⚡ Challenge Paper Preparation

Challenge Prep: Electrochemistry

IGCSE Chemistry 0620 — Topic 4

Electrochemistry is the topic where students mix everything up: anode vs cathode, oxidation vs reduction, electrolysis vs electrochemical cells, dilute vs concentrated solutions. Challenge papers exploit every single one of these confusions. The good news? Once you learn a few clear rules (like OILRIG and "PANIC" for electrodes), every question becomes a systematic checklist. This guide exposes the traps, walks you through the reasoning, and gives you hard practice questions so nothing on exam day catches you off guard.

⚠️ Common Traps & Misconceptions

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Ten traps that examiners love to set on electrochemistry questions. Know these cold before the exam.

⚠️ TRAP

Trap 1: Getting anode and cathode mixed up in electrolysis

The TrapStudents think the anode is negative and the cathode is positive in electrolysis, confusing it with electrochemical cells or just guessing randomly.

The TruthIn electrolysis: the anode is positive (+) and the cathode is negative (−). Remember: PANIC — Positive Anode, Negative Is Cathode. Positive ions (cations) are attracted to the negative cathode. Negative ions (anions) are attracted to the positive anode.

Why It MattersIf you get the electrodes wrong, every part of your answer flips: the wrong product at each electrode, the wrong half-equation, and the wrong type of reaction (oxidation/reduction). It's one mistake that destroys the entire question.

Example Question"In the electrolysis of molten lead bromide, name the product formed at the cathode and explain why it forms there."

⚠️ TRAP

Trap 2: Wrong products for dilute vs concentrated aqueous solutions

The TrapStudents learn the products for one concentration and apply them to all concentrations. For example, they say chlorine is always produced at the anode from sodium chloride solution.

The TruthFor aqueous solutions, the products depend on concentration:
• At the cathode: If the metal is more reactive than hydrogen (e.g. Na, K, Ca, Mg, Al), hydrogen gas is produced instead of the metal. If the metal is less reactive than hydrogen (e.g. Cu, Ag), the metal is deposited.
• At the anode: If the solution is concentrated with a halide (Cl⁻, Br⁻, I⁻), the halogen is produced. If the solution is dilute or contains sulfate/nitrate ions, oxygen is produced.

Why It MattersChallenge papers specify "dilute sodium chloride solution" or "concentrated sodium chloride solution" to test exactly this distinction. The cathode product is the same (H₂), but the anode product changes: Cl₂ from concentrated, O₂ from dilute.

Example Question"Compare the products of electrolysis of concentrated sodium chloride solution with dilute sodium chloride solution."

⚠️ TRAP

Trap 3: Forgetting to balance electrons in half-equations

The TrapStudents write Cl⁻ → Cl₂ without balancing the chloride ions or adding the electrons lost.

The TruthHalf-equations must balance for atoms, charge, and electrons. For chlorine at the anode: 2Cl⁻ → Cl₂ + 2e⁻. You need 2 chloride ions to make one Cl₂ molecule, and 2 electrons are released (oxidation). At the cathode: Pb²⁺ + 2e⁻ → Pb.

Why It MattersChallenge papers award separate marks for: correct formula of product, balanced atoms, balanced electrons. An unbalanced half-equation typically loses 2 of 3 marks.

Example Question"Write the half-equation for the reaction occurring at the anode during the electrolysis of molten sodium chloride."

⚠️ TRAP

Trap 4: Confusing electrolysis with electrochemical (galvanic) cells

The TrapStudents mix up the two types of electrochemical system. They apply electrolysis rules to a battery, or vice versa.

The TruthElectrolysis uses electrical energy to force a non-spontaneous reaction — it needs an external power supply. Electrochemical cells (batteries) generate electrical energy from a spontaneous chemical reaction — they produce electricity. In electrolysis the anode is +, in a simple cell the conventions can differ, but in both: oxidation always happens at the anode and reduction always happens at the cathode.

Why It MattersChallenge papers might show a diagram and ask you to identify whether it's electrolysis or an electrochemical cell. Look for: external battery = electrolysis. Two different metals in a solution without a battery = electrochemical cell.

Example Question"Explain the difference between the role of electrical energy in electrolysis and in a simple cell."

⚠️ TRAP

Trap 5: Not knowing when hydrogen forms vs the metal at the cathode

The TrapStudents assume the metal always forms at the cathode during electrolysis of aqueous solutions, regardless of the metal's reactivity.

