# 7.4 Oxidation and Reduction in Chemical Cells

F.5 Chemistry · Topic VII Redox Reactions, Chemical Cells and Electrolysis · Lesson 3

Half equations for metal couples, inert electrodes, the zinc-carbon cell and the fuel cell.

## Figure

Zinc-carbon cell: the zinc case (−) is oxidized; at the graphite rod (+) ammonium ions are reduced and MnO₂ removes the hydrogen.

## Key points

- **Negative electrode = anode ／ 陽極 = oxidation**; **positive electrode = cathode ／ 陰極 = reduction**. Electrons flow from − to + in the **external circuit**; only **ions** cross the salt bridge.
- **Metal–metal ion cells**: the metal **higher in the E.C.S.** is the negative electrode. Cr rod | FeSO~4~(aq) | Fe rod: electrons Cr → Fe, and the pale green fades (Fe^2+^ + 2e^−^ → Fe).
- **Inert electrodes ／ 惰性電極** (graphite, platinum) only **provide a surface**: Fe^2+^ | acidified Cr~2~O~7~^2−^ turns pale green → yellow-brown and orange → green.
- **Predicting** with the **E.C.S.**: the **stronger oxidizing agent is reduced** (+), the **stronger reducing agent oxidized** (−); write the half equations, then the overall equation.
- **Zinc-carbon cell ／ 鋅碳電池**: zinc cup (−); graphite rod (+) in **MnO~2~** + powdered graphite; moist **NH~4~Cl paste** as electrolyte. MnO~2~ removes the insulating H~2~. Drawbacks: **leaks** as the zinc thins; an **H~2~ layer** on rapid discharge drops the voltage.
- **Fuel cell ／ 燃料電池**: **not rechargeable**, but **fuel** and **oxidant** are fed in continuously. H~2~–O~2~ cell: porous **nickel or platinum** electrodes (conduct electrons, let gases through, **catalyze**) in **hot concentrated KOH(aq)**.
- Fuel cells are **steady**, **efficient** and **non-polluting** (water only), but **expensive**, and hydrogen is hard to **store and transport**. Uses: the Apollo spacecraft and the Space Shuttle, backup power, vehicles.

## Must know

| Cell | Negative electrode (anode): oxidation | Positive electrode (cathode): reduction | Remember |
|---|---|---|---|
| **Metal–metal cell (Mg \| Cu)** | Mg(s) → Mg^2+^(aq) + 2e^−^ | Cu^2+^(aq) + 2e^−^ → Cu(s) | The more reactive metal is the **negative** electrode |
| **Inert electrodes (KI(aq) \| Br~2~(aq))** | 2I^−^(aq) → I~2~(aq) + 2e^−^ | Br~2~(aq) + 2e^−^ → 2Br^−^(aq) | Graphite / platinum only **provide a surface**; the stronger oxidizing agent (E.C.S.) is reduced |
| **Zinc-carbon cell** | Zn(s) → Zn^2+^(aq) + 2e^−^ (zinc case) | 2NH~4~^+^(aq) + 2MnO~2~(s) + 2e^−^ → 2NH~3~(aq) + Mn~2~O~3~(s) + H~2~O(l) (graphite rod) | MnO~2~ **removes hydrogen**; the NH~4~Cl paste is the electrolyte |
| **H~2~–O~2~ fuel cell (hot conc. KOH)** | H~2~(g) + 2OH^−^(aq) → 2H~2~O(l) + 2e^−^ | O~2~(g) + 2H~2~O(l) + 4e^−^ → 4OH^−^(aq) | Works as long as hydrogen and oxygen are **supplied**; the only product is water |

Sodium–sulphur cell (about 370 °C, molten): Na → Na^+^ + e^−^; S + 2e^−^ → S^2−^; a porous device keeps the sodium and sulphur apart and lets ions pass; rechargeable, so power stations use it to store surplus electricity.

## Common mistakes

- ✗ "Electrons pass through the salt bridge or the electrolyte." ✓ Electrons move only in the **electrodes and wires**; **ions** move through the salt bridge, the electrolyte or a porous device to complete the circuit.
- ✗ "In a zinc-carbon cell the current flows from the zinc case to the carbon rod." ✓ Current flows **opposite** to the electrons: in the external circuit electrons go zinc → rod, so the current goes from the **carbon rod to the zinc case**.
- ✗ "MnO~2~ is oxidized; the zinc case is the positive electrode." ✓ MnO~2~ is the **oxidizing agent** and is **reduced** (to Mn~2~O~3~); the zinc case is the **negative** electrode and gets **thinner**.
- ✗ "An unused zinc-carbon cell does not change." ✓ The NH~4~Cl paste is slightly **acidic**, so it slowly **corrodes the zinc case** even when the cell is not in use: a short shelf life.
- ✗ "In a KOH fuel cell oxygen gains electrons, so the oxygen electrode is the anode (−)." ✓ Gaining electrons is **reduction**, at the **cathode (+)**. Hydrogen is oxidized at the anode (−), so electrons flow from the **hydrogen** electrode to the **oxygen** electrode.

## Quick check

1. A chromium rod and an iron rod stand in iron(II) sulphate solution, whose colour slowly fades. Give the direction of electron flow with a reason, and the half equation at the chromium rod.
2. Carbon rods in KI(aq) and in Br~2~(aq) are joined by a salt bridge. Which is the stronger oxidizing agent? Write both half equations and state what is seen in each beaker.
3. In a zinc-carbon cell, name the electrolyte and the oxidizing agent, write the half equation at the zinc case, and say how the case changes as the cell is used.
4. For an alkaline hydrogen-oxygen fuel cell, write the half equation at each electrode, give the direction of electron flow, and one advantage and one disadvantage.
5. Why must a sodium–sulphur cell run at about 370 °C, and what are the two functions of its porous device?

## Answers

1. From chromium to iron: chromium is higher in the E.C.S. and loses electrons more readily; iron(II) ions gain them at the iron rod (Fe^2+^ + 2e^−^ → Fe), so the colour fades. Cr → Cr^3+^ + 3e^−^.
2. Bromine. 2I^−^(aq) → I~2~(aq) + 2e^−^ (KI beaker turns brown); Br~2~(aq) + 2e^−^ → 2Br^−^(aq) (the yellow-brown colour fades).
3. Electrolyte: moist ammonium chloride paste; oxidizing agent: manganese(IV) oxide. Zn(s) → Zn^2+^(aq) + 2e^−^. The zinc case becomes thinner and corroded, from silvery to dull grey.
4. Anode: H~2~(g) + 2OH^−^(aq) → 2H~2~O(l) + 2e^−^; cathode: O~2~(g) + 2H~2~O(l) + 4e^−^ → 4OH^−^(aq); electrons flow from the hydrogen electrode to the oxygen electrode. Advantage: steady, efficient, non-polluting (water only). Disadvantage: expensive; hydrogen and oxygen are hard to store and transport.
5. To keep the sodium and sulphur molten so the particles stay mobile. The porous device separates sodium from sulphur (no direct reaction) and lets ions pass to complete the circuit.

## Did you know?

The zinc-carbon cell comes from Leclanché's wet cell (1866); in 1886 Carl Gassner replaced its liquid with a moist paste, making the first practical "dry cell".