Electrolysis

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Electrolysis Principles

Electrolysis is the breakdown of a molten or aqueous ionic compound by an electric current.
Covalent compounds and solid ionic compounds cannot undergo electrolysis because they have no free mobile ions.
Electrolyte: molten or dissolved ionic compound that conducts electricity.
Anode (positive electrode) attracts anions (negative ions).
Cathode (negative electrode) attracts cations (positive ions).
Use PANIC: Positive is Anode, Negative Is Cathode.
Electrons flow from the negative terminal of the power supply to the cathode; they do not pass through the electrolyte.
Reduction (gain of electrons) occurs at the cathode; oxidation (loss of electrons) occurs at the anode.

Ionic conductivity: solid vs molten/dissolved

Electrolysis of Molten Compounds

Molten binary ionic compounds decompose into their elements: metal at cathode, non-metal at anode.
Example: molten PbBr₂ → Pb (grey metal) at cathode, Br₂ (brown gas) at anode.
Ions must be free to move – compound must be molten (or in solution).
Electrodes must be inert (e.g., graphite or platinum) to avoid side reactions.
Half-equation at cathode: Pb²⁺ + 2e⁻ → Pb.
Half-equation at anode: 2Br⁻ → Br₂ + 2e⁻.

Electrolysis of molten lead(II) bromide

Electrolysis of Aqueous Sodium Chloride (Brine)

Brine contains Na⁺, Cl⁻, H⁺, OH⁻ ions.
At cathode: H⁺ is discharged (less reactive than Na⁺) → hydrogen gas.
At anode: Cl⁻ is discharged → chlorine gas.
Na⁺ and OH⁻ remain in solution → sodium hydroxide forms.
Industrial uses: Cl₂ → bleach; H₂ → margarine; NaOH → soap and detergents.

Electrolysis of aqueous sodium chloride (brine)

Electrolysis of Dilute Sulfuric Acid

Contains H⁺, SO₄²⁻, OH⁻ ions.
At cathode: H⁺ gains electrons → hydrogen gas.
At anode: OH⁻ loses electrons → oxygen gas (and water).
Volume of H₂ produced is twice the volume of O₂.
Gas tests: H₂ – lit splint gives 'squeaky pop'; O₂ – glowing splint relights.

Hoffman voltameter: dilute sulfuric acid

Electrolysis of Aqueous Solutions (Extended)

Water dissociates: H₂O ⇌ H⁺ + OH⁻ – these ions compete with solute ions.
At cathode: if metal is less reactive than hydrogen, metal is deposited; if more reactive, hydrogen gas is produced.
At anode: if halide ions (Cl⁻, Br⁻, I⁻) are present, halogen forms; otherwise oxygen from OH⁻.
Concentration matters: concentrated halide → halogen; dilute halide → oxygen.
Example: CuSO₄(aq) with graphite electrodes → Cu at cathode, O₂ at anode; blue colour fades.
With copper electrodes: anode dissolves (Cu → Cu²⁺ + 2e⁻), cathode gains mass (Cu²⁺ + 2e⁻ → Cu); solution concentration remains constant.

Apparatus for the electrolysis of copper(II) sulfate (graphite electrodes)

Ionic Half-Equations (Extended)

Oxidation Is Loss (of electrons), Reduction Is Gain (OIL RIG).
REDuction at CAThode (RED CAT); ANode for OXidation (AN OX).
Metal formation: Mⁿ⁺ + n e⁻ → M (reduction).
Hydrogen formation: 2H⁺ + 2e⁻ → H₂.
Halogen formation: 2X⁻ → X₂ + 2e⁻ (oxidation).
Oxygen formation: 4OH⁻ → O₂ + 2H₂O + 4e⁻.
Half-equations must balance atoms and charges.

Summary Table of Common Electrolyses

Molten PbBr₂: anode – Br₂; cathode – Pb.
Concentrated NaCl(aq): anode – Cl₂; cathode – H₂.
Dilute H₂SO₄: anode – O₂; cathode – H₂.
CuSO₄(aq) (inert electrodes): anode – O₂; cathode – Cu.

Summary of Common Electrolyses

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1.What is the name of the process in which a molten ionic compound is broken down by an electric current?
Easy
- AElectrolysis
- BElectroplating
- CElectrorefining
- DElectrolysis of aqueous solutions
2.Which electrode is the positive electrode in an electrolysis cell?
Easy
- AAnode
- BCathode
- CElectrolyte
- DTerminal
3.What type of ion is attracted to the cathode?
Easy
- ACation
- BAnion
- CElectron
- DNeutral atom
4.During electrolysis, what happens at the anode?
Easy
- AOxidation (loss of electrons)
- BReduction (gain of electrons)
- CNo reaction
- DFormation of metal
5.In the electrolysis of molten lead(II) bromide, what is produced at the cathode?
Easy
- ALead
- BBromine
- COxygen
- DHydrogen
6.During the electrolysis of concentrated aqueous sodium chloride (brine), which gas is produced at the anode?
Easy
- AChlorine
- BOxygen
- CHydrogen
- DNitrogen
7.In the electrolysis of dilute sulfuric acid, what is the volume ratio of hydrogen to oxygen produced?
Medium
- A2:1
- B1:2
- C1:1
- D3:1
8.Which of the following is a suitable material for inert electrodes?
Easy
- AGraphite
- BCopper
- CIron
- DZinc

