Solutions and Electrochemistry VI
Primary Cells
Sealed system
Stores electrical energy
Not rechargeable
Leclanche dry cells
Zinc chloride batteries
Alkaline batteries
Lithium iodine batteries
Zinc mercuric batteries
Zinc silver-oxide batteries
Leclanche Dry Cell
Zn ---> Zn2+ + 2e-
2NH4+ + 2MnO2 +2e- ----> Mn2O3 + H20 + 2NH3
Zn2+ + 4NH3 ---> [Zn(NH3)4]2+
Sparingly soluble
Not rechargeable
Torches / radios
Non-toxic
No liquid electrolyte
High energy density
Inexpensive
Low current delivery
Not rechargeable
Secondary Cells
Sealed system
Stores electrical energy
Rechargeable
Lead acid battery
6 x 2V cells in series in cars (12V)
Grids give large surface area
Pb + HSO4- ----> PbSO4 +H+ +2e-
PbO2 +H2SO4 + 2H+ +2e- ---->PbSO4 +2H2O
Overall: Pb + PbO2 +2SO4(2-) + 4H+ ----> 2PbSO4 + 2H2O
Cheap
High energy density
Robust
High current
Toxic
Corrosive
Liquid electrolyte
Low power density
Fuel Cells
Electrical energy supplied
Continuous supply if you keep adding reagents
Hydrogen Fuel Cell
Efficiency = Free energy of cell/ Enthalpy of combustion
Semi permeable Ni membrane coated with Pt
H2 ---> 2e- + 2H+
1/2O2 +2H+ +2e- ---> H2O
H2 +1/2O2 ---> H2O
Proton Exchange Membrane
Portable
Lightweight
Long life
Cheap to run
Not safe?
Catalyst poisoning
Exepsnive to prdocue