IGCSE Chemistry Paper 4 (Theory / Extended)

Topic 4: Electrochemistry -- Mock Exam 1

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 Compounds

Total: 12 marks

A teacher in Mumbai sets up an experiment to electrolyse molten lead(II) bromide (PbBr₂) using carbon electrodes and a d.c. power supply.

(a) [3]

(i) Define electrolysis. [2]

(ii) State why lead(II) bromide must be molten for electrolysis to occur. [1]

Model Answer -- 1(a)

Electrolysis is the decomposition / breakdown of an ionic compound [1]

using an electric current / when molten or in aqueous solution [1]

When molten, the ions are free to move and carry the electric current; when solid, ions are held in fixed positions and cannot move [1]

1 mark -- electrolysis decomposition ionic compound 1 mark -- using electric current molten aqueous solution 1 mark -- molten ions free move carry current solid ions fixed cannot move

(b) [3]

(i) State the name of the electrode connected to the positive terminal of the power supply. [1]

(ii) Describe what you would observe at the cathode during electrolysis. [1]

(iii) Describe what you would observe at the anode during electrolysis. [1]

Model Answer -- 1(b)

The positive electrode is the anode [1]

At the cathode: a silvery / grey metallic bead of lead is deposited [1]

At the anode: brown / orange-brown fumes of bromine gas are produced [1]

1 mark -- positive electrode anode 1 mark -- cathode silvery grey metallic lead deposited 1 mark -- anode brown orange fumes bromine gas produced

(i) Write the half-equation for the reaction at the cathode. [2]

(ii) Write the half-equation for the reaction at the anode. [2]

Model Answer -- 1(c)

Cathode: Pb²⁺ + 2e⁻ → Pb [1 for species, 1 for balanced]

Pb²+ + 2e− → Pb

Anode: 2Br⁻ → Br₂ + 2e⁻ [1 for species, 1 for balanced]

2Br− → Br₂ + 2e−

1 mark -- cathode lead ion Pb2+ gains electrons 1 mark -- cathode equation balanced 2 electrons 1 mark -- anode bromide ions Br- lose electrons 1 mark -- anode equation balanced 2Br- gives Br2 plus 2e-

(d) [2]

Using the half-equations from part (c):

(i) State which electrode shows reduction, and explain why. [1]

(ii) State which electrode shows oxidation, and explain why. [1]

Model Answer -- 1(d)

Reduction occurs at the cathode because the Pb²⁺ ions gain electrons / there is a gain of electrons [1]

Oxidation occurs at the anode because the Br⁻ ions lose electrons / there is a loss of electrons [1]

1 mark -- reduction cathode gain electrons 1 mark -- oxidation anode lose electrons

Question 2 -- Electrolysis of Aqueous Solutions

Total: 12 marks

A chemistry laboratory in Birmingham investigates the electrolysis of concentrated sodium chloride solution (brine) using inert carbon electrodes.

(a) [2]

State the four types of ions present in concentrated sodium chloride solution. Give the source of each ion.

Model Answer -- 2(a)

Na⁺ and Cl⁻ ions from sodium chloride [1]

H⁺ and OH⁻ ions from the water [1]

1 mark -- sodium Na+ chloride Cl- ions from sodium chloride 1 mark -- hydrogen H+ hydroxide OH- ions from water

(b) [4]

(i) At the cathode, hydrogen gas is produced rather than sodium metal. Explain why hydrogen is discharged in preference to sodium, using the reactivity series. [2]

(ii) At the anode, chlorine gas is produced rather than oxygen. Explain why chlorine is discharged in preference to oxygen from the concentrated solution. [2]

Model Answer -- 2(b)

Sodium is more reactive than hydrogen / sodium is higher in the reactivity series [1]

The less reactive cation is preferentially discharged at the cathode / hydrogen ions gain electrons more readily than sodium ions [1]

When the solution is concentrated, the halide ion (Cl⁻) is discharged in preference to OH⁻ [1]

Concentration overrides the normal rule: the high concentration of Cl⁻ ions means they are preferentially discharged [1]

1 mark -- sodium more reactive than hydrogen higher reactivity series 1 mark -- less reactive cation discharged cathode hydrogen ions gain electrons 1 mark -- concentrated halide chloride ion discharged preference hydroxide 1 mark -- concentration overrides high concentration chloride preferentially discharged

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

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

(iii) Name the solution remaining after electrolysis. [1]

(iv) State one industrial use of the solution named in part (iii). [1]

Model Answer -- 2(c)

Cathode: 2H⁺ + 2e⁻ → H₂ [1]

2H+ + 2e− → H₂

Anode: 2Cl⁻ → Cl₂ + 2e⁻ [1]

2Cl− → Cl₂ + 2e−

The remaining solution is sodium hydroxide (NaOH) [1]

Used in making soap / paper / bleach / ceramics / purifying bauxite [1]

1 mark -- cathode 2H+ plus 2e- gives H2 1 mark -- anode 2Cl- gives Cl2 plus 2e- 1 mark -- remaining solution sodium hydroxide NaOH 1 mark -- industrial use soap paper bleach ceramics bauxite

(d) [2]

If a dilute solution of sodium chloride were used instead, state how the products at the anode would differ. Explain your answer.

