IGCSE Chemistry Paper 4 (Theory / Extended)

Topic 4: Electrochemistry -- Mock Exam 2

Duration1 hour 15 minutes

Total Marks80

Questions7

Time Remaining75:00

Syllabus0620

Instructions

Answer all questions in the spaces provided.
Show all working for calculations.
Use appropriate scientific terminology.
Write balanced half-equations where required.
Your answers will be automatically graded when you submit.

Question Navigation

Question 1 -- Electrolysis of Molten Zinc Chloride

Total: 12 marks

In a school laboratory in Kolkata, a student electrolyses molten zinc chloride (ZnCl₂) using graphite electrodes and a d.c. power supply.

(a) [3]

(i) Define the term anode. [1]

(ii) Define the term cathode. [1]

(iii) State why graphite is a suitable material for electrodes. [1]

Model Answer -- 1(a)

The anode is the positive electrode [1]

The cathode is the negative electrode [1]

Graphite conducts electricity and is inert / does not react with the electrolyte or the products [1]

1 mark -- anode positive electrode 1 mark -- cathode negative electrode 1 mark -- graphite conducts electricity inert does not react

(b) [3]

(i) State the product formed at the cathode and describe what you would observe. [2]

(ii) State the product formed at the anode and describe what you would observe. [1]

Model Answer -- 1(b)

Zinc metal is formed at the cathode [1]

A shiny / silvery-grey deposit of metal is seen [1]

Chlorine gas is produced at the anode; a yellow-green gas with a pungent / choking smell is observed / gas bubbles seen [1]

1 mark -- zinc metal formed cathode 1 mark -- shiny silvery grey deposit metal 1 mark -- chlorine gas anode yellow green pungent smell bubbles

(i) Write the half-equation at the cathode. [1]

(ii) Write the half-equation at the anode. [1]

(iii) Explain why Zn²⁺ ions move towards the cathode during electrolysis. [1]

(iv) Explain why Cl⁻ ions move towards the anode during electrolysis. [1]

Model Answer -- 1(c)

Cathode: Zn²⁺ + 2e⁻ → Zn [1]

Zn²+ + 2e− → Zn

Anode: 2Cl⁻ → Cl₂ + 2e⁻ [1]

2Cl− → Cl₂ + 2e−

Zn²⁺ ions are positively charged and are attracted to the negatively charged cathode [1]

Cl⁻ ions are negatively charged and are attracted to the positively charged anode [1]

1 mark -- cathode Zn2+ plus 2e- gives Zn 1 mark -- anode 2Cl- gives Cl2 plus 2e- 1 mark -- Zn2+ positively charged attracted negatively charged cathode 1 mark -- Cl- negatively charged attracted positively charged anode

(d) [2]

(i) Explain why zinc chloride must be molten or dissolved in water for electrolysis to occur. [1]

(ii) Suggest why a covalent compound such as hexane (C₆H₁₄) cannot be electrolysed. [1]

Model Answer -- 1(d)

When molten or in solution, the ions are free to move and carry the electric current; when solid, the ions are in fixed positions [1]

Hexane is a covalent compound and contains no ions / no charged particles to carry the current [1]

1 mark -- molten solution ions free move carry current solid fixed positions 1 mark -- hexane covalent no ions no charged particles carry current

Question 2 -- Electrolysis of Copper(II) Sulfate Solution

Total: 12 marks

A student in Manchester electrolyses copper(II) sulfate solution using inert platinum electrodes.

(a) [2]

List the four types of ions present in the copper(II) sulfate solution. State the source of each.