The TruthIn aqueous solutions, both the metal ion and H⁺ ions (from water) are present at the cathode. The less reactive species is discharged preferentially. If the metal is more reactive than hydrogen (any metal above hydrogen in the reactivity series: K, Na, Ca, Mg, Al, Zn), hydrogen gas is produced. If the metal is less reactive than hydrogen (Cu, Ag, Au), the metal is deposited.

Why It MattersChallenge papers test this with metals students are less familiar with. The rule is simple: check the reactivity series. Above hydrogen = hydrogen gas. Below hydrogen = metal deposited.

Example Question"Predict the product at the cathode during electrolysis of aqueous magnesium sulfate. Explain your answer."

⚠️ TRAP

Trap 6: Mixing up oxidation and reduction at electrodes

The TrapStudents say "reduction happens at the anode" or "the cathode is where electrons are lost."

The TruthUse OILRIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). At the anode: negative ions lose electrons → oxidation. At the cathode: positive ions gain electrons → reduction. This is true for BOTH electrolysis and electrochemical cells — it never changes.

Why It MattersIf you label oxidation/reduction the wrong way round, the half-equations, electron flow direction, and product identification all become wrong. Memorise: Anode = Oxidation, Cathode = Reduction (alphabetical order: A before C, O before R).

Example Question"State whether oxidation or reduction occurs at the cathode during electrolysis. Explain your answer in terms of electron transfer."

⚠️ TRAP

Trap 7: Not knowing why cryolite is used in aluminium extraction

The TrapStudents say cryolite "makes the reaction faster" or "makes aluminium oxide conduct better" without explaining the actual reason.

The TruthAluminium oxide (Al₂O₃) has a melting point of about 2072°C — far too high to be practical. Dissolving Al₂O₃ in molten cryolite (Na₃AlF₆) lowers the working temperature to about 950°C. This saves enormous amounts of energy and money. Cryolite does NOT act as a catalyst and does NOT participate in the reaction.

Why It MattersThis is a classic 2-mark question. Mark 1: lowers the melting point / operating temperature. Mark 2: this saves energy / reduces costs. You need both points for full marks.

Example Question"Explain why cryolite is added to aluminium oxide before electrolysis in the extraction of aluminium."

⚠️ TRAP

Trap 8: Thinking fuel cells involve combustion

The TrapStudents describe a hydrogen fuel cell as "burning hydrogen to make electricity" or say the hydrogen "combusts."

The TruthIn a fuel cell, hydrogen and oxygen react electrochemically, not by combustion. The reaction happens at separate electrodes with an electrolyte between them. Hydrogen is oxidised at one electrode, oxygen is reduced at the other, and electrons flow through an external circuit. There is no flame, no burning, and no thermal energy conversion — chemical energy is converted directly to electrical energy.

Why It MattersChallenge papers ask you to compare fuel cells with combustion engines, or to explain the advantage of fuel cells. The key advantage is higher efficiency (no heat loss from burning), plus the only waste product is water.

Example Question"Explain why a hydrogen fuel cell is more efficient than burning hydrogen in a conventional engine to generate electricity."

⚠️ TRAP

Trap 9: Forgetting that electrolysis requires a liquid (molten or dissolved)

The TrapStudents don't explain why the substance must be molten or in solution, or they suggest electrolysing a solid ionic compound.

The TruthFor electrolysis to work, the ions must be free to move. In a solid ionic compound, ions are locked in fixed positions in the lattice and cannot carry charge. Only when the compound is melted (molten) or dissolved in water are the ions free to move towards the electrodes.

Why It MattersChallenge papers might ask "Explain why solid lead bromide does not undergo electrolysis" — testing whether you link ion mobility to the requirement for a liquid state.

Example Question"A student tries to electrolyse solid sodium chloride but no products form. Explain why."

⚠️ TRAP

Trap 10: Confusing electroplating with purification of metals

The TrapStudents mix up which metal is the anode and which is the cathode in electroplating, or confuse it with the purification of copper.

The TruthIn electroplating: the object to be coated is the cathode (metal ions from the solution are deposited onto it). The plating metal is the anode (it dissolves to replenish the solution). In copper purification: impure copper is the anode (dissolves), pure copper is the cathode (pure copper is deposited). In both cases: the cathode is where you want metal to appear.

