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IGCSE Chemistry 0620 · Chapter 4 notes · ~15 min read

Electrochemistry

How electricity splits compounds, how to predict what appears at each electrode in seconds, how electroplating works — and how a fuel cell turns hydrogen into electricity.

ElectrolysisProducts at electrodesElectroplatingIonic half-equationsFuel cells
RSby Riyaz Sir
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • Electrolysis = using electricity to break down an ionic compound that is molten or dissolved.
  • Cathode (−) attracts positive ions (cations). Anode (+) attracts negative ions (anions). Remember PANIC: Positive Anode, Negative Is Cathode.
  • At the cathode you get a metal or hydrogen. At the anode you get a non-metal (never hydrogen).
  • A hydrogen–oxygen fuel cell makes electricity from H₂ + O₂ and its only product is water.

By the end you can

  • Define electrolysis, electrolyte, anode and cathode, and say why the compound must be molten or dissolved.
  • Predict the products of electrolysis of molten compounds, concentrated sodium chloride and dilute sulfuric acid.
  • Describe electroplating: purpose, set-up and why the object is the cathode.
  • Predict products for copper(II) sulfate and halide solutions, and write half-equations. EXT
  • Describe a hydrogen–oxygen fuel cell and compare it with a petrol engine. EXT

1What happens in electrolysis

The key words

Electrolysis — the breakdown of an ionic compound, molten or in aqueous solution, by the passage of electricity.

Electrolyte — the molten or dissolved ionic compound that conducts electricity and is broken down.

Electrode — a rod (usually graphite or platinum) that carries current into or out of the electrolyte. Inert electrodes do not take part in the reaction.

Anode — the positive electrode. Anions (negative ions) move to it.

Cathode — the negative electrode. Cations (positive ions) move to it.

Memory trickPANIC — Positive Anode, Negative Is Cathode.

Why must the electrolyte be molten or dissolved?

  • Current in a wire is carried by moving electrons. Current in an electrolyte is carried by moving ions.
  • In a solid ionic compound the ions are locked in the lattice, so a solid does not conduct. When it is molten or dissolved, the ions are free to move.
  • Opposite charges attract: cations → cathode (−), anions → anode (+).
−+cathode (−)anode (+)+++−−e⁻e⁻electrolyte: molten or dissolved ionic compoundcarbon (graphite) electrodes are inert+cation−anioncathode: cations gainelectrons = reduction(metal or H₂ forms)anode: anions loseelectrons = oxidation(non-metal forms)OIL RIG
The electrolysis cell. Cations are attracted to the cathode and gain electrons (reduction). Anions go to the anode and lose electrons (oxidation).

What happens at the electrodes?

  • At the cathode, a positive ion gains electrons and becomes neutral. This is reduction.
  • At the anode, a negative ion loses electrons and becomes neutral. This is oxidation.
  • This is called discharging the ion: it loses its charge.
  • EXT Electrons are pushed round the circuit by the power supply. They leave the negative terminal and enter the cathode; they are taken from the anode and return to the positive terminal.
  • Remember OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain.
Picture itThink of the power supply as a pump. It pumps electrons into the cathode, which stuffs them into positive ions. At the anode negative ions give their spare electrons back to the pump.
Common mistake"Electrons travel through the electrolyte." They do not. Ions carry the current through the liquid. Electrons only travel in the wires and electrodes.
Common mistakeMixing up the electrodes: the anode is positive in electrolysis. Check using PANIC.

2Electrolysis of molten compounds

Molten lead(II) bromide, PbBr₂

−+Pbcathode (−)anode (+)molten lead(II) bromide, PbBr₂ (heated)cathode (reduction)Pb²⁺ + 2e⁻ → Pbmolten lead sinksanode (oxidation)2Br⁻ → Br₂ + 2e⁻brown bromine gassolid PbBr₂ does notconduct: the ions arefixed. It must be molten
Molten lead(II) bromide. Lead forms at the cathode; brown bromine gas forms at the anode.
  • The compound is heated until it melts, so the ions can move. Ions present: Pb²⁺ and Br⁻.
  • Cathode (−): Pb²⁺ is attracted here and gains electrons. Lead forms as a molten silver-grey metal, which sinks to the bottom.
  • Anode (+): Br⁻ is attracted here and loses electrons. Bromine is made — orange-brown fumes.
  • EXT Cathode (reduction): Pb²⁺ + 2e⁻ → Pb
  • EXT Anode (oxidation): 2Br⁻ → Br₂ + 2e⁻
Exam tipA molten binary ionic compound (two elements) always gives the metal at the cathode and the non-metal at the anode. Molten sodium chloride gives sodium and chlorine; molten aluminium oxide gives aluminium and oxygen.
Common mistakeUsing brine instead of molten NaCl. They give different products — molten NaCl has no water, so it gives sodium, not hydrogen.
  • Do it in a fume cupboard — bromine is toxic.

