IB Chemistry HLTopic 6 — Electron TransferPaper 1 & 2Core idea~11 min read
Primary Cells
On the last page, zinc handed its electrons straight to hydrogen ions in the same beaker. All that energy came out as heat and went nowhere useful. A voltaic cell does one clever thing: it separates the two halves so the electrons have to take the long way round, through a wire, doing work as they go.
📚 What you need to know
A voltaic (galvanic) cell turns a spontaneous redox reaction into electricity.
Each beaker is a half-cell; the metal strip in it is an electrode.
Oxidation at the anode (negative here); reduction at the cathode (positive here).
Electrons travel through the wire; ions travel through the salt bridge to keep charge balanced.
The voltage produced is the EMF or cell potential, and only differences can be measured.
Primary cells cannot be recharged. Fuel cells keep going as long as fuel is supplied.
Cell diagram convention: oxidation on the left, single line = phase boundary, double line = salt bridge.
The zinc-copper cell
The zinc electrode gets thinner and the copper one gets thicker. The blue of the copper sulfate slowly fades as Cu2+ ions are used up.
Why the salt bridge is not optional
Without it the cell stops almost immediately. Follow the charge:
Zinc dissolves, so the left solution builds up positive charge.
Copper ions plate out, so the right solution is left with excess negative charge.
That build-up opposes any further electron flow, and everything grinds to a halt.
The salt bridge fixes it by letting ions drift: anions move towards the anode and cations towards the cathode. It is usually filter paper soaked in potassium nitrate, chosen because nitrates are soluble so nothing precipitates.
Do not describe the salt bridge as “letting electrons through”. It carries ions, not electrons. Electrons only ever travel through the metal wire — that is the entire point of building the cell this way.
Single vertical line for a phase boundary, such as solid metal meeting its solution.
Double vertical line in the middle for the salt bridge.
Highest oxidation state next to the salt bridge on both sides.
Quote the EMF with the sign, taken as right minus left.
Written backwards? Cu(s) | Cu2+(aq) || Zn2+(aq) | Zn(s) describes the same physical cell but gives −1.10 V. The negative sign tells you the diagram is the wrong way round, not that the cell does not work.
Fuel cells
A fuel cell is a voltaic cell where the reactants are fed in continuously instead of being sealed inside. It never goes flat, because the “battery” is the fuel supply.
Cell potential: (+0.40) − (−0.83) = +1.23 V. Because there is no combustion, the bond energy goes to electricity rather than heat and light.
Feature
Advantage
Problem
Product
Only water, so no CO2 or nitrogen oxides at the point of use
The hydrogen itself is usually made from fossil fuels
Efficiency
No combustion step, so more of the energy becomes electricity
Expensive catalysts that can be poisoned or wear out
Fuel supply
Runs continuously, refuels in minutes
Hydrogen is flammable and needs heavy high-pressure tanks
Energy density
Very high energy per gram
Low energy per unit volume, so the tanks are bulky
Worked examples
WORKED EXAMPLE
A magnesium electrode is connected to a silver electrode. Write the cell diagram and state which way the electrons flow. (Mg2+/Mg = −2.37 V, Ag+/Ag = +0.80 V)
Step 1: The more negative half-cell is oxidisedMagnesium at −2.37 V, so magnesium goes on the left.Step 2: Build the diagramMg(s) | Mg2+(aq) || Ag+(aq) | Ag(s)Step 3: Cell potentialEθcell = (+0.80) − (−2.37) = +3.17 VElectrons flow from magnesium to silver through the wiremagnesium is the negative electrode because it is pushing electrons out
WORKED EXAMPLE
In a zinc-copper cell, describe two changes you would see in the beakers after several hours.
Step 1: Follow the anodeZinc is oxidised to Zn2+ and goes into solution.The zinc strip gets thinner and loses massStep 2: Follow the cathodeCu2+ ions are reduced and deposit as copper metal.The copper strip gets thicker and gains massZinc electrode erodes; copper electrode grows and the blue colour fadesthe fading blue is Cu2+ being removed from the solution
WORKED EXAMPLE
Explain why a hydrogen fuel cell is described as cleaner than a petrol engine, and give one reason that claim needs qualifying.
Step 1: What comes outThe only product is water, so no carbon dioxide at the point of use.Step 2: Why no nitrogen oxidesThere is no high-temperature combustion, so the nitrogen in the air is not oxidised.Step 3: The qualificationMost hydrogen is currently made from natural gas, which releases CO2 elsewhere.Clean at the point of use, but only as clean as the hydrogen supplyquestions on this always want both sides of the argument
💡 Exam tip
Say the salt bridge completes the circuit and balances the charge, and that it carries ions.
In a voltaic cell the anode is negative. Learn it as a pair with electrolysis, where it is positive.
For cell diagrams, put oxidation on the left and check your EMF comes out positive.
Describe observations concretely: electrode thinner, electrode thicker, blue colour fading.
For fuel cells, give a balanced pair of advantages and disadvantages if the question says “evaluate”.
Potassium nitrate is a good salt bridge because both its ions form soluble compounds.
⚠ Common mix-up
Saying electrons cross the salt bridge. They do not. Ions do.
Getting the anode charge wrong. Negative in a voltaic cell, positive in electrolysis.
Writing the cell diagram backwards and then reporting a negative EMF as the answer.
Calling a fuel cell rechargeable. It is refuelled, not recharged — nothing is reversed.
Claiming fuel cells are completely emission-free. It depends entirely on how the hydrogen was made.
Forgetting that a primary cell is dead once the reaction reaches equilibrium. That is what a flat battery is.
Up next: Secondary Cells — if you can force electrons back the other way, the reaction reverses and the cell recharges. That is the difference between a battery you throw away and one you plug in.
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