Why It MattersChallenge papers show a diagram and ask you to label the electrodes, state what happens at each, or explain why the anode gets lighter while the cathode gets heavier. If you swap them, all the explanations are backwards.

Example Question"A steel spoon is to be electroplated with silver. State which electrode the spoon should be connected to and explain why."

🧩 Multi-Step Reasoning Walkthroughs

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Five challenging questions broken down step by step. Try each step yourself before revealing the next.

Walkthrough 1 — Predicting Products of Aqueous ElectrolysisConcentrated copper(II) chloride solution (CuCl₂(aq)) is electrolysed using inert (carbon) electrodes. Predict the products at each electrode and write the half-equations.

Identify ions

CuCl₂ dissociates into Cu²⁺ and Cl⁻. But this is an aqueous solution, so water also provides H⁺ and OH⁻ ions. So four ions are present: Cu²⁺, H⁺, Cl⁻, OH⁻.

Cathode (negative electrode)

Both Cu²⁺ and H⁺ migrate to the cathode. Cu is below hydrogen in the reactivity series, so Cu²⁺ is discharged preferentially. Copper metal is deposited.
Half-equation: Cu²⁺ + 2e⁻ → Cu (reduction).

Anode (positive electrode)

Both Cl⁻ and OH⁻ migrate to the anode. The solution is concentrated, so halide ions are discharged preferentially. Chlorine gas is produced.
Half-equation: 2Cl⁻ → Cl₂ + 2e⁻ (oxidation).

Observable evidence

Cathode: brown/pink solid (copper) deposits on the electrode. Anode: greenish-yellow gas (chlorine) with a bleach-like smell forms as bubbles. The blue colour of the solution fades as Cu²⁺ ions are removed.

Final AnswerCathode: copper (Cu²⁺ + 2e⁻ → Cu). Anode: chlorine (2Cl⁻ → Cl₂ + 2e⁻).

Examiner's NoteThe systematic approach: (1) list all ions, (2) decide cathode product using reactivity series, (3) decide anode product using concentration + halide rule. If the solution were dilute CuCl₂, the anode product would be oxygen instead of chlorine.

Walkthrough 2 — Aluminium ExtractionAluminium is extracted from aluminium oxide by electrolysis. The aluminium oxide is dissolved in molten cryolite. The electrodes are made of carbon (graphite). Explain why: (a) cryolite is used, (b) the carbon anodes must be replaced regularly.

Temperature problem

Al₂O₃ has a melting point of about 2072°C. Heating to this temperature would require huge amounts of energy and cost. Dissolving Al₂O₃ in cryolite lowers the operating temperature to about 950°C, saving energy and reducing costs significantly.

Electrode reactions

Cathode: Al³⁺ + 3e⁻ → Al (molten aluminium sinks to the bottom and is tapped off).
Anode: 2O²⁻ → O₂ + 4e⁻ (oxygen gas is produced).

Carbon + oxygen

The oxygen produced at the anode reacts with the carbon (graphite) electrodes at the high operating temperature: C + O₂ → CO₂. The carbon anodes gradually burn away and must be replaced regularly.

Energy costs

Even with cryolite, the process requires huge amounts of electricity (to keep the mixture molten and to drive the electrolysis). Aluminium extraction plants are typically built near cheap hydroelectric power sources. The high energy requirement is why aluminium is more expensive than iron, even though aluminium ore is more abundant.

Final Answer(a) Cryolite lowers the melting point, saving energy/costs. (b) Oxygen at the anode reacts with the carbon electrodes (C + O₂ → CO₂), burning them away.

Examiner's NoteFor full marks on part (a), you need TWO points: (1) lowers the melting point and (2) saves energy/reduces cost. Just saying "lowers the melting point" without explaining why that matters is usually 1/2 marks. For part (b), name the specific reaction (carbon + oxygen → carbon dioxide).

Walkthrough 3 — Electroplating SetupDescribe how you would electroplate a brass key with a layer of nickel. State the cathode, anode, and electrolyte, and explain your choices.

Cathode = object to be plated

The brass key is the cathode. Metal ions from the solution will gain electrons and be deposited onto the cathode's surface. You want the nickel to coat the key, so the key must be the cathode.

Anode = plating metal

A piece of pure nickel is the anode. As the electrolysis proceeds, the nickel anode dissolves (oxidation: Ni → Ni²⁺ + 2e⁻), replenishing the nickel ions in the solution so the plating can continue.

