DSE Chemistry | Electrolysis Lesson-Prep Guide (Tutor Edition) | Discharge Order, Faraday Calculations, Plating

2026-11-01

Tutor lesson-prep guides · Secondary Chemistry

Students know the anode is oxidised but freeze when two ions compete. This lesson gives one fixed discharge table plus the concentration exception that overturns it.

Objectives — Students can: (1) use the discharge series to predict cathode and anode products; (2) convert current and time into charge, moles of electrons and mass deposited; (3) state the three conditions for electroplating and the key feature of the chlor-alkali cell.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Moles	Moles from mass	n = m / M
Redox	Reaction at the cathode	Reduction, gain of electrons
Charge units	Charge from 1 A for 1 s	1 C

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Cell layout	Supply, electrodes, electrolyte, ion movement	Label anode and cathode
10-24 min	Discharge order	Two tables, cations and anions	Predict four products
24-38 min	Charge calculations	Q = It, moles of electrons, then substance	Get 1.18 g of copper
38-50 min	Industrial uses	Plating, aluminium, chlor-alkali	State three conditions
50-56 min	Answer format	Half-equations, electron balance, units	Rewrite one answer
56-60 min	Exit check	Three questions marked on the spot	Independent work

03 Teaching script (read it out)

Scene 1: Discharge order: learn the table, then the exception

Cathode, reduction: K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Ag, Au; the further right, the easier to discharge. Anode, oxidation: sulphide, iodide, bromide, chloride, hydroxide, oxyanions; the further left, the easier. With inert electrodes water or hydroxide can be oxidised. Exception: concentrated sodium chloride gives chlorine at the anode, dilute gives oxygen.

Ask the student: with copper electrodes in dilute copper sulphate, what happens at the anode (copper dissolves, no oxygen).

Scene 2: Charge: one formula, three layers

2.0 A for 30 minutes: Q = It = 2.0 x 1800 = 3600 C, so moles of electrons = 3600 / 96500 = 0.0373 mol. Copper: Cu2+ + 2e- gives Cu, so 0.0373 / 2 = 0.0187 mol = 1.18 g. Anode: 2 H2O gives O2 + 4 H+ + 4 e-, so 0.0373 / 4 = 0.0093 mol = 0.22 dm3 at 24 dm3 per mole.

Ask the student: the same charge deposits how much silver, Ag+ + e- (about 4.02 g).

Scene 3: Industry: conditions matter more than the equation

Electroplating: the object is the cathode, the plating metal is the anode, the electrolyte contains that metal ion, and a small current gives an even thin layer. Aluminium extraction: alumina dissolved in molten cryolite, aluminium at the cathode, carbon anodes consumed and replaced. Chlor-alkali: concentrated brine gives hydrogen and sodium hydroxide at the cathode and chlorine at the anode, and the two compartments must stay separated.

Warn: “why separate them” is answered by “to stop chlorine reacting with sodium hydroxide”.

04 Question types and worked steps

Type	Example	Steps	Answer
Prediction	Products for a given electrolyte	Write ions, then apply the table	Metal or hydrogen at the cathode
Calculation	Q = It	2.0 x 1800	3600 C
Calculation	Mass deposited	Moles of electrons divided by n, times M	1.18 g
Application	Three plating conditions	Cathode, anode, electrolyte	Object is the cathode

Sanity check (teach this): The cathode gains mass unless an active anode or displacement interferes, and gas volumes use the room temperature molar volume, not 22.4 dm3. Moles of electrons always means charge divided by 96500.

05 Independent practice (3 questions)

Question	Answer	What to watch
Charge from 2.0 A for 30 minutes	3600 C	Convert time to seconds
Mass of copper from that charge	1.18 g	Cu2+ needs two electrons
Anode product from concentrated brine	Chlorine	Concentration beats the table

06 Common mistakes and what to say

Mistake	Student writes	Say this
Minutes used in Q = It	Charge reported as 60 C	“Current in A, time must be in seconds.”
Forgetting to divide by n	Copper reported as 2.37 g	“Cu2+ takes two electrons, so divide by two.”
Brine concentration ignored	Dilute brine gives chlorine	“Dilute gives oxygen; concentrated gives chlorine.”
Object wired as the anode	Object on the positive terminal	“To be plated it must be the cathode.”

07 Homework (paper, section and question numbers)

DSE chemistry practice paper (electrochemistry) — Section B electrolysis calculations
Secondary chemistry chapter index — All chapters

08 Exit check (last 5 minutes)

Question	Answer	Decision
Moles of electrons from 2.0 A for 30 minutes	0.0373 mol	All three right: move to redox titration
Mass of copper from the same charge	1.18 g	One wrong: reteach electron balance
Electrode used for the object in plating	The cathode	Two wrong: restart act one

09 Tutor FAQ

Must the whole discharge series be memorised?

Learn the common dozen; oxyanions are always hard to discharge, which covers the rest.

Why do carbon anodes wear away?

Oxygen formation oxidises the carbon to carbon dioxide, so anodes are replaced.

96500 or 96485?

Use 96500 for DSE and give three significant figures.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.