DSE Chemistry | Electrolysis and electroplating calculations - tutor prep guide

DSE Chemistry | Electrolysis and electroplating calculations – tutor prep guide | Half-equations and three-step conversion

2026-11-06

Tutor lesson-prep guides · Secondary Chemistry

Students can recite Q = It, yet stop dead at how much heavier the cathode gets in 30 minutes, because the middle step, moles of electrons, was never written down.

Objectives — Students can: (1) tell an electrolytic cell from a chemical cell by the direction of energy flow, while keeping oxidation at the anode and reduction at the cathode; (2) write both half-equations for a given electrolyte with electrons balanced; (3) convert charge to mass in three steps: Q = It, then moles of electrons, then moles of product divided by electrons per particle.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Redox	Does oxidation lose or gain electrons?	It loses them (OIL RIG)
Mole concept	How much charge is one mole of electrons?	96,500 coulombs, the Faraday constant
Electrode names	Which terminal does the cathode of an electrolytic cell connect to?	The negative terminal

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Two apparatus diagrams	A cell beside an electrolytic cell: which is driven by an external supply	Mark the electron flow
8-20 min	Fix anode and cathode	Oxidation at the anode, reduction at the cathode, in both	Ten quick oral rounds
20-32 min	Half-equations	Balance atoms first, then electrons	Write both equations for copper sulphate
32-44 min	Discharge order	Reactive anode dissolves first, otherwise chloride before hydroxide	Decide two items
44-56 min	Three-step conversion	Q=It, moles of electrons, moles of product, mass	Compute 2.0 A for 30 minutes
56-60 min	Exit check	Collect three items, mark at once	Work alone

03 Teaching script (read it out)

Scene 1: Sort the cell before naming the electrodes

A chemical cell runs on a spontaneous reaction; an electrolytic cell is driven by an external supply. The naming, however, never changes: the anode is where oxidation happens, the cathode where reduction happens.
The common error is assuming the cathode must be negative, because in an electrolytic cell it is, while in a simple cell the cathode is positive, so students carry the wrong rule into the other diagram.
Separate the two questions: first where do the electrons come from, then is this electrode oxidising or reducing, and only then attach a name. The name is the last step, not the first.

Ask the student to mark electron flow and all four electrode reactions on both diagrams and state the order of decisions.

Scene 2: Half-equations: atoms first, electrons second

In copper sulphate solution with inert electrodes the anode gives 2H2O to O2 + 4H+ + 4e- and the cathode gives Cu2+ + 2e- to Cu. With copper electrodes the anode instead dissolves, Cu to Cu2+ + 2e-, and the concentration stays roughly constant, which is exactly why electroplating works.
Fix the order: write reactants and products, balance atoms (metal, then oxygen, then hydrogen), top up oxygen with water, balance charge with electrons, then check both sides carry the same number of electrons.
Teach discharge order alongside: at the anode a reactive metal electrode dissolves first, otherwise chloride discharges before hydroxide; at the cathode the higher standard electrode potential goes first, so copper before hydrogen and hydrogen before sodium.

Ask the student to write the four half-equations for copper sulphate with copper and with carbon electrodes, and say which one changed.

Scene 3: Three-step conversion: from current to mass

Three lines, one step each: Q equals current times time in seconds; moles of electrons equal Q divided by 96,500; moles of product equal moles of electrons divided by electrons per particle. With 2.0 A for 30 minutes: Q is 3,600 C, electrons are 0.0373 mol, copper deposited is 0.0187 mol, which is 1.18 g.
Three traps to demonstrate live: time must be in seconds, since 30 minutes is not 30; the division by electrons per particle uses the coefficient in the equation, so dividing, not multiplying; and grams only appear after multiplying by molar mass.
Require the three lines in the answer even when only the mass is asked, because the marks live in the route.

Ask the student to say how far out the answer would be if 30 minutes were read as 30 seconds, and why.

04 Question types and worked steps

Type	Example	Steps	Answer
Half-equations	Write the anode and cathode reactions	Classify first, then balance atoms and electrons	Equal electrons on both sides, state symbols included
Discharge order	Which ion discharges first?	Compare potentials or the reactivity series, name the reason	A because-therefore sentence, not a bare answer
Quantitative calculation	Mass gained at the cathode in 30 minutes	Q=It, moles of electrons, moles, mass	Units shown: C, mol, g
Electroplating application	Why copper rather than carbon for the anode?	The anode dissolves and tops up the ions	Mention both concentration and continuity

Sanity check (teach this): Sanity check: electrode names must be derived from electron flow, never recited; a half-equation is only finished when both sides carry equal electrons; the three conversion lines stay on separate lines because errors sit in the skipped step, not the arithmetic. Make one deliberate slip with seconds so students learn to catch it.

05 Independent practice (3 questions)

Question	Answer	What to watch
Half-equations	Electrolyse sodium bromide solution with carbon electrodes	Whether the competition with water is handled
Moles of electrons	3.0 A for 20 minutes, find moles of electrons	Whether seconds are converted first
Mass	Continue: grams of silver deposited	Whether the electron count was treated as two

06 Common mistakes and what to say

Mistake	Student writes	Say this
Cathode must be negative	The cathode of a cell is the negative terminal	Where do the electrons come from? State the flow first, then name it. In a simple cell the cathode is positive.
Time left in minutes	Multiplying 2.0 by 30	Q is in coulombs and seconds are the base unit. How many seconds in 30 minutes? Write it first.
Multiplying by electrons	Moles of copper equal moles of electrons times two	How many electrons does one copper ion need? Two electrons per copper means divide by two.
Carbon anode for plating	Carbon works because it conducts	A carbon anode releases no copper ions, so what happens to the concentration? What keeps plating going?

07 Homework (paper, section and question numbers)

Secondary Chemistry index — All chapters for this subject
Prep guides index — Matching chapters in other subjects
Redo this chapter — Repeat the 2.0 A, 30-minute copper plating in three lines and write both half-equations

08 Exit check (last 5 minutes)

Question	Answer	Decision
Electrode reaction types	Oxidation at the anode, reduction at the cathode	Wrong means back to act one
Three conversion steps	Q=It, moles of electrons, moles of product	Missing one means repeat act three
Use of the electron count	Divide by the coefficient in the equation	Multiplying means repeat act three

09 Tutor FAQ

Must students learn standard electrode potentials?

The idea yes, the numbers no; discharge order needs the reactivity series plus a few common ions.

How does gas volume fit in?

One extra step on the same chain: convert moles of gas with 24 dm3 per mole and state the room conditions.

What if arithmetic is weak?

Insist on three labelled lines with units, so method marks survive; then add a power-of-ten check.

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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.