Electrolyte = solution of plating metal's salt

The electrolyte is a solution of a nickel salt, such as nickel sulfate (NiSO₄) solution. This provides Ni²⁺ ions that are deposited at the cathode.

The cycle

Cathode: Ni²⁺ + 2e⁻ → Ni (nickel deposited on key).
Anode: Ni → Ni²⁺ + 2e⁻ (nickel dissolves from anode).
The concentration of Ni²⁺ in the solution stays constant because the anode dissolves at the same rate as the cathode deposits.

Final AnswerCathode: brass key. Anode: pure nickel. Electrolyte: nickel sulfate solution.

Examiner's NoteThe three-part rule for electroplating: (1) object = cathode, (2) plating metal = anode, (3) electrolyte = solution containing ions of the plating metal. Examiners award 1 mark for each correctly identified component.

Walkthrough 4 — Comparing Molten vs Aqueous ElectrolysisCompare the products of electrolysis of molten sodium chloride with those of concentrated aqueous sodium chloride. Explain any differences.

Ion inventory

Molten NaCl: Only Na⁺ and Cl⁻ — no water present.
Aqueous NaCl: Na⁺, Cl⁻, H⁺ (from water), and OH⁻ (from water) — four different ions.

Cathode products

Molten: Only Na⁺ is available, so sodium metal is deposited. Na⁺ + e⁻ → Na.
Aqueous: Na⁺ and H⁺ compete. Na is more reactive than hydrogen, so H⁺ is discharged instead. Hydrogen gas forms. 2H⁺ + 2e⁻ → H₂.

Anode products

Both molten and concentrated aqueous: Chlorine gas is produced. 2Cl⁻ → Cl₂ + 2e⁻. (In concentrated solution, halide ions are preferentially discharged over OH⁻.)

The critical difference

The anode product is the same (Cl₂). The cathode product is different: sodium from molten NaCl, hydrogen from aqueous NaCl. The difference occurs because water provides H⁺ ions that compete with Na⁺ at the cathode, and hydrogen is less reactive, so it is discharged preferentially.

Final AnswerMolten NaCl: cathode = sodium, anode = chlorine. Concentrated NaCl(aq): cathode = hydrogen, anode = chlorine. The difference is at the cathode, because in aqueous solution H⁺ ions from water are discharged instead of Na⁺.

Examiner's NoteThis comparison question is worth 4-6 marks on challenge papers. You need to explain the reason for the difference (presence of water, reactivity series) — just listing the products without explanation loses 2+ marks.

Walkthrough 5 — Hydrogen Fuel CellA hydrogen fuel cell uses hydrogen and oxygen to produce electricity. Explain how the fuel cell works, write relevant equations, and state one advantage and one disadvantage compared to fossil fuel power stations.

Electrochemical reaction

Hydrogen gas is fed to one electrode and oxygen gas to the other. They react electrochemically (NOT by combustion). Chemical energy is converted directly to electrical energy. The only product is water.

Overall equation

2H₂ + O₂ → 2H₂O
This is the same overall equation as burning hydrogen, but in a fuel cell it happens electrochemically at separate electrodes, not as combustion.

Key advantage

The only product is water — no carbon dioxide, no sulfur dioxide, no pollutants. This makes fuel cells much cleaner than fossil fuels. Also, fuel cells are more efficient because chemical energy is converted directly to electrical energy without the thermal (heat) step that wastes energy in combustion engines.

Key disadvantage

Hydrogen is difficult and expensive to store and transport (it must be compressed or liquefied). Also, most hydrogen is currently produced from fossil fuels (steam reforming of methane), which still produces CO₂. Truly "green" hydrogen requires electrolysis of water using renewable electricity, which is not yet widespread.

Final AnswerH₂ and O₂ react electrochemically to produce electricity and water. Advantage: no pollutants / only water produced. Disadvantage: hydrogen is expensive to store/produce.

Examiner's NoteNever say "the hydrogen burns" or "combustion occurs" — that's the most common error. The key word is "electrochemically." For advantages, the most creditworthy points are "only water as waste product" and "higher efficiency." For disadvantages, say storage/transport difficulty or the source of the hydrogen.

🔍 Spot the Difference

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Pairs of questions that look nearly identical but require different answers. The difference is always in the details.

Question A

What forms at the cathode during electrolysis of molten lead bromide?

Lead metal. Only Pb²⁺ and Br⁻ ions are present. Pb²⁺ gains electrons at the cathode.

