IB Chemistry SLTopic 6 — Electron TransferPaper 1 & 2Materials~12 min read
Secondary Cells
A primary cell dies when a reactant runs out and that is the end of it. A secondary cell runs the same reaction backwards when you push electrons the other way — which is all that “rechargeable” means.
📚 What you need to know
A secondary cell can be recharged because its reaction is reversible.
Recharging applies a voltage greater than the cell’s own voltage, forcing electrons backwards.
Lead–acid: Pb and PbO2 electrodes in sulfuric acid, about 2 V per cell, six in series for a car battery.
NiCad: cadmium and a nickel hydroxide-oxide system, about 1.2 V, and it suffers the memory effect.
Lithium-ion: lithium cobalt oxide and graphite with a solid polymer electrolyte, about 3.5–4.0 V.
Every type has an environmental cost: lead and cadmium are toxic, and lithium is a finite resource.
Charging is discharging in reverse
When a rechargeable cell is used, its redox reaction runs the way it wants to go — downhill, spontaneously, pushing electrons round the circuit. Charging it means doing the opposite: connecting a supply that pushes harder than the cell does, so the electrons are driven back and the reactants re-form.
While a car is running, the alternator is doing exactly this to the battery: driving the discharge reaction backwards and rebuilding the electrodes.
Notice what the right-hand panel is. Driving a non-spontaneous redox reaction with an external supply is electrolysis — the subject of the next page. Charging a battery and electroplating a spoon are the same process wearing different clothes.
Lead–acid batteries
The oldest rechargeable design still in everyday use, and the one under the bonnet of most cars. Each cell has a lead negative electrode and a lead(IV) oxide positive electrode, both dipped in sulfuric acid.
Both electrodes end up coated in the same product, lead(II) sulfate, which is why the reaction reverses so neatly on charging.
Lead–acid batteries are designed to deliver a very large current for a few seconds — exactly what turning over an engine requires. What they are not is portable: lead is dense, the case is bulky, and both the lead compounds and the concentrated acid make disposal a serious problem.
NiCad cells last for hundreds of cycles and come in the same sizes as ordinary batteries, which made them the standard rechargeable for years. Two things count against them: cadmium is toxic, and they suffer the memory effect — recharge one repeatedly without letting it run down and it gradually loses capacity.
Lithium is the reason your phone is not the size of a brick. It is the least dense metal and has a very negative electrode potential, so a lithium cell delivers roughly three times the voltage of a NiCad for a fraction of the mass. Lithium ions shuttle between a lithium cobalt oxide electrode and a graphite one through a solid polymer electrolyte, which cannot leak.
Voltage is only half the story. A lead–acid cell wins outright on current delivery, and a lithium cell wins outright on energy per kilogram.
Lead–acid
NiCad
Lithium-ion
Voltage per cell
about 2 V
about 1.2 V
3.5–4.0 V
Best at
very high current, briefly
many charge cycles
energy per unit mass
Typical use
starting car engines
power tools, older devices
phones, laptops, electric cars
Main drawback
heavy, bulky, toxic lead
toxic cadmium, memory effect
degrades with cycles, fire risk
Environmental issue
lead and corrosive acid
cadmium contamination
finite lithium, must be recycled
WORKED EXAMPLE
Explain, in terms of the reactions taking place, the difference between a primary cell and a secondary cell.
Primary cellThe redox reaction runs in one direction only, and the products cannot be converted back into reactants.discarded once a reactant is used upSecondary cellThe reaction is reversible. Applying a voltage larger than the cell’s own voltage drives electrons the opposite way and re-forms the original reactants.can be recharged and used againSay reversible and mention the applied voltage. “It can be recharged” simply restates the question.
WORKED EXAMPLE
A lead–acid cell has the half-equations below. Pb(s) + SO42–(aq) → PbSO4(s) + 2e– PbO2(s) + 4H+(aq) + SO42–(aq) + 2e– → PbSO4(s) + 2H2O(l) Deduce the overall equation and identify the oxidation state change of lead at each electrode.
Step 1 — the electrons already matchTwo electrons in each half, so simply add and cancel them.Pb + PbO₂ + 4H⁺ + 2SO₄²⁻ → 2PbSO₄ + 2H₂OStep 2 — lead at the negative electrodePb: 0 → +2, oxidisedStep 3 — lead at the positive electrodePb: +4 → +2, reducedLead is both the oxidising and the reducing agent here, in different oxidation states, and both electrodes end up as PbSO₄.
WORKED EXAMPLE
A manufacturer is choosing a cell for a cordless drill and for a mobile phone. Suggest, with reasons, a suitable type for each.
The drillIt needs a large current in short bursts, survives being knocked about, and mass matters less.NiCad or a similar robust rechargeableThe phoneMass and thickness dominate, and it is charged daily without being fully discharged.lithium-ionThe clincher is the memory effect: a NiCad topped up every night would lose capacity, while a lithium-ion cell does not care.
💡 Exam tip
Define a secondary cell by its reversible reaction, not by the word “rechargeable”.
State that recharging needs a voltage greater than the cell’s own, applied in the opposite direction.
Learn the electrode materials and electrolyte for each of the three types.
For comparison questions, use voltage, mass, current delivery, lifetime and toxicity as your headings.
Give a balanced comment on the environment: every battery type has a disposal problem.
Cells in series add their voltages — that is where the 12 V of a car battery comes from.
⚠️ Common mix-up
Saying a secondary cell “makes more reactants”. It re-forms the ones that were used.
Thinking any cell can be recharged if you connect a supply to it. Only a reversible reaction can.
Attributing the memory effect to lithium-ion cells. It is a NiCad problem.
Claiming lithium-ion cells are environmentally harmless because they contain no lead or cadmium.
Confusing 2 V per cell with 12 V per battery when writing about lead–acid.
Up next: Electrolytic Cells — recharging a battery is one example of forcing a reaction to run uphill. Now for the general case, and what it lets you take apart.
Want this explained one-to-one?
Book a free session with an experienced IB Chemistry tutor and get your trickiest topics made simple.