Plus de questions

9.In the electrolysis of aqueous copper(II) sulfate with graphite electrodes, what is observed at the anode?
Medium
- ABubbles of oxygen gas
- BCopper metal deposited
- CBubbles of chlorine gas
- DSolution turns green
10.What is the correct balanced half-equation for the formation of hydrogen gas at the cathode during electrolysis?
Medium
- A2H⁺ + 2e⁻ → H₂
- BH⁺ + e⁻ → H
- C2H₂O + 2e⁻ → H₂ + 2OH⁻
- DH₂O + e⁻ → H⁺ + OH⁻
11.In the electrolysis of aqueous sodium chloride, why is hydrogen produced at the cathode instead of sodium?
Medium
- ASodium is more reactive than hydrogen
- BHydrogen is more reactive than sodium
- CSodium ions are not present
- DThe solution is acidic
12.Which of the following is a use of chlorine produced during the electrolysis of brine?
Easy
- AMaking bleach
- BMaking margarine
- CMaking soap
- DMaking detergents
13.During the electrolysis of concentrated aqueous sodium chloride (brine), which product is formed at the cathode?
Easy
- ASodium metal
- BChlorine gas
- CHydrogen gas
- DOxygen gas
14.A student electrolyses dilute sulfuric acid using platinum electrodes. A gas is collected at the anode that relights a glowing splint. What is this gas?
Medium
- AHydrogen
- BChlorine
- COxygen
- DSulfur dioxide
15.Why must lead(II) chloride be molten for electrolysis to occur?
Easy
- ATo allow ions to move freely
- BTo increase the voltage
- CTo prevent side reactions
- DTo make it conduct heat
16.The half-equation for the formation of oxygen at the anode during electrolysis of dilute sulfuric acid is: 4OH⁻ → O₂ + 2H₂O + 4e⁻. How many electrons are released per molecule of oxygen?
Medium
- A2
- B4
- C1
- D8
17.In the electrolysis of molten aluminium oxide, what is the ionic half-equation at the negative electrode?
Medium
- AAl³⁺ + 3e⁻ → Al
- BAl → Al³⁺ + 3e⁻
- C2O²⁻ → O₂ + 4e⁻
- DAl³⁺ + e⁻ → Al²⁺
18.A student electrolyses dilute sodium chloride solution. Which gas is produced at the anode?
Medium
- AChlorine
- BHydrogen
- COxygen
- DSodium vapour
19.In an experiment to electrolyse molten lead(II) bromide, a student uses graphite electrodes. Which statement explains why graphite is a suitable material for the electrodes?
Medium
- AGraphite is a non-metal that conducts electricity and is inert.
- BGraphite is a metal that conducts electricity and is reactive.
- CGraphite is an insulator that does not react with the electrolyte.
- DGraphite is a covalent compound that conducts electricity when molten.
20.A student electrolyses dilute sulfuric acid using platinum electrodes. After some time, the volume of hydrogen collected is 20 cm³. What is the volume of oxygen collected under the same conditions?
Medium
- A10 cm³
- B20 cm³
- C40 cm³
- D5 cm³
21.During the electrolysis of concentrated aqueous sodium chloride (brine), a student tests the gas produced at the anode with damp litmus paper. What is the expected result?
Medium
- AThe damp litmus paper turns red and then bleaches to white.
- BThe damp litmus paper turns blue.
- CThe damp litmus paper turns red and stays red.
- DThe damp litmus paper turns green.
22.In the electrolysis of aqueous copper(II) sulfate using graphite electrodes, the blue colour of the solution fades. Which statement explains this observation?
Medium
- ACopper(II) ions are reduced to copper metal at the cathode, removing Cu²⁺ from the solution.
- BCopper(II) ions are oxidised to copper metal at the cathode, removing Cu²⁺ from the solution.
- CSulfate ions are reduced to sulfur dioxide at the anode, removing SO₄²⁻ from the solution.
- DWater molecules are reduced to hydrogen gas at the cathode, consuming H₂O.
23.Which of the following is a correct test for hydrogen gas?
Easy
- AA lit splint produces a squeaky pop.
- BA glowing splint relights.
- CDamp litmus paper turns red then white.
- DLimewater turns milky.
24.A student sets up an electrolysis cell with molten magnesium bromide. Which pair of products is formed at the electrodes?
Easy
- ACathode: magnesium; Anode: bromine
- BCathode: bromine; Anode: magnesium
- CCathode: magnesium; Anode: oxygen
- DCathode: hydrogen; Anode: bromine
25.In the electrolysis of dilute sulfuric acid, the ionic half-equation at the anode is 4OH⁻ → O₂ + 2H₂O + 4e⁻. Which statement about this half-equation is correct?
Medium
- AHydroxide ions are oxidised because they lose electrons.
- BHydroxide ions are reduced because they gain electrons.
- COxygen is produced at the cathode.
- DThe half-equation shows reduction because electrons are on the right.