Question B

What forms at the cathode during electrolysis of aqueous lead nitrate?

Lead metal. Lead is below hydrogen in the reactivity series, so Pb²⁺ is discharged preferentially over H⁺.

Key DifferenceSame cathode product (lead), but for different reasons. Molten: lead is the only cation. Aqueous: lead wins because it's less reactive than hydrogen. The explanation changes even though the answer is the same — and examiners award marks for the explanation.

Question A

What forms at the anode during electrolysis of concentrated NaCl(aq)?

Chlorine gas (Cl₂). Concentrated halide solution → halogen at anode.

Question B

What forms at the anode during electrolysis of dilute NaCl(aq)?

Oxygen gas (O₂). Dilute solution → OH⁻ from water is discharged instead.

Key DifferenceThe single word "concentrated" vs "dilute" changes the anode product entirely. Concentrated halide = halogen produced. Dilute = oxygen produced (from discharge of OH⁻ ions). Always check the concentration before predicting the anode product.

Question A

What forms at the cathode during electrolysis of aqueous copper sulfate?

Copper metal is deposited. Cu is less reactive than H, so Cu²⁺ is discharged.

Question B

What forms at the cathode during electrolysis of aqueous sodium sulfate?

Hydrogen gas. Na is more reactive than H, so H⁺ from water is discharged instead.

Key DifferenceSame electrolysis setup, same sulfate anion, but different metal. Cu is below H in the reactivity series → copper deposited. Na is above H → hydrogen gas. The reactivity series determines the cathode product in aqueous electrolysis.

Question A

In copper purification, what is the anode made of?

Impure copper. It dissolves, releasing Cu²⁺ ions into solution. Impurities fall to the bottom as "sludge."

Question B

In copper electroplating of a steel spoon, what is the anode made of?

Pure copper. It dissolves to replenish Cu²⁺ in solution, maintaining a constant plating rate.

Key DifferenceBoth use copper as the anode, but for different purposes. In purification: impure copper anode, pure copper cathode — the goal is to transfer copper from impure to pure. In electroplating: pure copper anode, steel spoon cathode — the goal is to coat the spoon.

Question A

In a hydrogen fuel cell, how is electricity produced?

H₂ and O₂ react electrochemically at separate electrodes. Chemical energy converts directly to electrical energy.

Question B

In a hydrogen-powered combustion engine, how is electricity produced?

H₂ is burned (combustion) to produce heat, which drives a turbine/generator. Chemical → thermal → kinetic → electrical energy.

Key DifferenceSame reactants (H₂ + O₂), same product (H₂O), but different mechanisms. Fuel cell = electrochemical (direct conversion, more efficient). Engine = combustion (via heat, less efficient due to energy losses). Never describe a fuel cell as "burning."

🔗 Electrochemistry Concept Map

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Click each node to see how the subtopics connect.

⭐ DECISION TREE

Predicting Electrolysis Products — the systematic approach

Step 1: Molten or Aqueous? ▶

Molten: Only the ions from the compound are present. Products are straightforward — the metal forms at the cathode, the non-metal forms at the anode.

Aqueous: Water adds extra ions (H⁺ and OH⁻), creating competition at both electrodes. You must use the reactivity series and concentration rules.

Step 2: Cathode Product (aqueous) ▶

Rule: The less reactive cation is discharged. Compare the metal with hydrogen in the reactivity series.

Metal ABOVE hydrogen (K, Na, Ca, Mg, Al, Zn) → H₂ gas produced

Metal BELOW hydrogen (Fe*, Cu, Ag, Au) → metal deposited. (*Iron is borderline — usually H₂ from dilute solutions)

Step 3: Anode Product (aqueous) ▶

Concentrated halide (Cl⁻, Br⁻, I⁻) → halogen gas (Cl₂, Br₂, I₂)

Dilute halide or non-halide (SO₄²⁻, NO₃⁻) → oxygen gas (from discharge of OH⁻)

OILRIG — Oxidation and Reduction ▶

Oxidation Is Loss of electrons — happens at the ANODE (A and O both come first alphabetically)

Reduction Is Gain of electrons — happens at the CATHODE

This rule applies to BOTH electrolysis AND electrochemical cells — it never changes.