Half-equationsEXTENDED

Half-equation — an equation showing what happens at one electrode. Balance the atoms and the charge. Electrons (e⁻) are shown.

  • Cathode: metal ion + electrons → metal atom (electrons on the left).
  • Anode: non-metal ion → molecule + electrons (electrons on the right).
Worked example

EXT Write the half-equations for the electrolysis of molten aluminium oxide, Al₂O₃ (ions Al³⁺ and O²⁻).

  1. Cathode: Al³⁺ gains 3 electrons: Al³⁺ + 3e⁻ → Al
  2. Anode: O²⁻ loses 2 electrons. Oxygen forms as O₂ molecules, so use 2 ions: 2O²⁻ → O₂ + 4e⁻
  3. Check the charge on the left of the anode equation: 2 × (−2) = −4, and on the right: 4 × (−1) = −4 ✓

3Electrolysis of aqueous solutions

Water adds two extra ions

  • In a solution there are two sources of ions: the dissolved compound and water. Water splits very slightly into H⁺ and OH⁻.
  • So at each electrode there are two ions competing to be discharged.
ElectrodeRule
cathode (−)the metal is formed if it is less reactive than hydrogen (for example copper, silver). If the metal is more reactive than hydrogen (sodium, potassium, calcium, magnesium …) then hydrogen is formed instead
anode (+)a halogen (Cl₂, Br₂, I₂) forms if a halide ion is present in concentrated solution. Otherwise oxygen forms from OH⁻ (this is the case for sulfate, nitrate and dilute halide solutions)
  • Never at the anode: hydrogen. Never at the cathode: a non-metal other than hydrogen.

Dilute sulfuric acid

−+cathode (−)anode (+)H₂O₂2 volumes1 volumedilute sulfuric acid, H₂SO₄(aq)ions: H⁺ OH⁻ SO₄²⁻cathode: H⁺ discharged2H⁺ + 2e⁻ → H₂anode: OH⁻ discharged4OH⁻ → 2H₂O + O₂ + 4e⁻overall: water is split2H₂O → 2H₂ + O₂
Dilute sulfuric acid is really the electrolysis of water. Hydrogen forms at the cathode and oxygen at the anode, in a 2 : 1 volume ratio.
  • Ions present: H⁺, OH⁻ (from water) and SO₄²⁻ (from the acid).
  • Cathode: H⁺ is discharged → hydrogen gas (test: lit splint → "squeaky pop").
  • Anode: OH⁻ is discharged (sulfate is not) → oxygen gas (test: glowing splint relights).
  • EXT Cathode: 2H⁺ + 2e⁻ → H₂. Anode: 4OH⁻ → 2H₂O + O₂ + 4e⁻.
  • Overall, water is split: 2H₂O → 2H₂ + O₂. Twice as much hydrogen as oxygen by volume, because 2H₂O gives 2H₂ and only 1 O₂. The acid is not used up — it just makes the water conduct, and it gets more concentrated.
Worked example

In the electrolysis of dilute sulfuric acid, 18 cm³ of oxygen is collected. What volume of hydrogen is collected?

  1. The volume ratio is H₂ : O₂ = 2 : 1
  2. volume of H₂ = 2 × 18 = 36 cm³

Concentrated sodium chloride solution (brine)

−+cathode (−)anode (+)H₂Cl₂concentrated sodium chloride solutionNaCl(aq) contains:Na⁺ H⁺ Cl⁻ OH⁻cathode: H⁺ discharged2H⁺ + 2e⁻ → H₂anode: Cl⁻ discharged2Cl⁻ → Cl₂ + 2e⁻left behind: Na⁺ + OH⁻= NaOH(aq)
Electrolysis of concentrated sodium chloride solution. Hydrogen forms at the cathode, chlorine at the anode, and sodium hydroxide solution is left behind.
  • Ions present: Na⁺, H⁺, Cl⁻ and OH⁻.
  • Cathode: sodium is more reactive than hydrogen, so H⁺ is discharged → hydrogen. No sodium metal forms.
  • Anode: the solution is concentrated, so Cl⁻ is discharged → chlorine gas (test: bleaches damp blue litmus paper).
  • EXT Cathode: 2H⁺ + 2e⁻ → H₂. Anode: 2Cl⁻ → Cl₂ + 2e⁻.
  • The ions left in solution are Na⁺ and OH⁻ — that is sodium hydroxide solution.
  • This is a big industry (the chlor-alkali industry). Chlorine: water treatment, bleach, plastics (PVC). Sodium hydroxide: soap, paper, making aluminium oxide. Hydrogen: fuel and margarine.
Common mistakeSaying sodium forms at the cathode in brine. It cannot — sodium is more reactive than hydrogen, so H₂ is formed.