Model Answer -- 2(a)

Cu²⁺ and SO₄²⁻ ions from copper(II) sulfate [1]

H⁺ and OH⁻ ions from water [1]

1 mark -- Cu2+ sulfate SO42- ions from copper sulfate 1 mark -- H+ hydroxide OH- ions from water

(b) [3]

(i) State the product at the cathode. [1]

(ii) Explain, using the reactivity series, why this product is formed at the cathode rather than the alternative. [2]

Model Answer -- 2(b)

Copper metal is deposited at the cathode [1]

Copper is less reactive than hydrogen / lower in the reactivity series than hydrogen [1]

The less reactive cation is preferentially discharged, so Cu²⁺ ions gain electrons in preference to H⁺ ions [1]

1 mark -- copper metal deposited cathode 1 mark -- copper less reactive than hydrogen lower reactivity series 1 mark -- less reactive cation preferentially discharged Cu2+ gains electrons preference H+

(i) State the product at the anode. [1]

(ii) Explain why this product is formed rather than the alternative. [1]

(iii) Write the half-equation for the reaction at the anode. [1]

Model Answer -- 2(c)

Oxygen gas is produced at the anode [1]

SO₄²⁻ ions are not discharged because they are too stable; OH⁻ ions are discharged instead [1]

4OH⁻ → 2H₂O + O₂ + 4e⁻ [1]

4OH− → 2H₂O + O₂ + 4e−

1 mark -- oxygen gas produced anode 1 mark -- sulfate ions not discharged too stable hydroxide ions discharged instead 1 mark -- equation 4OH- gives 2H2O plus O2 plus 4e-

(d) [4]

(i) State how the colour of the solution changes during prolonged electrolysis. [1]

(ii) Explain why this colour change occurs. [2]

(iii) State the products at both electrodes once all the copper has been removed. [1]

Model Answer -- 2(d)

The blue colour fades / becomes paler and eventually becomes colourless [1]

Cu²⁺ ions are responsible for the blue colour [1]

Cu²⁺ ions are removed from solution as they are deposited as copper at the cathode, but no new Cu²⁺ ions are added (inert electrodes) [1]

Hydrogen at the cathode and oxygen at the anode (effectively electrolysis of dilute sulfuric acid) [1]

1 mark -- blue colour fades paler colourless 1 mark -- Cu2+ ions responsible blue colour 1 mark -- Cu2+ removed deposited copper cathode no new Cu2+ added inert electrodes 1 mark -- hydrogen cathode oxygen anode electrolysis dilute sulfuric acid

Question 3 -- Aluminium Extraction: Costs and Environment

Total: 12 marks

The NALCO aluminium smelter in Angul, Odisha is one of India's largest aluminium producers. The plant uses the Hall-Héroult process to extract aluminium from purified bauxite.

(a) [4]

(i) Name the compound from which aluminium is extracted after purification of bauxite. [1]

(ii) Name the substance in which it is dissolved. [1]

(iii) State the material used for the cathode lining of the cell. [1]

(iv) State what happens to the aluminium once it is formed. [1]

Model Answer -- 3(a)

Aluminium oxide / Al₂O₃ [1]

Cryolite / Na₃AlF₆ [1]

Carbon / graphite [1]

Molten aluminium sinks to the bottom of the cell and is tapped off / drained [1]

1 mark -- aluminium oxide Al2O3 1 mark -- cryolite 1 mark -- cathode carbon graphite 1 mark -- molten aluminium sinks bottom tapped drained

(b) [2]

Write the half-equations for the reactions at:

(i) the cathode [1]

(ii) the anode [1]

Model Answer -- 3(b)

Cathode: Al³⁺ + 3e⁻ → Al [1]

Al³+ + 3e− → Al

Anode: 2O²⁻ → O₂ + 4e⁻ [1]

2O²− → O₂ + 4e−

1 mark -- cathode Al3+ plus 3e- gives Al 1 mark -- anode 2O2- gives O2 plus 4e-

Aluminium is much more expensive to produce than iron, even though aluminium is more abundant in the Earth's crust.