Model Answer -- 2(d)

Oxygen would be produced at the anode instead of chlorine [1]

In dilute solution, the concentration of Cl⁻ ions is low, so OH⁻ ions are preferentially discharged instead [1]

1 mark -- oxygen produced anode instead chlorine 1 mark -- dilute solution low concentration chloride hydroxide ions preferentially discharged

Question 3 -- Extraction of Aluminium

Total: 12 marks

Aluminium is extracted from its ore bauxite at a smelting plant in Jharsuguda, Odisha. The purified ore, aluminium oxide (Al₂O₃), is dissolved in molten cryolite and electrolysed.

(a) [2]

(i) Explain why aluminium cannot be extracted by reduction with carbon. [1]

(ii) State the purpose of cryolite in the extraction process. [1]

Model Answer -- 3(a)

Aluminium is too reactive / too high in the reactivity series to be reduced by carbon [1]

Cryolite lowers the melting point of aluminium oxide from about 2072 °C to about 950 °C, reducing energy costs [1]

1 mark -- aluminium too reactive high reactivity series reduced carbon 1 mark -- cryolite lowers melting point reducing energy costs

(b) [3]

(i) State the material used for the electrodes. [1]

(ii) State where the aluminium collects and explain why. [1]

(iii) Identify the gas produced at the anode. [1]

Model Answer -- 3(b)

The electrodes are made of carbon / graphite [1]

Aluminium collects at the bottom of the cell as molten aluminium because it is denser than the electrolyte and sinks [1]

Oxygen gas is produced at the anode [1]

1 mark -- electrodes carbon graphite 1 mark -- aluminium collects bottom cell denser sinks 1 mark -- oxygen gas produced anode

(i) Write the half-equation for the reaction at the cathode. [2]

(ii) Write the half-equation for the reaction at the anode. [2]

Model Answer -- 3(c)

Cathode: Al³⁺ + 3e⁻ → Al [1 for species, 1 for balanced]

Al³+ + 3e− → Al

Anode: 2O²⁻ → O₂ + 4e⁻ [1 for species, 1 for balanced]

2O²− → O₂ + 4e−

1 mark -- cathode aluminium ion Al3+ gains electrons 1 mark -- cathode balanced 3 electrons 1 mark -- anode oxide ions O2- lose electrons oxygen formed 1 mark -- anode balanced 2O2- gives O2 plus 4e-

(d) [3]

The carbon anodes must be replaced regularly.

(i) Explain why the anodes need to be replaced. [2]

(ii) Write the equation for the reaction that causes this problem. [1]

Model Answer -- 3(d)

The oxygen produced at the anode reacts with the carbon electrodes at high temperature [1]

This burns away / oxidises the carbon anode, making it smaller over time [1]

C + O₂ → CO₂ [1]

1 mark -- oxygen reacts carbon electrodes high temperature 1 mark -- burns away oxidises carbon anode smaller 1 mark -- equation C plus O2 gives CO2

Question 4 -- Purification of Copper by Electrolysis

Total: 12 marks

A copper refinery in Tuticorin, Tamil Nadu uses electrolysis to purify copper for electrical wiring. The electrolyte is copper(II) sulfate solution.

(a) [3]

(i) State what the anode is made of. [1]

(ii) State what the cathode is made of. [1]

(iii) State the electrolyte used. [1]

Model Answer -- 4(a)

The anode is made of impure copper [1]

The cathode is made of pure copper / a thin sheet of pure copper [1]

The electrolyte is copper(II) sulfate solution [1]

1 mark -- anode impure copper 1 mark -- cathode pure copper thin sheet 1 mark -- electrolyte copper sulfate solution

(b) [4]

(i) Describe what happens to the anode during electrolysis. [1]

(ii) Describe what happens to the cathode during electrolysis. [1]

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

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

Model Answer -- 4(b)

The anode dissolves / decreases in mass / gets smaller as copper atoms lose electrons and enter the solution as Cu²⁺ ions [1]

The cathode increases in mass / gets bigger as Cu²⁺ ions from solution gain electrons and are deposited as pure copper [1]

Anode: Cu → Cu²⁺ + 2e⁻ [1]

Cu → Cu²+ + 2e−

Cathode: Cu²⁺ + 2e⁻ → Cu [1]

Cu²+ + 2e− → Cu

1 mark -- anode dissolves decreases mass copper atoms lose electrons 1 mark -- cathode increases mass copper ions gain electrons deposited pure 1 mark -- anode equation Cu gives Cu2+ plus 2e- 1 mark -- cathode equation Cu2+ plus 2e- gives Cu

The impure copper contains small amounts of gold and silver.