Industrial Applications ▶

Aluminium extraction: Al₂O₃ dissolved in cryolite, carbon anodes burn away

Electroplating: Object = cathode, plating metal = anode, electrolyte = salt of plating metal

Copper purification: Impure Cu = anode, pure Cu = cathode, CuSO₄ electrolyte

Brine electrolysis: Produces Cl₂, H₂, NaOH — all industrially important

Electrochemical Cells & Fuel Cells ▶

Simple cell: Two different metals in an electrolyte. The more reactive metal is the negative terminal (it oxidises). Voltage depends on reactivity difference.

Fuel cell: H₂ + O₂ → H₂O electrochemically. Not combustion. Direct chemical → electrical energy conversion. Only waste = water.

❌ "Why Is This Wrong?" Exercises

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Spot the error in each student's answer. Think about it yourself before revealing the flaw.

Exercise 1: "State the product at the cathode during electrolysis of molten sodium chloride."

Student's Answer"Chlorine gas is produced at the cathode."

The FlawThe student put the product at the wrong electrode. Chlorine (Cl₂) forms at the anode, not the cathode. Na⁺ ions are attracted to the cathode (negative electrode) where they gain electrons to form sodium metal.

Correct AnswerSodium metal forms at the cathode. Na⁺ + e⁻ → Na.

Key RulePANIC: Positive Anode, Negative Is Cathode. Positive ions go to the negative cathode. Negative ions go to the positive anode.

Exercise 2: "Write the half-equation for the production of chlorine at the anode."

Student's Answer"Cl⁻ → Cl + e⁻"

The FlawChlorine exists as Cl₂ (diatomic molecule), not as individual Cl atoms. The equation must show 2 chloride ions combining to form one Cl₂ molecule.

Correct Answer2Cl⁻ → Cl₂ + 2e⁻

Key RuleAll halogens are diatomic. Half-equations must balance for atoms, charge, AND electrons. Check: left side has 2− charge, right side has 0 + 2− = 2−. Balanced.

Exercise 3: "Explain why cryolite is used in the extraction of aluminium."

Student's Answer"Cryolite is a catalyst that speeds up the electrolysis reaction."

The FlawCryolite is not a catalyst. It does not speed up the reaction. Its role is to lower the melting point of aluminium oxide from ~2072°C to ~950°C, which saves energy and reduces costs.

Correct Answer"Cryolite lowers the melting point of aluminium oxide, reducing the temperature needed for electrolysis and saving energy/costs."

Key RuleFor 2 marks: (1) lowers the melting point, (2) saves energy/reduces costs. Saying "catalyst" is a common but incorrect response.

Exercise 4: "Explain how a hydrogen fuel cell produces electricity."

Student's Answer"Hydrogen is burned in the fuel cell and the heat produced is used to generate electricity."

The FlawHydrogen is not burned in a fuel cell. The reaction is electrochemical, not combustion. Chemical energy is converted directly to electrical energy without producing heat as an intermediate step.

Correct Answer"Hydrogen and oxygen react electrochemically at separate electrodes. Chemical energy is converted directly to electrical energy. The only product is water."

Key RuleFuel cell = electrochemical (direct conversion). Engine = combustion (chemical → heat → kinetic → electrical). Never use "burn" or "combustion" for fuel cells.

Exercise 5: "What product forms at the cathode during electrolysis of aqueous magnesium chloride?"

Student's Answer"Magnesium metal is deposited at the cathode."

The FlawMagnesium is more reactive than hydrogen in the reactivity series. In aqueous solution, H⁺ ions from water are discharged instead of Mg²⁺. Magnesium metal would only form from molten MgCl₂.

Correct AnswerHydrogen gas is produced. 2H⁺ + 2e⁻ → H₂.

Key RuleIn aqueous electrolysis, check the reactivity series. Metals above hydrogen = hydrogen gas at cathode. Only metals below hydrogen (Cu, Ag, Au) are deposited from aqueous solution.

Exercise 6: "In electroplating a ring with gold, which electrode should the ring be?"

Student's Answer"The ring should be the anode because gold needs to go onto it."

The FlawThe ring should be the cathode, not the anode. Metal ions are deposited at the cathode (reduction: Au³⁺ + 3e⁻ → Au). If the ring were the anode, it would dissolve instead of being coated.

Correct AnswerThe ring is the cathode. The gold metal is the anode (it dissolves to provide Au³⁺). The electrolyte is a gold salt solution.

Key RuleObject to be plated = cathode. Plating metal = anode. Electrolyte = salt of the plating metal.

Exercise 7: "Explain why solid sodium chloride does not undergo electrolysis."