(i) Explain why the electricity costs for aluminium extraction are very high. [2]

(ii) State why aluminium smelters are often located near hydroelectric power stations. [1]

Model Answer -- 3(c)

The electrolytic cell requires very high temperatures to keep the electrolyte molten / large amounts of electricity are needed to maintain the process [1]

The process runs continuously 24 hours a day / large current is needed for the electrolysis itself [1]

Hydroelectric power provides cheap / renewable electricity, reducing the overall cost of production [1]

1 mark -- high temperatures keep electrolyte molten large electricity maintain 1 mark -- process runs continuously large current needed electrolysis 1 mark -- hydroelectric cheap renewable electricity reducing cost

(d) [3]

(i) State one environmental problem caused by the extraction of aluminium. [1]

(ii) State two advantages of recycling aluminium rather than extracting it from its ore. [2]

Model Answer -- 3(d)

Mining bauxite causes habitat destruction / deforestation / landscape scarring / CO₂ emissions from burning carbon anodes [1]

Recycling uses much less energy / only about 5% of the energy needed for extraction [1]

Recycling conserves bauxite ore / reduces mining / reduces landfill waste [1]

1 mark -- mining habitat destruction deforestation CO2 emissions carbon anodes 1 mark -- recycling less energy 5 percent 1 mark -- recycling conserves bauxite reduces mining reduces landfill

Question 4 -- Electroplating with Chromium

Total: 12 marks

A car parts factory in Pune electroplates steel bumpers with a thin layer of chromium to protect them from corrosion. The electrolyte contains chromium(III) ions.

(a) [3]

(i) Define electroplating. [1]

(ii) State two reasons why the factory electroplates the bumpers with chromium. [2]

Model Answer -- 4(a)

Electroplating is the process of coating a metal object with a thin layer of another metal using electrolysis [1]

To prevent corrosion / rusting of the steel [1]

To give a shiny, attractive appearance / improve aesthetics [1]

1 mark -- electroplating coating metal object thin layer another metal electrolysis 1 mark -- prevent corrosion rusting steel 1 mark -- shiny attractive appearance aesthetics

(b) [3]

(i) State which electrode the steel bumper should be connected to. [1]

(ii) State what the other electrode should be made of. [1]

(iii) Name a suitable electrolyte for this process. [1]

Model Answer -- 4(b)

The steel bumper should be connected as the cathode / negative electrode [1]

The anode should be made of chromium metal [1]

Chromium(III) sulfate solution / chromium(III) chloride solution / a solution containing Cr³⁺ ions [1]

1 mark -- steel bumper cathode negative electrode 1 mark -- anode chromium metal 1 mark -- chromium sulfate chloride solution containing Cr3+ ions

Write the half-equation for the reaction at:

(i) the cathode [1]

(ii) the anode [1]

Model Answer -- 4(c)

Cathode: Cr³⁺ + 3e⁻ → Cr [1]

Cr³+ + 3e− → Cr

Anode: Cr → Cr³⁺ + 3e⁻ [1]

Cr → Cr³+ + 3e−

1 mark -- cathode Cr3+ plus 3e- gives Cr 1 mark -- anode Cr gives Cr3+ plus 3e-

(d) [4]

(i) Explain why the surface of the steel bumper must be cleaned thoroughly before electroplating. [1]

(ii) Explain why using a chromium anode is better than using an inert anode such as platinum. [1]

(iii) State and explain one way to increase the rate at which the coating is deposited. [2]

Model Answer -- 4(d)

Grease or dirt prevents the chromium coating from sticking properly / ensures an even, adherent coating [1]

A chromium anode dissolves to replace the Cr³⁺ ions used up, keeping the concentration of the electrolyte constant; an inert anode would deplete the solution [1]

Increase the current [1]

A larger current causes more ions to be discharged per second / increases the rate of electron transfer at the cathode [1]

1 mark -- grease dirt prevents coating sticking even adherent 1 mark -- chromium anode dissolves replace Cr3+ ions concentration constant inert deplete 1 mark -- increase current 1 mark -- larger current more ions discharged per second rate electron transfer

Question 5 -- Copper Purification

Total: 10 marks

A copper refinery near Swansea, Wales purifies blister copper (about 99% pure) to produce 99.99% pure copper for use in electronics.