26.A student electrolyses dilute barium chloride solution using inert electrodes. Which gas is produced at the anode?
Medium
- AOxygen
- BChlorine
- CHydrogen
- DBarium
27.Which of the following substances can undergo electrolysis?
Easy
- AMolten sodium chloride
- BSolid sodium chloride
- CSugar solution
- DPure water
28.In the electrolysis of molten aluminium oxide (Al₂O₃), what is the ionic half-equation at the negative electrode?
Easy
- AAl³⁺ + 3e⁻ → Al
- BAl³⁺ → Al + 3e⁻
- C2O²⁻ → O₂ + 4e⁻
- DAl + 3e⁻ → Al³⁺
29.A student uses copper electrodes to electrolyse aqueous copper(II) sulfate. Which observation is correct?
Medium
- AThe blue colour of the solution remains unchanged.
- BThe blue colour of the solution fades.
- CThe anode increases in mass.
- DOxygen gas is produced at the anode.
30.What is the definition of electrolysis?
Easy
- AThe breakdown of a molten ionic compound by an electric current
- BThe breakdown of any compound by heat
- CThe movement of electrons through a solid metal
- DThe reaction between an acid and a base
31.What is the name of the negative electrode in an electrolytic cell?
Easy
- AAnode
- BCathode
- CElectrolyte
- DTerminal
32.In the electrolysis of molten lead(II) bromide, what is observed at the anode?
Easy
- AA grey solid deposits
- BBubbles of a brown gas
- CBubbles of a colourless gas
- DThe electrode dissolves
33.Which of the following is a correct test for oxygen gas?
Easy
- AA lit splint gives a squeaky pop
- BA glowing splint relights
- CDamp litmus paper turns red then white
- DLimewater turns milky
34.In the electrolysis of dilute sulfuric acid using inert electrodes, the half-equation at the anode is: 4OH⁻ → O₂ + 2H₂O + 4e⁻. Which statement about this half-equation is correct?
Medium
- AOxidation occurs because electrons are gained
- BReduction occurs because electrons are lost
- COxidation occurs because electrons are lost
- DReduction occurs because electrons are gained
35.What is the name of the negative electrode in an electrolysis cell?
Easy
- AAnode
- BCathode
- CElectrolyte
- DAnion
36.Ionic compounds in the solid state can conduct electricity.
Easy
True or false?
37.Match each term to its correct definition.
Easy
- Electrode
- Electrolyte
- Cation
- A rod that conducts electricity into or out of an electrolyte
- An ionic compound that conducts electricity when molten or in solution
- A positively charged ion attracted to the cathode
38.During electrolysis, reduction occurs at the anode.
Easy
True or false?
39.During the electrolysis of dilute sulfuric acid using inert electrodes, which half-equation correctly represents the reaction at the anode?
Medium
- A2H⁺ + 2e⁻ → H₂
- B4OH⁻ → O₂ + 2H₂O + 4e⁻
- C2Cl⁻ → Cl₂ + 2e⁻
- DCu²⁺ + 2e⁻ → Cu
40.Place the following steps for the electrolysis of molten lead(II) bromide in the correct order.
Medium
- Heat lead(II) bromide until molten
- Add two graphite electrodes connected to a power supply
- Turn on the power supply
- Observe brown gas at the anode and grey deposit at the cathode
41.In the electrolysis of concentrated aqueous sodium chloride (brine), the product at the cathode is chlorine gas.
Easy
True or false?
42.Which of the following statements about the electrolysis of concentrated aqueous sodium chloride (brine) are correct? (Select all that apply)
Hard
- AChlorine gas is produced at the anode.
- BHydrogen gas is produced at the cathode.
- CSodium metal is produced at the cathode.
- DSodium hydroxide solution remains in the electrolysis chamber.
- EThe electrolyte contains Na⁺, Cl⁻, H⁺, and OH⁻ ions.
43.During electrolysis of dilute sulfuric acid using inert electrodes, the volume of hydrogen produced is twice the volume of oxygen.
Medium
True or false?
44.A student electrolyses aqueous copper(II) sulfate using graphite electrodes. Which observation is correct?
Medium
- AThe blue colour of the solution becomes darker.
- BThe cathode gains a pink-brown coating.
- CBubbles of hydrogen gas are seen at the anode.
- DThe anode increases in mass.
45.Complete the sentence.
Easy
In electrolysis, negatively charged ions are called ____ and are attracted to the ____.
46.State the test for hydrogen gas and the expected result.
Easy
47.Complete the ionic half-equation for the formation of oxygen gas at the anode during electrolysis of dilute sulfuric acid.
Medium
4OH⁻ → ____ + 2H₂O + 4e⁻
48.Explain why the concentration of copper(II) sulfate solution remains constant when copper electrodes are used instead of graphite electrodes for electrolysis.
Hard

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