AQA A Level Chemistry

Revision Notes

5.4.4 Commercial Cells

Commercial Cells

  • Electrochemical cells can be used as a commercial source of electrical energy
  • Cells can be non-rechargeable (irreversible), rechargeable or fuel cells
  • Type of cells used in commercial applications depend on
    • the voltage required
    • the current needed
    • the size of the cell
    • the cost
  • Although it is commonly used incorrectly, the term battery should be used to refer to a collection of cells
  • A car battery is correct, because it is a collection of six cells joined together

Non-rechargeable cells

The Daniell cell

  • The Daniell cell was one of the earliest electrochemical cells and consisted of a simple metal-metal ion system
  • It was invented by British chemist John Daniell in 1836
  • The cell consists of
    • a zinc rod immersed in a solution of zinc sulfate
    • a copper cylinder filled with copper sulfate solution
    • a porous pot that separates the copper sulfate from the zinc sulfate

Daniell cell, downloadable AS & A Level Chemistry revision notes

A Daniell cell

  • The zinc acts as the negative electrode and the copper is the positive electrode
  • The half-cell reactions are

Zn (s) →   Zn2+ (aq)  +  2e           E = -0.76 V 

Cu2+ (aq)  + 2e→ Cu (s)              E = +0.34 V

  • The cell generates an emf of 1.1 V and the overall reaction is

Zn (s)  + CuSO4 (aq)→ Cu (s)  + ZnSO4 (aq)      Ecell = +1.10 V

  • However, the cell is impractical to use as a portable device because of the hazardous liquids in the cell

Zinc-carbon cells

  • Zinc-carbon cells are the most common type of non-rechargeable cells, consisting of
    • a zinc casing which acts as the negative electrode
    • a paste of ammonium chloride which acts as an electrolyte as well as the positive electrode
    • a carbon rod which acts as an electron carrier in the cell

Zinc-carbon cell, downloadable AS & A Level Chemistry revision notes

The zinc-carbon cell

  • The half-cell reactions are

Zn (s) →   Zn2+ (aq)  +  2e                               E = -0.76 V 

2NH4+ (aq) + 2e→ 2NH3 (g) + H2 (g)              E = +0.74 V

  • The cell generates an emf of 1.50 V and the overall reaction is

2NH4+ (aq) + Zn (s)  → 2NH3 (g) + H2 (g)  + Zn2+ (aq)    Ecell = +1.50 V

  • As the cell discharges, the zinc casing eventually wears away and the corrosive contents of the electrolyte paste can leak out, which is an obvious disadvantage of zinc-carbon cells
  • The cell provides a small current and is relative cheap compared to other cells
  • Extra long life cells have a similar chemistry, but supply a higher current and use zinc chloride in the paste; they are suitable for torches, radios and clocks
  • Another variation on the cell uses an alkaline paste in the electrolyte and they have a much longer operating life, but are noticeably more expensive than regular zinc-carbon cells

Rechargeable Cells

  • Rechargeable cells employ chemical reactions which can be reversed by applying a voltage greater than the cell voltage, causing electrons to push in the opposite direction
  • There are many types of rechargeable cells, but common ones include lead-acid batteries, NiCad cells and lithium cells which are covered in more detail in the next section

Lead-acid batteries

  • Lead-acid batteries consist of six cells joined together in series
  • The cells use lead metal as the negative electrode and and lead(IV) oxide as the positive electrode
  • The electrolyte is sulfuric acid

Lead acid cell, downloadable AS & A Level Chemistry revision notes

A lead-acid battery

  • The half-cell reactions are

Pb (s) +  SO42- (aq)  →   PbSO4 (s)  +  2e                                                 E = -0.36 V 

PbO2 (s) +  4H+ (aq) +  SO42- (aq) +  2e →  PbSO4 (s)  + 2H2O (l)         E = +1.70 V

  • The cell generates an emf of about 2 V and the overall reaction is

PbO2 (s) +  4H+ (aq) +  2SO42- (aq) +  Pb (s) →  2PbSO4 (s)  + 2H2O (l)       Ecell = +2.06 V

  • In a commercial car battery, the six cells in series give a combined voltage of about 12 V
  • When the car is in motion, the generator provides a push of electrons that reverses the reaction and regenerates lead and lead(IV) oxide
  • Lead-acid batteries are designed to produce a high current for a short period of time, hence their use in powering a starter motor in car engines
  • The disadvantage of lead-acid batteries is that:
    • They are very heavy
    • They contain toxic materials: lead and lead(IV) oxide
    • The sulfuric acid electrolyte is very corrosive
  • This presents challenges of disposal when lead-acid batteries come to the end of their useful life

NiCad cells

  • NiCad stands for nickel-cadmium and these cells are available in many standard sizes and voltages so they can replace almost any application of traditional zinc-carbon cells
  • Although they are more expensive cells, the fact they can be recharged hundreds of times means they are commercially viable
  • The positive electrode consists of cadmium and the negative electrode is made of a nickel(II) hydroxide-oxide system
  • The half-cell reactions are

Cd (s) +  2OH–  (aq) → Cd(OH)2 (s)  +  2e                            E = -0.82 V 

NiO(OH) (s) + H2O (l) + e→ Ni(OH)2 (s) +   OH  (aq)         E =  +0.38 V

  • The overall reaction in the cell is

2NiO(OH) (s) + 2H2O (l) + Cd (s) → 2Ni(OH)2 (s) + Cd(OH)2 (s)         E = +1.2 V

  • Cadmium is a toxic metal so the disposal of old NiCad cells is also an environmental issue

 

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