(a) [4]

(i) State what the impure copper is used as in the electrolytic cell. [1]

(ii) Write the half-equation for the reaction at the anode. [1]

(iii) Write the half-equation for the reaction at the cathode. [1]

(iv) Explain why the mass of the cathode increases during electrolysis. [1]

Model Answer -- 5(a)

The impure copper is the anode / positive electrode [1]

Anode: Cu → Cu²⁺ + 2e⁻ [1]

Cu → Cu²+ + 2e−

Cathode: Cu²⁺ + 2e⁻ → Cu [1]

Cu²+ + 2e− → Cu

Pure copper is deposited on the cathode as Cu²⁺ ions gain electrons and form copper atoms [1]

1 mark -- impure copper anode positive electrode 1 mark -- anode Cu gives Cu2+ plus 2e- 1 mark -- cathode Cu2+ plus 2e- gives Cu 1 mark -- pure copper deposited cathode Cu2+ ions gain electrons form copper

(b) [3]

The anode initially has a mass of 150 g. After electrolysis, the cathode has gained 120 g and the anode now has a mass of 25 g.

(i) Calculate the mass lost by the anode. [1]

(ii) Explain why the mass lost by the anode (125 g) is greater than the mass gained by the cathode (120 g). [2]

Model Answer -- 5(b)

Mass lost = 150 − 25 = 125 g [1]

The anode contains impurities (e.g. gold, silver, iron) that do not dissolve or are not deposited at the cathode [1]

These impurities fall to the bottom as anode sludge / some impurities dissolve but are not deposited [1]

1 mark -- mass lost 150 minus 25 equals 125 1 mark -- anode contains impurities gold silver iron not dissolved not deposited cathode 1 mark -- impurities fall bottom anode sludge

(i) State one use of pure copper that requires high electrical conductivity. [1]

(ii) Explain, in terms of structure, why copper is a good conductor of electricity. [2]

Model Answer -- 5(c)

Electrical wiring / cables / circuit boards / electronic components [1]

Copper has metallic bonding with a sea of delocalised electrons [1]

These delocalised electrons are free to move through the structure, carrying the electric charge / current [1]

1 mark -- electrical wiring cables circuit boards electronic 1 mark -- metallic bonding sea delocalised electrons 1 mark -- delocalised electrons free move carrying charge current

Question 6 -- Electrochemical Cells and the Reactivity Series

Total: 12 marks

A student in Edinburgh investigates how the voltage of a simple cell depends on the metals used. She sets up cells using different pairs of metals in dilute sulfuric acid.

(a) [4]

The student records the following results:

Metal pair	Voltage / V
Zinc and Copper	1.10
Magnesium and Copper	2.71
Iron and Copper	0.78
Magnesium and Zinc	1.61

(i) State the pattern between the voltage produced and the metals used. [2]

(ii) Predict which pair of metals would give the highest voltage from this list: iron, lead, magnesium, tin. Explain your answer. [2]

Model Answer -- 6(a)

The greater the difference in reactivity between the two metals, the higher the voltage [1]

Metals further apart in the reactivity series produce a larger voltage [1]

Magnesium and lead (or magnesium and tin) would give the highest voltage [1]

Because magnesium is the most reactive and lead/tin is the least reactive, giving the greatest difference in reactivity [1]

1 mark -- greater difference reactivity higher voltage 1 mark -- further apart reactivity series larger voltage 1 mark -- magnesium lead or magnesium tin highest voltage 1 mark -- magnesium most reactive lead tin least reactive greatest difference

(b) [4]

For the magnesium-copper cell:

(i) State which metal is the negative electrode and explain why. [2]

(ii) Write the half-equation for the reaction at the magnesium electrode. [1]

(iii) Describe what you would observe at the copper electrode during operation. [1]

Model Answer -- 6(b)

Magnesium is the negative electrode [1]

Because magnesium is more reactive / loses electrons more readily, so it pushes electrons into the external circuit [1]

Mg → Mg²⁺ + 2e⁻ [1]

Mg → Mg²+ + 2e−

Bubbles of hydrogen gas are produced on the surface of the copper [1]

1 mark -- magnesium negative electrode 1 mark -- magnesium more reactive loses electrons readily pushes electrons external circuit 1 mark -- equation Mg gives Mg2+ plus 2e- 1 mark -- bubbles hydrogen gas surface copper

The student finds that the voltage decreases over time.

