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