S5 Chemistry: Chemical Cells — Tutor Lesson Plan (45 min, electrode script, four errors)

2026-11-02

Tutor lesson-prep guides · Secondary chemistry (DSE)

Students can name the anode and still mark the wrong electrode as positive, because the electron-flow convention feels backwards. This lesson fixes the convention first, then hangs the voltage on it.

Objectives — the student can identify the negative electrode of a simple cell from the reactivity of the metal pair, describe electron flow in the external circuit and ion flow in the electrolyte, calculate charge and energy from current and time, and state why a fuel cell is not a store of energy.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Reactivity series	Which is more reactive, zinc or copper?	Zinc, it loses electrons more readily
Ionic equations	Write the half-equation for zinc losing two electrons	Zn -> Zn2+ + 2e-
Current and charge	What are the units of current and charge?	Ampere and coulomb; charge = current x time

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Hook	Two metals and a lemon, or a zinc-copper cell with a voltmeter	Predict which electrode is negative before naming it
5-14 min	Scene 1	The more reactive metal loses electrons and becomes negative	Label one cell diagram from scratch
14-23 min	Scene 2	Electron flow outside, ion flow inside, and the salt bridge	Trace one electron from metal to metal
23-31 min	Scene 3	Charge, energy and amount of substance	Calculate energy released in kJ
31-39 min	Scene 4	Fuel cells: reactants in, products out, no recharge	Compare with a rechargeable battery
39-45 min	Exit check	Three decisions in section 08	Marked at once, error named

03 Teaching script (read it out)

Scene 1: The negative electrode is the one that dissolves

The more reactive metal is oxidised: zinc atoms lose electrons and the zinc strip loses mass. Those electrons leave through the wire, so zinc is the negative electrode and is the anode. In a zinc-copper cell the reading is about 1.10 V. Ask the student to say “zinc dissolves, so zinc is negative” five times before any cell diagram is drawn.

Test the rule: swap zinc for magnesium and ask what happens to the voltage and to the polarity.

Scene 2: Outside electrons move, inside ions move

Electrons travel through the wire from the negative electrode to the positive one. In the electrolyte, positive ions move towards the positive electrode and negative ions towards the negative electrode, so the circuit is closed through the solution or the salt bridge. The electrolyte conducts by ion movement, never by electron movement.

Question: “If the salt bridge is removed, what happens to the voltage?” No closed circuit, so no reading.

Scene 3: Charge and energy come from one multiplication each

A cell delivering 0.50 A for 2 hours passes Q = I x t = 0.50 x 7200 = 3600 C. The energy released is E = Q x V = 3600 x 1.50 = 5400 J, that is 5.4 kJ. The same charge is 0.037 mol of electrons, so the moles of zinc consumed follow from the two-electron half-equation. A fuel cell keeps running only while reactants are supplied, so it is a converter, not a store.

Ask: double the current and halve the time. Which quantities change?

04 Question types and worked steps

Type	Example	Steps	Answer
Polarity	Which electrode is negative in a magnesium-copper cell?	The more reactive metal: magnesium, which is oxidised	Magnesium, the anode
Energy calculation	A cell gives 0.50 A at 1.50 V for 2 hours. Find the energy in kJ.	Q = I x t, then E = Q x V, then divide by 1000	5.4 kJ
Explanation	Why does a fuel cell not need recharging?	Fuel and oxidant are supplied continuously, so reactants are not stored inside	Not a store, a continuous converter

Sanity check (teach this): Every answer must name which species loses electrons before naming the polarity. If the student says “copper is positive because it is the cathode”, ask which metal is being used up. Energy answers should be in the range 1 to 10 kJ for a lab-scale cell; a value in megajoules means the time was left in minutes.

05 Independent practice (3 questions)

Question	Answer	What to watch
A zinc-iron cell is built. Which electrode is negative and why?	Zinc, it is more reactive and is oxidised to Zn2+	Reason must mention oxidation, not just “it is first”
State what happens to the voltage when the salt bridge is removed.	The circuit is broken, so the reading falls to zero	Closed circuit language
A cell supplies 0.50 A at 1.50 V for one hour. Find the charge and the energy.	1800 C and 2.7 kJ	Both units stated

06 Common mistakes and what to say

Mistake	Student writes	Say this
Making copper the negative electrode	Zinc is oxidised, so zinc is negative	Ask which metal loses mass during the experiment
Saying electrons travel through the electrolyte	Electrons move in the wire only; ions carry charge in solution	Ask what would happen to a bulb if the wire were cut
Treating a fuel cell as a battery that needs charging	A fuel cell is supplied with reactants continuously	Ask where the hydrogen comes from
Leaving time in minutes in the charge calculation	Convert to seconds first	Write the unit next to every number before dividing

07 Homework (paper, section and question numbers)

HKDSE Chemistry Paper 1 multiple-choice practice paper — Questions on redox, cells and electrolysis; then redo section 05 questions 1 to 3
Secondary chemistry chapter index — Read the redox chapter before the next lesson

08 Exit check (last 5 minutes)

Question	Answer	Decision
In a zinc-silver cell, which electrode is negative?	Zinc, it is the more reactive metal and is oxidised	All correct: move to electrolysis
Where do electrons flow in the external circuit?	From the negative electrode to the positive electrode	One wrong: redraw the cell diagram
Why must a fuel cell be supplied continuously?	The reactants are not stored inside the cell	Two wrong: repeat scene 4

09 Tutor FAQ

Do students need standard electrode potentials?

No at S5. They need the reactivity rule and the direction of electron flow. Potentials only matter if a question gives them.

Should I teach electrolysis in the same lesson?

No. Cells and electrolysis use the same vocabulary with reversed energy flow; teach them a week apart and compare explicitly.

How precise should the voltage be?

Quote the value the question gives. Only say “about 1.1 V” for a zinc-copper cell when no figure is supplied.

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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.