Dry cells

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Dry cells
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Simple chemical cell
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Simple chemical cell

• Overall equation (Redox reaction)
• Zn(s) CuSO4(aq) ? ZnSO4(aq) Cu(s)
• Ionic equation
• Zn(s) Cu2(aq) ? Zn2(aq) Cu(s)
• Half equations
• Zn(s) ? Zn2(aq) 2e- Oxidation
• Cu2(aq) 2e- ? Cu(s) Reduction

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Daniell cell
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Daniell cell

• Anode (oxidation / -ve electrode)
• Zn(s) ? Zn2(aq) 2e-
• Cathode (reduction / ve electrode)
• Cu2(aq) 2e- ? Cu(s)
• Overall cell equation (Redox reaction)
• Zn(s) Cu2(aq) ? Zn2(aq) Cu(s)

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Daniell cell

• Disadvantages
• Liquid electrolyte may spill out ? inconvenient
  to be used
• Require porous pot for separation of two
  different electrolytes
• Low voltage 1.1V

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Dry cell

• Use a paste of electrolyte instead of aqueous
  electrolyte.
• Zinc-carbon cell
• Alkaline manganese cell
• Silver oxide cell / Button cell
• Nickel-cadmium cell (rechargeable)

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Zinc-carbon cell
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Zinc-carbon cell

• Anode (oxidation / -ve electrode) zinc
• Cathode (reduction / ve electrode) carbon
• Electrolyte moist paste of ammonium chloride
• Oxidizing agent manganese(IV) oxide
• Additive carbon powder is added to increase the
  conductivity

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Zinc-carbon cell

• Anode (oxidation / -ve electrode)
• Zn(s) ? Zn2(aq) 2e-
• Cathode (reduction / ve electrode)
• 2NH4(aq) 2e ? 2NH3(aq) H2(g)
• Hydrogen accumulates at electrode and decreases
  the current of the cell. This problem is solved
  by manganese(IV) oxide, an oxidizing agent that
  removes the hydrogen.
• 2MnO2(s) H2(g) ? Mn2O3(s) H2O(?)

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Zinc-carbon cell

• Ammonia is taken up by zinc ions.
• Zn2(aq) 2NH3(aq) 2Cl(aq) ?
  Zn(NH3)2Cl2(s)
• Overall cell equation
• 2MnO2(s) 2NH4Cl(aq) Zn(s) ? Zn(NH3)2Cl2(s)
  H2O(?) Mn2O3(s)
• The overall voltage of this cell is 1.5 volts.

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Zinc-carbon cell

• Disadvantages
• If current is drawn from the cell rapidly, the
  gaseous product cannot be removed fast enough.
  The voltage drops as a result. It is restored
  after standing.
• The lifetime of the cell is relatively short.
  There is a slow direct reaction between the zinc
  electrode and ammonium ions. After some time, the
  zinc case becomes thinner and the paste leaks
  out. The leakage problem can be solved by
  enclosing the whole cell in a steel or plastic
  case.

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Alkaline manganese cell
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Alkaline manganese cell

• Anode (oxidation / -ve electrode) zinc powder
• Cathode (reduction / ve electrode)
  manganese(IV) oxide
• Electrolyte potassium hydroxide

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Alkaline manganese cell

• Anode (oxidation / -ve electrode)
• Zn(s) 2OH(aq) ? ZnO(s) H2O(l) 2e
• Cathode (reduction / ve electrode)
• 2MnO2(s) H2O(l) 2e ? Mn2O3(s) 2OH(aq)
• The overall cell reaction is
• Zn(s) 2MnO2(s) ? ZnO(s) Mn2O3(s)

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Alkaline manganese cell

• Its lifetime is longer than that of a zinc-carbon
  cell.
• The outer steel case is not involved in the
  reaction. Therefore, this cell does not leak.
• It is used when larger currents are needed, for
  example in motorized toys.
• It is much more expensive than a zinc-carbon
  cell.
• It gives 1.5 V.

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Silver oxide cell
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Silver oxide cell

• Anode (oxidation / -ve electrode) zinc powder
• Cathode (reduction / ve electrode) silver oxide
• Electrolyte potassium hydroxide

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Silver oxide cell

• Anode (oxidation / -ve electrode)
• Zn(s) 2OH(aq) ? ZnO(s) H2O(l) 2e
• Cathode (reduction / ve electrode)
• Ag2O(s) H2O(l) 2e ? 2Ag(s) 2OH(aq)
• The overall cell reaction is
• Zn(s) Ag2O(s) ? ZnO(s) 2Ag(s)

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Silver oxide cell

• The silver oxide cell is small.
• It lasts for a long time.
• It also gives a steady current.
• It is more expensive than other types of dry
  cell.
• It gives 1.5 V.

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Nickel-cadmium cell (Ni-Cd)

• Anode (oxidation / -ve electrode) cadmium
• Cathode (reduction / ve electrode) nickel(IV)
  oxide
• Electrolyte potassium hydroxide

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Nickel-cadmium cell (Ni-Cd)

• It is classified as a secondary cell
  (rechargeable cell).
• It gives a larger current.
• It is more expensive.
• It gives 1.25 volts.
• When the cell is recharged, an electric current
  is passed through it in the direction opposite to
  that of the cell reaction.

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Pollution problems

• Cells contain toxic materials.
• mercury in zinc-carbon cell
• cadmium in nickel-cadmium cell
• Materials inside the cells do not decompose even
  after a long time.