S5 Chemistry: Chemical Cells and Fuel Cells prep guide | Electrode potentials, salt bridge and efficiency

2026-11-01

Tutor lesson-prep guides · Secondary Chemistry

Students can say a battery has a positive and a negative terminal, then freeze when asked which way the electrons flow or what the salt bridge is for. This lesson frames a cell as two half-reactions competing for electrons, and every conclusion has to point back to an electrode potential.

Objectives — Students can: (1) calculate a cell voltage with Ecell = Ecathode – Eanode and judge spontaneity; (2) explain electron flow, ion flow and how the salt bridge keeps the solutions neutral; (3) separate primary cells, electrolytic cells and fuel cells, and compute maximum electrical work with Q = nF.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Redox	What does OIL RIG stand for	Oxidation loses, reduction gains electrons
Ions and electrolytes	What is inside a salt bridge	Mobile ions
Metal reactivity	Which loses electrons more readily, zinc or copper	Zinc

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Half-reactions and potentials	E: Cu2+/Cu +0.34, Zn2+/Zn -0.76 V	EMF 1.10 V
10-24 min	Electron flow and salt bridge	Electrons outside, ions inside	Current runs opposite
24-36 min	Judging spontaneity	Ecell > 0 means spontaneous	Negative means driven
36-48 min	Fuel cells	H2/O2 cell at 1.23 V	237 kJ per mole
48-56 min	Efficiency and uses	Combustion enthalpy 286 kJ/mol	Efficiency 83.0%
56-60 min	Exit check	Three questions collected	Done independently

03 Teaching script (read it out)

Scene 1: Two half-reactions competing for electrons

In the Daniell cell the Zn2+/Zn couple sits at -0.76 V and Cu2+/Cu at +0.34 V. The higher potential wins electrons, so copper ions take the electrons zinc releases.
Ecell = 0.34 – (-0.76) = 1.10 V, and the positive sign means the reaction is spontaneous.
Zinc is the negative electrode where oxidation happens: Zn to Zn2+ plus 2e-. Copper is the positive electrode where reduction happens: Cu2+ plus 2e- to Cu.
Electrons travel through the external circuit from zinc to copper, while conventional current is drawn the other way. Writing those two directions the same way is the single most common slip.

Task: write both half-equations with 2 electrons on each side, then check the voltage with Ecell.

Scene 2: Salt bridge and the two circuits

Electrons only travel the external circuit; ions only travel the internal one through the salt bridge or a porous pot.
At the copper side Cu2+ is consumed, leaving excess negative charge; at the zinc side Zn2+ keeps building up. Without a salt bridge that charge separation cancels the potential difference within moments and the current stops.
Potassium nitrate in the bridge supplies K+ towards the zinc side and NO3- towards the copper side, so both solutions stay neutral and the current can keep flowing.
Keep the contrast sharp: a primary cell is spontaneous with Ecell above zero, an electrolytic cell is driven by an external supply and converts electrical energy into chemical energy.

Task: explain the effect of removing the salt bridge in three steps: charge builds up, the potential difference collapses, the current stops.

Scene 3: Fuel cells: why the efficiency beats an engine

The hydrogen-oxygen fuel cell gives 1.23 – 0.00 = 1.23 V and its only product is water.
Each mole of H2 releases 2 moles of electrons, so Q = 2 x 96485 = 192970 C and the maximum work is 192970 x 1.23 = 237353 J, about 237 kJ.
The combustion enthalpy of hydrogen is 286 kJ/mol, so the theoretical efficiency is 237 over 286, which is 83.0%. A petrol engine reaches roughly 27.5%, so the fuel cell is about 3.0 times better.
The reason is that no combustion step intervenes: chemical energy goes straight to electrical energy, avoiding the heat and sound losses of burning.

Task: find the maximum work from 6.54 g of zinc (Ar 65.4) in a 1.10 V cell: 0.100 mol, 19,297 C, 21.2 kJ.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Cell voltage	Ecell = higher E minus lower E, write half-equations first	1.10 V
Structure	Electron and ion flow	Electrons outside, ions through the bridge	Electrons zinc to copper
Comparison	Primary versus electrolytic	Spontaneity and external supply	Only Ecell above zero is spontaneous
Data	Fuel-cell efficiency	Q = nF, W = QV, divide by combustion enthalpy	83.0%

Sanity check (teach this): Check three directions every time: the cell voltage must be positive, so a negative answer means the electrodes were swapped; electron flow runs opposite to conventional current; and the electron count must be multiplied in full, two per mole for both hydrogen and zinc.
Reality check on the numbers: 1.10 V and 1.23 V are standard-condition values at 1 M, 298 K and 1 atm. A working cell delivers 10 to 20 percent less because of internal resistance and polarisation, so claiming the real cell matches theory ignores losses. Students should multiply out 192970 C rather than memorise it.

05 Independent practice (3 questions)

Question	Answer	What to watch
E(Ag+/Ag) = +0.80 V, E(Zn2+/Zn) = -0.76 V, voltage of the silver-zinc cell	1.56 V	0.80 minus -0.76
Charge released by 6.54 g of zinc	19297 C	0.100 mol times 2F
Maximum work per mole of hydrogen in a fuel cell	237 kJ	Q times V
What happens to the current if the salt bridge is removed	It stops	Charge builds up, potential collapses

06 Common mistakes and what to say

Mistake	Student writes	Say this
Electron and current direction mixed up	Saying electrons flow from copper to zinc	“Electrons are negative, so the current is drawn the opposite way. Write the two directions separately.”
Voltage worked out the wrong way round	Getting -1.10 V	“Higher potential minus lower potential. A negative answer means you chose the wrong cathode.”
Salt bridge described as carrying electrons	Saying the bridge conducts electrons	“The bridge moves ions only. Electrons never swim through it, and that sentence loses the mark.”
Efficiency given without a denominator	Answering 237% or 100%	“Efficiency needs a denominator: work on top, combustion enthalpy underneath.”

07 Homework (paper, section and question numbers)

Secondary chemistry chapter index — All chapters
Chemistry practice paper — Electrochemistry questions 12 to 17

08 Exit check (last 5 minutes)

Question	Answer	Decision
Voltage of the silver-zinc cell	1.56 V	All correct: move to rates of reaction
Two jobs of the salt bridge	Carry ions, keep neutrality	Wrong: reteach the two circuits
Efficiency formula for a fuel cell	Work divided by combustion enthalpy	Two wrong: back to act one

09 Tutor FAQ

Why is the standard hydrogen electrode the zero point?

A single electrode potential cannot be measured on its own, so the hydrogen electrode is defined as 0.00 V and every other couple is compared against it.

Is a fuel cell a primary cell?

Yes, the reaction is spontaneous and no external supply is needed; it differs in that reactants are fed in continuously instead of being sealed inside.

What can replace a salt bridge?

A porous pot or a filter paper soaked in electrolyte; what matters is allowing ion movement without mixing the two solutions.

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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.