SPM Chemistry Form 5 Notes – Terminology and Concepts: Oxidation and Reduction (Part 12 – Final)

by BerryBerryTeacher

in Berry Reference (Notes)

Can you name the common chemical cells that you encounter on a daily basis? Most Berry Readers won’t even realise the prevalence of chemical cells which they use in daily life. Maybe calling them ‘batteries’ will make it more familiar to all of you. So this post on Berry Berry Easy is the final post (part 12) in the extremely long topic of “Oxidation and Reduction” for SPM Chemistry Form 5. Fortunately, this is probably the easiest sub-topic in this whole topic. Not much to worry about besides knowing which type of cells are useful for which applications and why. Also important would be to link the anodes and cathodes to their appropriate cells. (Note that (although not in syllabus) is that modern car batteries are shifting away from lead-acid accumulators.) In terms of possibilities of appearing in exams, this subtopic usually appear in low-marks questions. So it’s still worthwhile to understand and memorise the relationship between anode/cathode type to its working principles and examples.

SPM Chemistry Form 5 – Terminology and Concepts: Oxidation and Reduction (Part 12 – Final)

Rusting of Iron Nails

Rusting of Iron Nails

More Chemical Cells

1. Primary cells – are not rechargeable and can be used only once.

2. Secondary cells – are rechargeable when cells are exhausted and can be reused again.

A. Dry Cell

  • Anode (-): Zinc / Zn(s) –> Zn2+(aq) + 2e / reducing agent
  • Cathode (+): Graphite (carbon) rod / 2NH4+(aq) + 2e –> 2NH3(g) + H2(g) / oxidising agent
  • Electrolyte: Moist paste of ammonium chloride, zinc chloride and a little water.
  • Overall reaction: Zn(s) + 2NH4+(aq) –> Zn2+(aq) + 2NH3(g) + H2(g)
  • Uses: touchlight, toys, clock, remote control and radio.

B. Alkaline Cell

  • Anode (-): Zinc / Zn(s) –> Zn2+(aq) + 2e / reducing agent
  • Cathode (+): Manganese(IV) oxide / 2MnO2(s) + H2O(l) +2e –> Mn2O3(s) + 2OH-(aq) / oxidising agent
  • Electrolyte: Potassium hydroxide paste.
  • Overall reaction: Zn(s) + 2MnO2(s) + H2O(l) –> Zn2+(aq) + Mn2O3(s) + 2OH-(aq)
  • Heavy use and longer shelf life.
  • Zinc corrodes more slowly.
  • Higher power.
  • More stable current and voltage.

C. Mercury Cell

  • Anode (-): Zinc / Zn(s) –> Zn2+(aq) + 2e / reducing agent
  • Cathode (+): Mercury(II) oxide / Hg2+(aq) + 2e –> Hg(l) / oxidising agent
  • Electrolyte: Potassium hydroxide paste.
  • Overall reaction: Zn(s) + Hg2+(aq) –> Zn2+(aq) + Hg(l)
  • Danger to the environment and mercury need to recycle.
  • Uses: Watches, camera and small devices.

D. Lead-acid Accumulator

  • Anode (-): Lead / Pb(s) + SO42-(aq) –> PbSO4(s) + 2e / reducing agent
  • Cathode (+): Lead(IV) oxide / PbO2(s) + 4H+(aq) + SO42-(aq) + 2e –> PbSO4(s) + 2H2O(l) / oxidising agent
  • Electrolyte: Sulphuric acid.
  • Overall reaction: / Pb(s) + PbO2(s) + 4H+(aq) + 2 SO42-(aq) –> 2PbSO4(s) + 2H2O(l)
  • Uses: Automobiles.

E. Nickel-Cadmium Cell

  • Anode (-): Cadmium / Cd(s) + 2OH-(aq) –> Cd(OH)2(s) + 2e / reducing agent
  • Cathode (+): Nickel(IV) oxide / NiO2(s) + 2H2O(l) + 2e –> Ni(OH)2(s) + 2OH-(aq) / oxidising agent
  • Electrolyte: Porous separator soaked in potassium hydroxide solution.
  • Overall reaction: Cd(s) + NiO2(s) + 2H2O(l) –> Cd(OH)2(s) + Ni(OH)2(s)
  • Suffer from memory effect – hold less charge.
  • Toxic heavy metal.
  • Expensive.
  • Uses: Toys, laptops, and mobile phones.

F. Rechargeable Chemical Cell

i) Nickel-metal hydride (NiMH)

  • Anode (-): hydrogen-absorbing alloy.
  • Cathode (+): Nickel(IV) oxide.
  • Contains rare earth elements such as titanium, vanadium, zirconium, cobalt, manganese and aluminium that are more environmentally friendly.
  • Higher capacity than NiCd.
  • Higher self-discharge rate.
  • Uses: digital cameras and mobile phones.

ii) Lithium-ion (Li-Ion)

  • Smaller and lighter.
  • Anode (-): Carbon.
  • Cathode (+): Metal oxide (cobalt oxide / manganese oxide).
  • Electrolyte: Lithium salt in an organic solvent (ether).
  • Inflammable and can easily explode when exposed to high temperature.
  • Uses: Portable electronic.

iii) Lithium-polymer (Li-Poly)

  • Very small, thin and light.
  • Anode (-): Carbon.
  • Cathode (+): Metal oxide.
  • Electrolyte: Lithium salt in a solid polymer composite (polyacrylonitrile).
  • Not flammable.
  • Uses: MP3, PDAs and laptops.

G. Other Chemical Cells

i) Fuel Cells

  • Anode (-): Fuel (hydrogen / hydrocarbon / alcohol).
  • Cathode (+): Oxygen.
  • Non-polluting product.
  • Uses: space vehicles and military applications.

ii) Solar Cells

  • Made of semiconductor materials (crystalline silicon).
  • Solar energy converted to electric energy.
  • Non-polluting product.
  • High cost.
  • Uses: space satellites, irrigation pumps, calculator and telecommunications.

Finally, you are done with the long topic of oxidation and reduction for SPM Chemistry Form 5. Do revise all parts to get a complete comprehension of the topic.

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