Electrochemistry · Lesson 10 of 13
Primary Cells and Rechargeable Batteries
“Explain the construction and reactions of dry, mercury, lead-storage and nickel–cadmium cells, including how charging reverses electrode processes.”
• Distinguish primary and secondary cells using their practical reversibility. • Connect components of dry and mercury cells to their electrode reactions. • Balance lead-storage discharge and charging reactions. • Explain how battery voltage stability and lifetime depend on chemistry and operation.
A useful battery needs more than a favourable reaction
A practical battery must supply useful voltage and current in a manageable package. Its reactants, electrolyte, separators and current collectors must remain in contact without short-circuiting. High available charge, low mass, stable voltage, durability, cost and safe operation are different design goals. A cell is one electrochemical unit; a battery may contain one or several connected cells in common usage. Connecting cells in series adds their voltages, while the chemical capacity still depends on the amount and accessibility of active material.
A primary cell is designed for use without routine electrical recharging. A secondary cell has electrode reactions and construction that support repeated charging and discharging. Practical rechargeability requires more than writing a chemical equation backwards.
The zinc–manganese dioxide dry cell
The familiar dry cell contains a zinc container that acts as the anode. A central graphite rod acts mainly as a current collector for the cathode mixture of MnO₂ and carbon. Between the electrode regions is a moist paste containing NH₄Cl and ZnCl₂. The cell is called dry because the electrolyte is a paste rather than a freely pouring liquid; ions still need a moist conducting medium.
Zinc changes from oxidation state 0 to +2. Manganese changes from +4 in MnO₂ to +3 in MnO(OH), so each manganese centre accepts one electron. Ammonia can coordinate with Zn²⁺, changing the distribution of dissolved species. For example Zn²⁺ + 4NH₃ ⇌ [Zn(NH₃)₄]²⁺ shows the complexation step; it is not by itself the complete cell reaction. The nominal cell voltage is about 1.5 V and changes as materials are consumed and reaction products accumulate.
Problem
Verify atoms and charge in MnO₂ + NH₄⁺ + e⁻ → MnO(OH) + NH₃.
- 1.The left side has Mn₁O₂N₁H₄. MnO(OH) contributes Mn₁O₂H₁ and NH₃ contributes N₁H₃, so the right totals match.
- 2.The +1 charge of NH₄⁺ and −1 of the electron cancel, giving net zero on both sides.
- 3.Manganese reduction from +4 to +3 accounts for the one accepted electron. The graphite collector need not be consumed in this simplified cathode process.
Mercury-cell voltage stability
A mercury cell uses zinc amalgam as its reducing material and HgO mixed with carbon as its cathode material, with an alkaline paste such as KOH and ZnO. It is a historical example of a compact primary cell, often described with nominal voltage about 1.35 V. The chemistry illustrates voltage stability, but mercury-containing cells require controlled handling and waste management; the description is not an instruction to construct or open one.
Adding cancels water, hydroxide and electrons, giving Zn(Hg) + HgO(s) → ZnO(s) + Hg(l). No dissolved ion concentration appears explicitly in this net reaction, helping maintain an approximately steady emf while the relevant phases and activities remain established. “Constant voltage” is an approximation: loading, material depletion, amalgam composition, temperature and internal resistance can still affect the terminal voltage.
Lead storage battery during discharge
A lead storage cell has a lead plate and a lead grid carrying lead dioxide, immersed in sulphuric acid, commonly described as about 38% acid solution in the charged state. During discharge the lead plate is the negative anode and lead dioxide is the positive cathode. Both become coated with PbSO₄, while acid is consumed and water is produced. A single cell provides roughly 2 V under ordinary battery conditions; several cells can be connected to obtain a larger nominal voltage.
Problem
Explain how both a lead plate and a lead-dioxide plate can produce PbSO₄ during discharge.
- 1.In metallic lead the oxidation state is 0, so its conversion to PbSO₄, where lead is +2, is oxidation.
