More Chemical Cells

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) + SO₄^2-(aq) –> PbSO₄(s) + 2e / reducing agent
• Cathode (+): Lead(IV) oxide / PbO₂(s) + 4H⁺(aq) + SO₄^2-(aq) + 2e –> PbSO₄(s) + 2H₂O(l) / oxidising agent
• Electrolyte: Sulphuric acid.
• Overall reaction: / Pb(s) + PbO₂(s) + 4H⁺(aq) + 2 SO₄^2-(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.