Copper(II) sulfate solutionEXTENDED

carbon (inert) electrodescopper electrodes−+−+cathodeanodecathodeanodecathode: Cu²⁺ + 2e⁻ → Cu(brown copper deposited)anode: 4OH⁻ → 2H₂O + O₂ + 4e⁻(O₂ bubbles)blue colour fadescathode: Cu²⁺ + 2e⁻ → Cu(copper grows thicker)anode: Cu → Cu²⁺ + 2e⁻(copper dissolves)blue colour stays the same
Copper(II) sulfate solution. Left: carbon electrodes. Right: copper electrodes.
  • Carbon (inert) electrodes: cathode → copper is deposited (copper is less reactive than hydrogen). Anode → oxygen (sulfate is not discharged, so OH⁻ is). The blue colour fades, as Cu²⁺ ions are removed, and the solution becomes acidic.
  • Copper electrodes: the cathode gains a layer of copper. The anode dissolves: Cu → Cu²⁺ + 2e⁻. For every Cu²⁺ made at the anode, one is used up at the cathode, so the blue colour stays the same. The mass of the anode goes down by the same amount that the cathode goes up.
  • Cathode: Cu²⁺ + 2e⁻ → Cu. Anode (carbon): 4OH⁻ → 2H₂O + O₂ + 4e⁻. Anode (copper): Cu → Cu²⁺ + 2e⁻.
Exam tip"Active" electrodes (copper) take part in the reaction. "Inert" electrodes (carbon, platinum) do not. Always read the electrode material in the question!

Halide solutions: dilute or concentrated?EXTENDED

  • Concentrated halide solution (chloride, bromide, iodide): the halogen is formed at the anode.
  • Dilute halide solution: there are many more water molecules than halide ions, so OH⁻ is discharged instead and oxygen is formed.
  • The cathode product is decided in the same way as before (metal or hydrogen).
Worked example

EXT Predict the products when concentrated aqueous potassium bromide is electrolysed using inert electrodes.

  1. Ions: K⁺, H⁺, Br⁻, OH⁻
  2. Cathode: potassium is more reactive than hydrogen → hydrogen forms
  3. Anode: bromide is concentrated → bromine forms (orange-brown)
  4. The solution left is potassium hydroxide

Summary table

ElectrolyteCathode (−)Anode (+)Left in the solution
molten PbBr₂leadbromine–
concentrated NaCl(aq)hydrogenchlorineNaOH(aq)
dilute H₂SO₄(aq)hydrogenoxygenmore concentrated acid
EXT CuSO₄(aq), carbon electrodescopperoxygenpaler blue, more acidic
EXT CuSO₄(aq), copper electrodescopper (cathode gets heavier)copper dissolves (anode gets lighter)stays blue
Exam tipMethod for any electrolysis: (1) list all the ions, including H⁺ and OH⁻ for solutions; (2) cathode: metal or H₂?; (3) anode: halide (concentrated) or OH⁻ → O₂?; (4) what stays in solution?

4Electroplating

Coating one metal with another

Electroplating — using electrolysis to coat the surface of an object with a thin layer of a metal.

−+Ag++cathode (−)anode (+)objectsilversilver nitrate solution, AgNO₃(aq)cathode (reduction)Ag⁺ + e⁻ → Aga layer of silver growsanode (oxidation)Ag → Ag⁺ + e⁻the silver bar slowly dissolvesplating metal = anodeobject = cathodesolution holds ions ofthe plating metal
Silver plating. The object to be plated is the cathode, the plating metal is the anode, and the electrolyte contains ions of the plating metal.
  • Object to be plated = cathode (−).
  • Plating metal = anode (+).
  • Electrolyte = a solution of a salt of the plating metal (silver nitrate for silver; copper(II) sulfate for copper).
  • Metal ions from the solution gain electrons at the cathode and form a thin, even layer on the object.
  • EXT Cathode: Ag⁺ + e⁻ → Ag. Anode: Ag → Ag⁺ + e⁻. The anode replaces the ions used, so the solution stays the same strength.
  • Why plate? To protect from corrosion (chromium, zinc or tin plating on steel) or to improve appearance (silver or gold plating on cutlery and jewellery).
Picture itImagine the silver bar slowly "posting" its atoms to the spoon: each silver atom dissolves from the bar as an ion and lands on the spoon as a new atom.
Common mistakeMaking the object the anode. If it were the anode, the object itself would dissolve!