(i) Explain what happens to the gold and silver during electrolysis. [1]

(ii) Explain why these metals do not dissolve at the anode. [1]

Model Answer -- 4(c)

Gold and silver fall to the bottom of the cell as an anode sludge / anode mud [1]

They are less reactive than copper / lower in the reactivity series, so they do not form ions and dissolve [1]

1 mark -- gold silver fall bottom anode sludge mud 1 mark -- less reactive than copper lower reactivity series do not form ions

(d) [3]

(i) Explain why copper must be purified for use in electrical wiring. [1]

(ii) State one property of copper, other than electrical conductivity, that makes it suitable for wiring. [1]

(iii) Explain why the concentration of the copper(II) sulfate solution remains constant throughout the process. [1]

Model Answer -- 4(d)

Impurities reduce the electrical conductivity of copper / pure copper conducts electricity more efficiently [1]

Copper is ductile / can be drawn into thin wires / is malleable / flexible [1]

For every Cu²⁺ ion removed from solution at the cathode, another Cu²⁺ ion enters the solution from the anode, so the concentration stays the same [1]

1 mark -- impurities reduce conductivity pure copper conducts efficiently 1 mark -- ductile drawn wires malleable flexible 1 mark -- Cu2+ removed cathode another Cu2+ enters from anode concentration stays same

Question 5 -- Electroplating

Total: 10 marks

A jewellery manufacturer in Jaipur uses electroplating to coat iron bracelets with a thin layer of silver. The electrolyte is silver nitrate solution.

(a) [3]

(i) State two reasons why the manufacturer electroplates the bracelets with silver. [2]

(ii) State what the cathode should be in this electroplating cell. [1]

Model Answer -- 5(a)

To improve appearance / make the bracelet look more attractive [1]

To prevent corrosion / rusting of the iron [1]

The cathode is the iron bracelet / the object to be plated [1]

1 mark -- improve appearance attractive 1 mark -- prevent corrosion rusting iron 1 mark -- cathode iron bracelet object plated

(b) [2]

(i) State what the anode should be made of. [1]

(ii) Explain why this material is chosen for the anode. [1]

Model Answer -- 5(b)

The anode should be made of pure silver [1]

The silver anode dissolves to replace the Ag⁺ ions being deposited from the solution, keeping the concentration of the electrolyte constant [1]

1 mark -- anode pure silver 1 mark -- silver dissolves replace ions concentration constant

Write the half-equation for the reaction at:

(i) the cathode [1]

(ii) the anode [1]

Model Answer -- 5(c)

Cathode: Ag⁺ + e⁻ → Ag [1]

Ag+ + e− → Ag

Anode: Ag → Ag⁺ + e⁻ [1]

Ag → Ag+ + e−

1 mark -- cathode Ag+ plus e- gives Ag 1 mark -- anode Ag gives Ag+ plus e-

(d) [3]

(i) State one factor that affects the thickness of the silver coating. [1]

(ii) Explain why the electrolyte must contain ions of the plating metal. [1]

(iii) Suggest why the manufacturer uses a low current during electroplating rather than a high current. [1]

Model Answer -- 5(d)

Time / duration of electrolysis / current size / concentration of electrolyte [1]

The metal ions in the electrolyte are the source of the metal atoms that are deposited on the cathode / the ions gain electrons to form the metal coating [1]

A low current produces a smoother, more even coating; a high current can produce a rough, uneven or powdery deposit [1]

1 mark -- time duration current concentration affects thickness 1 mark -- metal ions source deposited cathode gain electrons form coating 1 mark -- low current smoother even coating high current rough uneven powdery

Question 6 -- Electrochemical Cells

Total: 12 marks

A student in London sets up a simple electrochemical cell using a strip of zinc and a strip of copper dipping into dilute sulfuric acid. A voltmeter is connected between the two metals.