Student's Answer"Because sodium chloride is not a conductor of electricity."

The FlawThis is technically true but doesn't explain why. The explanation must be in terms of ion mobility: the ions are held in fixed positions in the lattice and cannot move to the electrodes.

Correct Answer"In solid NaCl, the Na⁺ and Cl⁻ ions are held in fixed positions in the giant ionic lattice and cannot move. Electrolysis requires ions that are free to move to the electrodes."

Key RuleThe mark scheme requires a reason in terms of ion mobility, not just a statement about conductivity. "Cannot conduct" is the observation; "ions fixed in the lattice" is the explanation.

Exercise 8: "During electrolysis of dilute sulfuric acid, what forms at the anode?"

Student's Answer"Sulfur is produced at the anode because the sulfate ions go there."

The FlawSulfate ions (SO₄²⁻) are NOT discharged during electrolysis of dilute sulfuric acid. OH⁻ ions from water are discharged instead, producing oxygen gas. SO₄²⁻ and NO₃⁻ are never discharged at the anode.

Correct AnswerOxygen gas is produced. 4OH⁻ → 2H₂O + O₂ + 4e⁻.

Key RuleAt the anode: concentrated halide → halogen. Everything else (dilute halide, sulfate, nitrate) → oxygen from OH⁻.

✍️ Ultra-Detailed Practice Questions

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Ten Cambridge-style challenge questions. Write your answer, then reveal the model answer with mark scheme and examiner's notes.

Question 1

[3 marks]

Molten zinc chloride is electrolysed using inert electrodes. State the product at each electrode and write the half-equation for the reaction at the cathode.

Model AnswerCathode: zinc metal [1]
Anode: chlorine gas [1]
Cathode half-equation: Zn²⁺ + 2e⁻ → Zn [1]

Examiner's NotesMolten = only ions from the compound. No water, so no competition. Zinc (cation) at cathode, chlorine (anion) at anode. Half-equation must show correct charge and electrons.

Question 2

[4 marks]

Concentrated aqueous sodium chloride (brine) is electrolysed. (a) Name the product at each electrode. (b) Name the useful solution that remains after electrolysis and state one use.

Model Answer(a) Cathode: hydrogen gas (Na is above H in reactivity series) [1]
Anode: chlorine gas (concentrated halide solution) [1]
(b) Sodium hydroxide solution remains [1]. Use: making soap / making bleach / making paper [1]

Examiner's NotesBrine electrolysis produces three useful products: H₂, Cl₂, and NaOH. The NaOH forms because Na⁺ and OH⁻ ions are left behind in solution after H⁺ and Cl⁻ are discharged. Common error: saying sodium metal forms at the cathode.

Question 3

[4 marks]

Describe how copper is purified by electrolysis. State the anode, cathode, electrolyte, and explain what happens during the process.

Model AnswerAnode: impure copper [1]. Cathode: pure copper [1]. Electrolyte: copper(II) sulfate solution [1].
Process: The impure copper anode dissolves (Cu → Cu²⁺ + 2e⁻), releasing Cu²⁺ ions into solution. These ions migrate to the cathode where pure copper is deposited (Cu²⁺ + 2e⁻ → Cu). Impurities from the anode fall to the bottom as "anode sludge." [1]

Examiner's NotesKey details: the anode gets lighter, the cathode gets heavier, and the concentration of Cu²⁺ in the solution stays constant. The "anode sludge" often contains valuable metals like gold and silver, which can be recovered.

Question 4

[4 marks]

Ananya sets up a simple electrochemical cell using a strip of zinc and a strip of copper in dilute sulfuric acid, connected by a wire. (a) Which metal is the negative terminal? (b) Explain why a voltage is produced. (c) What would happen to the voltage if magnesium replaced zinc?

Model Answer(a) Zinc is the negative terminal (it is more reactive) [1]
(b) The two metals have different reactivities, so electrons flow from the more reactive metal (zinc) through the wire to the less reactive metal (copper), creating a potential difference (voltage) [1]
(c) The voltage would increase [1] because magnesium is more reactive than zinc, so the difference in reactivity between the two metals is greater [1]

Examiner's NotesKey principle: voltage depends on the difference in reactivity between the two metals. Bigger difference = bigger voltage. Same metal on both sides = zero voltage. The more reactive metal always oxidises (loses electrons).