5Hydrogen–oxygen fuel cells

Electricity straight from a fuel

Fuel cell — a cell that is supplied continuously with a fuel and oxygen, and uses the reaction between them to generate electricity directly.

Melectrons flowionsmovethroughH₂ inO₂ (air) inH₂O outhydrogenoxygenelectrolyteoverall reaction:2H₂ + O₂ → 2H₂Ohydrogen is oxidised atone electrode, oxygen isreduced at the other;electrons flow throughthe wire and power the motoronly product: water(no CO₂ at the point of use)
A hydrogen–oxygen fuel cell. Hydrogen is oxidised at one electrode and oxygen is reduced at the other. The electrons go through the wire and power the motor.
  • Overall reaction: 2H₂ + O₂ → 2H₂O.
  • The only product is water. There is no carbon dioxide from the cell itself.
  • It is not the same as burning hydrogen. In a fuel cell the energy comes out as electricity, not heat.

Fuel cell or petrol engine?EXTENDED

Hydrogen fuel cellPetrol engine
productwater onlyCO₂, CO, NOₓ and soot
efficiencyhigherlower — much energy is wasted as heat
noisequietnoisy
fuelH₂ is hard to store (needs high pressure or a big tank) and is flammableliquid fuel is easy to store and refuel
fuel sourcehydrogen must be made first (often from natural gas, which gives CO₂)made from crude oil — non-renewable
cost and infrastructureexpensive cells, few filling stationscheap, an established network
Exam tipWhen a question says "evaluate" give both sides: advantages (clean, efficient, quiet) and disadvantages (storage, safety, hydrogen has to be made, expensive).
Common mistake"Fuel cells are carbon-free." They produce no CO₂ at the point of use, but making the hydrogen may produce CO₂.

6Test yourself

Test yourself — cover the answers first

Quick check

  1. What is electrolysis?
  2. Why does solid sodium chloride not conduct, but molten sodium chloride does?
  3. Which electrode is positive? Which ions go to it?
  4. Name the products of electrolysis of molten lead(II) bromide.
  5. What is the test for hydrogen, and for chlorine?
  6. What is the only product of a hydrogen–oxygen fuel cell?
  7. EXT Write the half-equation for the formation of chlorine at an anode.
  8. EXT During electrolysis of copper(II) sulfate with copper electrodes, what happens to the blue colour?

Exam-style

  1. Describe the electrolysis of concentrated sodium chloride solution. Name the product at each electrode and the substance left in solution. [4]
  2. Explain how a steel spoon can be silver plated. [4]
  3. EXT Write half-equations for electrolysis of dilute sulfuric acid and explain why hydrogen has twice the volume of oxygen. [4]
  4. EXT Give one advantage and one disadvantage of a hydrogen fuel cell compared with a petrol engine. [2]
Show answers
  1. breakdown of an ionic compound, molten or in solution, using electricity
  2. in the solid the ions cannot move; when molten they are free to move and carry charge
  3. anode; negative ions (anions)
  4. lead at the cathode, bromine at the anode
  5. hydrogen: lit splint gives a squeaky pop; chlorine: bleaches damp litmus paper
  6. water
  7. 2Cl⁻ → Cl₂ + 2e⁻
  8. it stays the same (copper dissolves at the anode and is deposited at the cathode)
  9. cathode: hydrogen (H⁺ discharged, sodium is more reactive than hydrogen)
    anode: chlorine (Cl⁻ discharged)
    sodium hydroxide solution is left
    because Na⁺ and OH⁻ ions remain in solution
  10. the spoon is the cathode (−)
    a bar of silver is the anode (+)
    electrolyte is silver nitrate solution
    silver ions gain electrons at the spoon and a layer of silver is deposited
  11. cathode: 2H⁺ + 2e⁻ → H₂
    anode: 4OH⁻ → 2H₂O + O₂ + 4e⁻
    4 electrons make 2 H₂ at the cathode but only 1 O₂ at the anode
    so the volume ratio is 2 : 1
  12. advantage: only water is produced / no CO₂ at the point of use / more efficient
    disadvantage: hydrogen is difficult to store / flammable / expensive / must be made first
Practise this chapter
ElectrolysisHydrogen–oxygen fuel cells
Next chapterChemical energetics →
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