.
(b) B1 bromine forms at the anode; an orange (red-brown) colouration appears in the solution around the anode oe
(b) B1 test: hold a piece of damp (blue or red) litmus paper close to the anode, or dip it into a sample of solution taken from near the anode oe
(b) B1 positive result: the damp litmus paper is bleached (turns white), confirming a halogen (bromine) is present
(b) Answer: Bromine forms at the anode, seen as an orange colouration in the solution near the electrode. Holding damp litmus paper close to the anode tests for this: if it bleaches (turns white), this confirms bromine is present.

Question 18

(a) M1 the collection tube was not completely filled with solution before being inverted over the electrode oe
(a) A1 so it also contained air, which diluted the oxygen collected and prevented a clear positive result oe (accept: too little gas had been collected before testing; or too long a delay before testing let air diffuse in)
(a) Answer: The tube used to collect the gas was probably not completely filled with solution before being inverted over the electrode, so it also contained air; this diluted the oxygen collected, which prevented a clear positive result.
(b) M1 completely fill the collection tube with copper sulfate solution before inverting it over the electrode, so no air is trapped inside oe
(b) A1 test the gas promptly, once a reasonable volume has collected, using a freshly lit/glowing splint, to avoid gas escaping or air diffusing in oe
(b) Answer: Completely fill the collection tube with solution before inverting it over the electrode, so no air is trapped, and test the gas promptly once enough has collected, using a freshly glowing splint.

Question 19

(a) B1 silver metal forms at the cathode
(a) M1 silver is less reactive than hydrogen oe
(a) A1 so Ag+ ions are discharged (gain electrons/are reduced) at the cathode in preference to hydrogen oe
(a) Answer: Silver metal forms at the cathode, because silver is less reactive than hydrogen, so Ag+ ions are discharged (reduced) at the cathode in preference to hydrogen.
(b) B1 oxygen gas forms at the anode
(b) M1 no halide ion is present (only nitrate ions), and nitrate ions are never discharged oe
(b) A1 so hydroxide ions (from the water) are oxidised at the anode instead, forming oxygen gas oe
(b) Answer: Oxygen gas forms at the anode, because no halide ion is present (only nitrate, which is never discharged), so hydroxide ions from the water are oxidised at the anode instead.
(c) M1 the cathode half equation is multiplied by 4, so that 4 electrons are transferred in both half equations: 4Ag+ + 4e- -> 4Ag
(c) M1 the two half equations are added together and the electrons cancel
(c) A1 correct overall balanced equation: 4Ag+ + 4OH- -> 4Ag + O2 + 2H2O cao
(c) Answer: 4Ag+ + 4OH- -> 4Ag + O2 + 2H2O

Question 19

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Electrolysis - Worksheets, Questions and Revision

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