- 2.In PbO₂ lead is +4, so its conversion to the same +2 state in PbSO₄ is reduction.
- 3.The identical final lead oxidation state is reached from opposite directions. This is why one plate supplies electrons and the other consumes them.
Charging reverses the chemical changes
A charger imposes a voltage that drives the reverse reaction. At the negative plate, PbSO₄ gains two electrons to regenerate Pb and release SO₄²⁻. At the positive plate, PbSO₄ is oxidised in water to regenerate PbO₂, SO₄²⁻ and H⁺ while releasing two electrons to the charger. The negative plate is now the cathode and the positive plate the anode. It is clearer to identify the physical plates first, then name their processes, rather than retaining discharge labels during charging.
Nickel–cadmium as another secondary system
A nickel–cadmium cell has cadmium-containing negative active material, nickel oxyhydroxide positive active material and an alkaline electrolyte. Plates may be wound in a jelly-roll structure with a separator soaked in hydroxide electrolyte. The separator allows ionic conduction while preventing direct electronic contact between plates. The cell is rechargeable because the active materials can be regenerated under controlled charging, though cycle life is finite and cadmium demands appropriate waste management.
A simplified alternative representation uses Ni(OH)₃ and CdO: Cd + 2Ni(OH)₃ → CdO + 2Ni(OH)₂ + H₂O. This also balances atoms and electron changes but uses different hydration conventions. Solid electrode materials are more complex than a single molecular species, so do not mix half-reactions from different conventions without balancing the associated water.
An equation that is formally reversible does not guarantee a primary cell can safely be recharged. Electrode shape changes, unwanted gases, side reactions and separator damage can prevent useful cycling.
Quick Quiz
Quiz
What is the main role of the graphite rod in the common zinc–MnO₂ dry cell?
MnO₂ participates in cathodic reduction; the graphite rod provides a conductive collector.
Why is a dry cell still able to conduct ions?
Dry means the electrolyte is not a freely flowing liquid, not that all water is absent.
What happens to sulphuric-acid concentration in a lead cell during discharge?
The net discharge reaction consumes H₂SO₄ and produces water.
During lead-battery charging, which process occurs at the negative plate?
The charger delivers electrons to the negative plate, making it the cathode during charging.
What helps the mercury cell maintain a fairly steady emf?
The condensed-phase net chemistry supports a relatively stable reaction quotient while the relevant phases persist.
Practice Problems
- Compare primary and secondary cells, and explain why writing a reverse reaction is not enough to establish rechargeability.
- Add the simplified dry-cell half-reactions and identify the manganese oxidation-state change.
- Show that hydroxide cancels in the mercury-cell net reaction and explain the voltage implication.
- Write both charging half-reactions of a lead cell and identify each physical plate’s role.
- Check atoms and oxidation states in Cd + 2NiO(OH) + 2H₂O → Cd(OH)₂ + 2Ni(OH)₂.
- Explain why a larger battery can store more energy without a larger single-cell voltage.
1. Primary cells are designed for discharge without routine recharging; secondary cells support repeated controlled reversal of their electrode changes. 2. Useful charging requires materials and structure that survive the process, not only a favourable reverse equation. 3. Gas evolution, irreversible compounds, loss of contact and separator damage can make a formally reversible reaction unsuitable for repeated use.
Key Takeaways
• Primary cells are designed for use without routine recharging. • Secondary cells support repeated reversal through suitable chemistry and construction. • In a dry cell zinc is oxidised while MnO₂ is reduced. • Graphite mainly collects current from the dry-cell cathode mixture. • Mercury-cell condensed-phase chemistry helps maintain a steady emf. • Lead-cell discharge converts both plates toward PbSO₄ and consumes acid. • Charging regenerates Pb, PbO₂ and acid while reversing process labels. • Nickel–cadmium cells couple cadmium oxidation to nickel(III) reduction.