Question 5

[3 marks]

Write the half-equations for the electrolysis of dilute sulfuric acid using inert electrodes. State whether each is oxidation or reduction.

Model AnswerCathode: 2H⁺ + 2e⁻ → H₂ — reduction (gain of electrons) [1]
Anode: 4OH⁻ → 2H₂O + O₂ + 4e⁻ — oxidation (loss of electrons) [1]
Overall: 2H₂O → 2H₂ + O₂ [1]

Examiner's NotesThe H₂:O₂ volume ratio is 2:1 (matching the formula of water, H₂O). This observation is often tested: "The volume of gas at the cathode is double the volume at the anode. Explain why."

Question 6

[3 marks]

During the electrolysis of aluminium oxide dissolved in cryolite, the carbon anodes must be replaced regularly. Explain why, and write an equation for the reaction that occurs.

Model AnswerOxygen gas is produced at the anode [1]. At the high operating temperature, the oxygen reacts with the carbon anodes [1]: C + O₂ → CO₂ (or 2C + O₂ → 2CO). The anodes gradually burn away and must be replaced [1].

Examiner's NotesMust state: (1) oxygen is produced, (2) it reacts with carbon at high temperature, (3) carbon dioxide forms. An equation is needed for full marks. This is distinct from the graphite simply "wearing out" mechanically.

Question 7

[4 marks]

Predict the products at each electrode when dilute copper(II) sulfate solution is electrolysed using inert (platinum) electrodes. Write the half-equations.

Model AnswerCathode: copper metal (Cu is less reactive than H) [1]
Cu²⁺ + 2e⁻ → Cu [1]
Anode: oxygen gas (sulfate is not a halide, so OH⁻ is discharged) [1]
4OH⁻ → 2H₂O + O₂ + 4e⁻ [1]

Examiner's NotesAs electrolysis proceeds, Cu²⁺ ions are removed from solution, so the blue colour fades. Eventually, when all Cu²⁺ is deposited, H⁺ ions start being discharged instead, and hydrogen gas appears at the cathode. Challenge papers sometimes ask about this colour change.

Question 8

[3 marks]

Give one advantage and two disadvantages of using hydrogen fuel cells in cars compared to petrol engines.

Model AnswerAdvantage: The only waste product is water — no CO₂ or other pollutants [1]
Disadvantage 1: Hydrogen is difficult/expensive to store (needs high pressure or very low temperature) [1]
Disadvantage 2: Most hydrogen is currently produced from fossil fuels, which produces CO₂ / there is limited hydrogen refuelling infrastructure [1]

Examiner's NotesAlso acceptable: fuel cells are more efficient (direct energy conversion). For disadvantages: hydrogen is highly flammable (safety concern), fuel cells are currently expensive to manufacture, platinum catalysts are costly. Give specific reasons, not vague statements.

Question 9

[4 marks]

Dev wants to electroplate his iron bicycle bell with chromium to prevent rusting. State the cathode, anode, and electrolyte he should use. Explain why electroplating prevents rusting.

Model AnswerCathode: iron bicycle bell (the object to be plated) [1]
Anode: pure chromium (the plating metal) [1]
Electrolyte: chromium salt solution (e.g. chromium(III) sulfate) [1]
The chromium layer acts as a barrier between the iron and air/water, preventing oxygen and water from reaching the iron surface, so rusting cannot occur [1]

Examiner's NotesThe explanation of why it prevents rusting must mention the barrier function. Just saying "it coats the iron" without explaining how that prevents rusting is insufficient. Chromium is also more resistant to corrosion than iron, making it an excellent plating choice.

Question 10

[5 marks]

A student electrolises aqueous potassium iodide (KI) solution using inert electrodes. (a) Predict the products at each electrode. (b) Write the half-equations. (c) State one observation at the anode.

Model Answer(a) Cathode: hydrogen gas (K is above H in reactivity series) [1]
Anode: iodine (I⁻ is a halide ion, discharged preferentially) [1]
(b) Cathode: 2H⁺ + 2e⁻ → H₂ [1]
Anode: 2I⁻ → I₂ + 2e⁻ [1]
(c) A brown/dark colour appears around the anode (iodine in solution is brown) [1]

Examiner's NotesObservations earn marks! At the cathode: bubbles of gas (hydrogen). At the anode: brown coloration (iodine). You could test for hydrogen with a burning splint (squeaky pop) and for iodine by adding starch (turns blue-black). Challenge papers love observation + test questions.