(a) [3]

(i) State what an electrochemical cell converts. [1]

(ii) Explain why a voltage is produced when two different metals are used. [2]

Model Answer -- 6(a)

An electrochemical cell converts chemical energy into electrical energy [1]

The two metals have different reactivities / different tendencies to lose electrons [1]

This difference in reactivity creates a potential difference / voltage between the electrodes, causing electrons to flow through the external circuit [1]

1 mark -- converts chemical energy electrical energy 1 mark -- different metals different reactivities tendencies lose electrons 1 mark -- difference reactivity creates potential difference voltage electrons flow external circuit

(b) [4]

(i) State the direction of electron flow in the external circuit. [1]

(ii) Explain which metal acts as the negative electrode. [1]

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

(iv) Write the half-equation for the reaction at the copper electrode. [1]

Model Answer -- 6(b)

Electrons flow from zinc to copper through the external circuit [1]

Zinc is the negative electrode because it is more reactive / higher in the reactivity series, so it loses electrons more readily [1]

Zinc: Zn → Zn²⁺ + 2e⁻ [1]

Zn → Zn²+ + 2e−

Copper: 2H⁺ + 2e⁻ → H₂ [1]

2H+ + 2e− → H₂

1 mark -- electrons flow zinc copper external circuit 1 mark -- zinc negative electrode more reactive higher reactivity series 1 mark -- zinc equation Zn gives Zn2+ plus 2e- 1 mark -- copper equation 2H+ plus 2e- gives H2

(i) Predict how the voltage would change if the zinc were replaced by magnesium. Explain your answer. [2]

(ii) Predict the effect on the voltage if both electrodes were made of copper. [1]

Model Answer -- 6(c)

The voltage would increase [1]

Magnesium is more reactive than zinc / further apart in reactivity series from copper, so there is a greater difference in reactivity, producing a higher voltage [1]

No voltage / zero reading, because both electrodes are the same metal so there is no difference in reactivity [1]

1 mark -- voltage increase 1 mark -- magnesium more reactive greater difference reactivity higher voltage 1 mark -- no voltage zero same metal no difference reactivity

(d) [2]

State what you would observe happening to:

(i) the zinc strip over time [1]

(ii) the surface of the copper strip [1]

Model Answer -- 6(d)

The zinc strip dissolves / gets smaller / decreases in size as zinc atoms form Zn²⁺ ions [1]

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

1 mark -- zinc dissolves gets smaller decreases size 1 mark -- bubbles gas hydrogen produced surface copper

Question 7 -- Hydrogen Fuel Cells

Total: 10 marks

The Delhi Metro Rail Corporation is evaluating hydrogen fuel cell buses for use on feeder routes. A fuel cell uses hydrogen and oxygen to generate electricity.

(a) [3]

(i) State what energy conversion takes place in a hydrogen fuel cell. [1]

(ii) Write the overall equation for the reaction in a hydrogen fuel cell. [1]

(iii) State the only product formed. [1]

Model Answer -- 7(a)

Chemical energy is converted into electrical energy [1]

2H₂ + O₂ → 2H₂O [1]

The only product is water [1]

1 mark -- chemical energy converted electrical energy 1 mark -- equation 2H2 plus O2 gives 2H2O 1 mark -- only product water

(b) [3]

State three advantages of using hydrogen fuel cells for buses compared to using petrol or diesel engines.

Model Answer -- 7(b)

No carbon dioxide is produced / does not contribute to global warming / climate change [1]

No sulfur dioxide or nitrogen oxides produced / does not cause acid rain or air pollution [1]

The only waste product is water, which is not a pollutant / no particulates produced [1]

1 mark -- no carbon dioxide global warming climate change 1 mark -- no sulfur dioxide nitrogen oxides acid rain pollution 1 mark -- only waste water not pollutant no particulates

State two disadvantages or challenges of using hydrogen fuel cells for transport.

Model Answer -- 7(c)

Hydrogen is difficult to store / must be stored under high pressure or as a liquid at very low temperatures [1]

Hydrogen is highly flammable / explosive, posing safety risks / lack of hydrogen refuelling infrastructure / hydrogen is expensive to produce [1]

1 mark -- hydrogen difficult store high pressure low temperature 1 mark -- flammable explosive safety infrastructure expensive produce

(d) [2]

State two differences between a hydrogen fuel cell and a conventional rechargeable battery.

Model Answer -- 7(d)

A fuel cell uses an external supply of fuel (hydrogen) which is continuously fed in, whereas a battery stores its chemicals internally [1]

A fuel cell produces electricity as long as fuel is supplied, whereas a battery runs out / needs recharging [1]

1 mark -- fuel cell external supply fuel continuously fed battery stores chemicals internally 1 mark -- fuel cell produces electricity long fuel supplied battery runs out recharging

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