(i) Suggest why the voltage decreases. [1]

(ii) The student repeats the experiment using salt solution instead of sulfuric acid. Predict how this might affect the voltage. [1]

Model Answer -- 6(c)

The more reactive metal dissolves / is used up, or the acid concentration decreases / products build up on the electrodes [1]

The voltage would be similar / the electrolyte provides ions for the circuit but the voltage mainly depends on the metals used [1]

1 mark -- reactive metal dissolves used up acid concentration decreases products build up electrodes 1 mark -- voltage similar electrolyte provides ions voltage depends metals used

(d) [2]

(i) The student tries replacing one metal with a graphite rod. Explain why this still produces a voltage. [1]

(ii) Predict the effect on voltage if she uses two graphite rods as electrodes. Explain your answer. [1]

Model Answer -- 6(d)

Graphite conducts electricity and has a different reactivity from the metal, so a potential difference is still created between the two electrodes [1]

No voltage / zero, because both electrodes are the same material with the same reactivity, so no potential difference is created [1]

1 mark -- graphite conducts different reactivity potential difference still created 1 mark -- no voltage zero both same material same reactivity no potential difference

Question 7 -- Hydrogen Fuel Cells

Total: 10 marks

Transport for London is trialling hydrogen fuel cell buses on Route 7. The buses use hydrogen gas stored in high-pressure tanks on the roof.

(a) [3]

(i) State the two reactants in a hydrogen fuel cell. [1]

(ii) Write the overall equation for the reaction. [1]

(iii) Explain why a hydrogen fuel cell is described as an electrochemical cell, not as combustion. [1]

Model Answer -- 7(a)

Hydrogen and oxygen [1]

2H₂ + O₂ → 2H₂O [1]

In a fuel cell, the chemical energy is converted directly into electrical energy without combustion / burning; electrons flow through an external circuit rather than the reaction producing heat and light [1]

1 mark -- hydrogen oxygen 1 mark -- equation 2H2 plus O2 gives 2H2O 1 mark -- chemical energy directly electrical energy without combustion electrons external circuit

(b) [3]

Compare the environmental impact of a hydrogen fuel cell bus with a diesel bus. Give three points.

Model Answer -- 7(b)

Fuel cell produces only water; diesel produces CO₂ which contributes to global warming / climate change [1]

Diesel produces sulfur dioxide / nitrogen oxides which cause acid rain; fuel cell produces no acidic gases [1]

Diesel produces particulates which cause respiratory problems / smog; fuel cell produces no particulates [1]

1 mark -- fuel cell only water diesel CO2 global warming climate change 1 mark -- diesel sulfur dioxide nitrogen oxides acid rain fuel cell no acidic gases 1 mark -- diesel particulates respiratory smog fuel cell no particulates

(i) Explain why storing hydrogen in high-pressure tanks is necessary but poses challenges. [1]

(ii) State one way hydrogen gas can be produced for use in fuel cells, and identify a potential environmental concern with this method. [1]

Model Answer -- 7(c)

Hydrogen is a gas at room temperature with very low density, so it must be compressed to reduce volume for storage; however high-pressure tanks are heavy, expensive, and hydrogen is highly flammable / risk of explosion [1]

Steam reforming of natural gas / electrolysis of water; steam reforming produces CO₂ as a by-product / electrolysis requires electricity which may come from fossil fuels [1]

1 mark -- hydrogen gas low density compressed reduce volume heavy expensive flammable explosion 1 mark -- steam reforming natural gas electrolysis water CO2 by-product fossil fuels

(d) [2]

(i) State one advantage of fuel cells over conventional batteries in terms of operation. [1]

(ii) Explain why a fuel cell is more efficient than an internal combustion engine. [1]

Model Answer -- 7(d)

Fuel cells can operate continuously as long as fuel (hydrogen) and oxygen are supplied; batteries need recharging or replacing [1]

A fuel cell converts chemical energy directly to electrical energy, whereas a combustion engine loses much energy as heat / has intermediate steps (chemical → heat → kinetic → electrical), making it less efficient [1]

1 mark -- fuel cells operate continuously fuel supplied batteries recharging replacing 1 mark -- fuel cell directly electrical energy combustion engine loses energy heat intermediate steps less efficient

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