DSE Chemistry: Electrolysis and Electroplating, a Tutor Prep Guide

2026-11-04

Tutor lesson-prep guides · Secondary chemistry

Students can write Cu2+ plus two electrons gives Cu, then panic when two ions compete at one electrode, and forget the electron count in the mass calculation. This lesson reduces the chapter to two lines: order and charge.

Objectives — Students can: (1) decide which reaction occurs at each electrode from the discharge order and justify it; (2) write balanced half equations and check charge and atoms; (3) convert charge to mass using Q = It, n(e) = Q/F and the electron count; (4) find the mass of copper deposited by 2.0 A for 30 minutes in four minutes (about 1.18 g).

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Ions and electrons	How many electrons does Cu2+ need to become Cu?	Two
Redox at electrodes	Is the cathode oxidation or reduction?	Reduction, it gains electrons
Charge	One amp for one second is how many coulombs?	One coulomb

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Three-question check	Electron counts, cathode reaction, charge	Answer orally
8-22 min	Discharge order	Compare four ion pairs and concentration effects	Rank and justify
22-34 min	Half equations	Model the balancing steps, check charge	Write two alone
34-48 min	Charge to mass	Q = It, then n(e), then divide by electrons	One full run
48-56 min	Timed practice	One plating and one brine question	Work unaided
56-60 min	Exit check	Three quick questions	Answer orally

03 Teaching script (read it out)

Scene 1: Rank the ions before writing anything

The core skill is ranking, not memorising equations: at the cathode the easier reduction wins, at the anode the easier oxidation wins. Four pairs cover most of the syllabus: Cu2+ before H+ at the cathode, Cl- before OH- at the anode in concentrated brine, and OH- discharging to oxygen in dilute solution. Once the ranking is settled the equation is a five-second job. Require two written lines first: cathode discharges X, anode discharges Y.

Ask for the ranking orally every time; correct a wrong ranking before any equation is written.

Scene 2: Three checks for every half equation

The routine is fixed: (1) write reactant and product; (2) add electrons until charge balances, as in Cu2+ + 2e- gives Cu where the left is plus two minus two, zero on both sides; (3) count atoms. Plating questions need both electrodes: the object is the cathode where metal is deposited, and the anode is the plating metal dissolving. The two half equations must add up to the overall cell reaction with electrons cancelling. A student who writes Cu gives Cu2+ + 2e- at the cathode can be asked which electrode gains electrons, and will usually correct it alone.

After writing, force one verbal charge check: total charge on the left equals the right.

Scene 3: The charge line: coulombs to moles to grams

Three steps: Q = It, then n(e) = Q/96500, then divide by the electron count and multiply by molar mass. Here 2.0 A x 1800 s = 3600 C, so n(e) = 3600/96500 = 0.0373 mol. Copper is divalent, so n(Cu) = 0.0187 mol and the mass is 0.0187 x 63.5 = 1.18 g. The same charge plates 4.03 g of silver because silver is monovalent, so the electrons are not halved. The two commonest slips are forgetting the electron count and leaving the time in minutes.

Make students write Q = It with numbers filled in before allowing the next step.

04 Question types and worked steps

Type	Example	Steps	Answer
Discharge order	What forms at each electrode	Rank by ease of oxidation or reduction	Ranking justified
Half equations	Cathode or anode reaction	Species, electrons, atom check	Charge balances
Plating mass	Mass deposited	Q = It, n(e), divide by valency, multiply by molar mass	1.18 g
Products	Brine electrolysis products	H2 at cathode, Cl2 concentrated or O2 dilute	With a reason

Sanity check (teach this): Self-check: 2.0 A x 1800 s = 3600 C, which is 0.0373 mol of electrons. Copper takes two, so the metal is 0.0187 mol and weighs 1.18 g, while silver weighs 4.03 g, about 3.4 times more for the same charge. That ratio is a fast arithmetic check. An answer ten times too small means minutes were used as seconds; an answer twice too large means the valency was ignored.

05 Independent practice (3 questions)

Question	Answer	What to watch
Electrolyse copper sulphate with 2.0 A for 30 minutes: mass of copper?	1.18 g	Q = It first
Same charge for silver plating: mass?	4.03 g	Divided by one, not two
Concentrated brine: what forms at the anode and why?	Chlorine, Cl- oxidises before OH-	Gave a reason

06 Common mistakes and what to say

Mistake	Student writes	Say this
Minutes used as seconds	Q = 2.0 x 30 = 60 C	The t in Q = It is in seconds; minutes are off by sixty.
Electron count ignored	n(Cu) taken as 0.0373 mol giving 2.4 g	Cu2+ needs two electrons; using one doubles the mass.
Electrodes swapped	Oxidation written at the cathode	The cathode gains electrons, so it is reduction.
Unbalanced charge	Cu2+ + e- gives Cu	Left is plus one, right is zero, so the equation is not balanced.

07 Homework (paper, section and question numbers)

Chemistry subject index — All chapters
Practice paper — Electrolysis: questions 4 and 6 with mass

08 Exit check (last 5 minutes)

Question	Answer	Decision
Discharge order	Cu2+ at cathode, Cl- in concentrated brine	Cannot rank: reteach act one
Electron counts	Cu two, Ag one	Wrong: rewrite the half equation
Q = It	3600 C using seconds	Minutes used: redo two questions

09 Tutor FAQ

What makes an electrode inert?

Carbon or platinum does not react, so only ions discharge; a copper anode dissolves instead.

Which electrode holds the object being plated?

The cathode, because metal ions are reduced onto its surface.

Must Faraday constant be memorised?

It is normally given as 96500 C/mol; remember it is the charge of one mole of electrons.

Find a tutor — free Tutor Registration →

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.