S5 Chemistry Practice Book · Unit 13: Electrolysis

Unit 13 Electrolysis

Key unit competence: Predict the products of electrolysis of given electrolytes and work out quantities of products using Faraday's laws.

Key ideas

Electrolysis is the decomposition of an electrolyte (molten or in solution) by a direct current. The cathode (−) gains electrons: reduction; the anode (+) loses electrons: oxidation. Cations move to the cathode, anions to the anode.
Selective discharge depends on: position in the electrochemical series (less reactive metals and H⁺ are discharged first), concentration (concentrated Cl⁻ is discharged before OH⁻) and the nature of the electrodes (an active copper anode dissolves).
Concentrated NaCl (brine), inert electrodes: cathode 2H₂O + 2e⁻ → H₂ + 2OH⁻; anode 2Cl⁻ → Cl₂ + 2e⁻; NaOH remains. Dilute NaCl: oxygen at the anode. Acidified water: H₂ and O₂ in a 2 : 1 volume ratio.
CuSO₄(aq) with inert electrodes: copper at the cathode, oxygen at the anode; the blue colour fades and the solution becomes acidic (H₂SO₄). With copper electrodes: the anode dissolves (Cu → Cu²⁺ + 2e⁻), copper deposits at the cathode, the colour stays the same: used to purify copper.
Faraday's first law: mass deposited ∝ charge passed (Q = It). Second law: for the same charge, the moles deposited are inversely proportional to the charge on the ion. F = 96 500 C mol⁻¹ (charge of one mole of electrons). Mass = (Q ÷ F) × (M ÷ n).
Applications: electroplating (object at the cathode), refining of copper, extraction of reactive metals (Na, Mg, Al), the chlor-alkali industry (NaOH, Cl₂, H₂), anodising aluminium, and producing hydrogen from water.

Common misconceptions

"In electrolysis, electrons flow through the solution." No: Electrons flow in the wires; ions carry the charge in the electrolyte.
"The most reactive metal ion is discharged first at the cathode." No: The LEAST reactive ion (lowest in reactivity) is discharged first, e.g. H⁺ before Na⁺.
"The anode is always the negative electrode." No: In electrolysis the anode is positive (oxidation happens there).

Worked examples

Example 1. Explain the products of the electrolysis of concentrated copper(II) chloride solution with carbon electrodes.

Solution

Ions present: Cu²⁺, H⁺ (cathode); Cl⁻, OH⁻ (anode).

Cathode: copper is less reactive than hydrogen, so Cu²⁺ + 2e⁻ → Cu: pink-brown deposit.

Anode: Cl⁻ is concentrated, so 2Cl⁻ → Cl₂ + 2e⁻: pale green gas that bleaches damp litmus.

Overall: CuCl₂ → Cu + Cl₂; the green-blue solution fades.

Example 2. Describe how a spoon is electroplated with silver.

Solution

The spoon (cleaned) is the cathode; a pure silver bar is the anode.

The electrolyte contains silver ions, e.g. silver nitrate or a silver cyanide complex.

Cathode: Ag⁺ + e⁻ → Ag coats the spoon. Anode: Ag → Ag⁺ + e⁻ replaces ions in solution.

A small current for a long time gives an even, adherent coat.

Practice questions

State Faraday's first law of electrolysis.
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The mass of a substance liberated at an electrode is directly proportional to the quantity of electricity passed.
Common mistake: Mixing it up with the second law.
State Faraday's second law.
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When the same charge passes through different electrolytes, the moles liberated are inversely proportional to the charges on the ions.
Common mistake: Saying equal masses are deposited.
Write the half-equation at the cathode in the electrolysis of brine.
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2H₂O + 2e⁻ → H₂ + 2OH⁻ (or 2H⁺ + 2e⁻ → H₂).
Common mistake: Writing Na⁺ + e⁻ → Na.
Write the half-equation at a copper anode in CuSO₄(aq).
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Cu → Cu²⁺ + 2e⁻
Common mistake: Writing oxygen as the product.
Name three factors that affect selective discharge.
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Position in the electrochemical series, concentration of ions, nature of the electrodes.
Common mistake: Listing the colour or size of the beaker.
Calculate the charge passed by 0.50 A in 10 minutes.
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Q = 0.50 × 600 = 300 C.
Common mistake: Using 10 instead of 600 seconds.
Give three uses of electrolysis.
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Electroplating, refining copper, extracting Na/Al/Mg, producing NaOH and Cl₂ (any three).
Common mistake: Listing distillation.
Why is aluminium not extracted with carbon?
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Aluminium is more reactive than carbon, so carbon cannot reduce Al₂O₃ at practical temperatures.
Common mistake: Saying aluminium ore contains no oxygen.
Why must the anodes be replaced in aluminium extraction?
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Oxygen formed at the hot carbon anodes burns them to CO₂.
Common mistake: Saying aluminium dissolves the anodes.
What volume of hydrogen (room conditions) is made when 0.0200 mol of electrons pass?
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0.0100 mol × 24 dm³ = 0.240 dm³.
Common mistake: Forgetting that 2 electrons give 1 H₂.

Matching and drag-and-drop

Matching

Match each item in column A with the correct answer in column B. Write the letter next to each number.

Column A	Column B
1. When CuSO₄(aq) is electrolysed with carbon electrodes, the blue colour:	a. H⁺
2. In electroplating, the object to be plated is made the:	b. fades
3. Purification of copper by electrolysis uses an impure copper:	c. anode
4. Which ion is discharged first at the cathode from a solution of K⁺ and H⁺?	d. cathode

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1 → b (fades) · 2 → d (cathode) · 3 → c (anode) · 4 → a (H⁺)

Cu²⁺ ions are removed at the cathode.
Metal ions are reduced onto it.
Impure Cu dissolves; pure Cu deposits on the cathode.
Potassium is more reactive, so H⁺ is reduced.

Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
power supply only · 2 : 1 · anode · 2
1. The number of faradays needed to deposit one mole of copper from Cu²⁺ is .
2. Electrolysis of acidified water gives hydrogen and oxygen in the volume ratio .
3. During electrolysis, anions move towards the .
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1: 2 · 2: 2 : 1 · 3: anode
- Cu²⁺ + 2e⁻ → Cu.
- 2H₂O → 2H₂ + O₂.
- Negative ions are attracted to the positive anode.

Matching

Match each item in column A with the correct answer in column B. Write the letter next to each number.

Column A	Column B
1. Q = It. A current of 2.0 A for 100 s passes a charge of:	a. the decomposition of an electrolyte by electricity
2. In electrolysis, reduction takes place at the:	b. 200 C
3. Electrolysis is:	c. cathode
4. Aluminium is extracted by electrolysis because:	d. it is too reactive to be reduced by carbon

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1 → b (200 C) · 2 → c (cathode) · 3 → a (the decomposition of an electrolyte by electricity) · 4 → d (it is too reactive to be reduced by carbon)

Q = 2.0 × 100 = 200 C.
Cations gain electrons at the cathode.
Direct current decomposes the electrolyte.
Reactive metals need electrolysis.

Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
one electron · oxygen · 3 × 96 500 C · one mole of electrons
1. The Faraday constant is the charge on .
2. The charge on one mole of Al³⁺ ions is .
3. At the anode in the electrolysis of dilute sulfuric acid, the gas formed is .
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1: one mole of electrons · 2: 3 × 96 500 C · 3: oxygen
- F = 96 500 C mol⁻¹.
- Three faradays.
- OH⁻ (or water) is oxidised; sulfate is not.

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