ATAR Chemistry Unit 3
Extended Response – Electrochemical Cells
Primary cells
A primary cell is a non-rechargeable galvanic cell and only contains a fixed amount of oxidant and
reductant; once the active species and consumed then it is considered ‘flat’. Some examples of primary
cells include the zinc-carbon dry cell, alkaline dry cell, and lithium cell.
Dry cell (Zinc-carbon battery)
The dry cell uses zinc as the reducing agent and manganese oxide
as the oxidising agent. A graphite rod and powdered graphite
conduct electrons to the surface of individual manganese dioxide
particles within the cathode paste. Ammonium chloride is also
incorporated into the cathode paste and serves as a salt bridge as
well as providing protons needed in the reduction of manganese
dioxide. However, the use of ammonium ions is problematic as
over time the acidic nature causes the zinc anode to dissolve
forming zinc ions, resulting in a low shelf life.
Anode half reaction: Zn(s) → Zn2+(aq) + 2 e- E° = -0.7 V
Cathode half reaction: 2MnO2(s) + 2NH4+(aq) + 2 e- → Mn2O3(s) + 2NH3(aq) + H2O(l) E° = 1.28 V
Overall reaction: 2MnO2(s) + 2NH4+(aq) + Zn(s) → Zn2+ + Mn2O3(s) + 2NH3(aq) + H2O(l) E° = 1.5* V
* the theoretical voltage of each cell should be -0.76 + 1.23 = 1.99 V, but considering many practical
conditions, the actual output is no more than 1.5 V.
Uses: remote controls, portable radios, toys, torches, smoke detectors
Advantages: inexpensive, available in many sizes, negligible environmental impact (non-hazardous)
Disadvantages: lower energy density, at high currents, NH3 builds up causing drop in voltage, and short
shelf life due to zinc anode reacting with the acidic NH4+ ions
Alkaline dry cell
As in the dry cell, zinc is used as the reducing agent and manganese
dioxide as the oxidising agent. Instead of an ammonium chloride
electrolyte it uses potassium hydroxide in the electrolyte paste. This
considerably improves its shelf life by illuminating the effect of
ammonium ions that dissolve the zinc anode of a dry cell. The zinc
anode is in the form of powdered zinc instead of zinc foil as in the
dry cell. Alkaline cells contain a greater mass of reductant an oxidant
than a dry cell of similar size.
Anode half reaction: Zn(s) + 2OH-(aq) → ZnO(s) + H2O(l) + 2e- E° = 1.28 V
Cathode half reaction: 2MnO2(s) + H2O(l) + 2e- → Mn2O3(s) + 2OH(aq) E° = 0.15 V
Overall reaction: Zn(s) + 2MnO2(s) → ZnO(s) + Mn2O3(s) E° = 1.43 V
Uses: toys, portable radios, CD players, electronic games, torches
Advantages: longer shelf life by eliminating ammonium ions that dissolve zinc anode, more constant
voltage than other dry cells, higher energy density, negligible environmental impact (non-hazardous)
Disadvantages: expensive
Extended Response – Electrochemical Cells
Primary cells
A primary cell is a non-rechargeable galvanic cell and only contains a fixed amount of oxidant and
reductant; once the active species and consumed then it is considered ‘flat’. Some examples of primary
cells include the zinc-carbon dry cell, alkaline dry cell, and lithium cell.
Dry cell (Zinc-carbon battery)
The dry cell uses zinc as the reducing agent and manganese oxide
as the oxidising agent. A graphite rod and powdered graphite
conduct electrons to the surface of individual manganese dioxide
particles within the cathode paste. Ammonium chloride is also
incorporated into the cathode paste and serves as a salt bridge as
well as providing protons needed in the reduction of manganese
dioxide. However, the use of ammonium ions is problematic as
over time the acidic nature causes the zinc anode to dissolve
forming zinc ions, resulting in a low shelf life.
Anode half reaction: Zn(s) → Zn2+(aq) + 2 e- E° = -0.7 V
Cathode half reaction: 2MnO2(s) + 2NH4+(aq) + 2 e- → Mn2O3(s) + 2NH3(aq) + H2O(l) E° = 1.28 V
Overall reaction: 2MnO2(s) + 2NH4+(aq) + Zn(s) → Zn2+ + Mn2O3(s) + 2NH3(aq) + H2O(l) E° = 1.5* V
* the theoretical voltage of each cell should be -0.76 + 1.23 = 1.99 V, but considering many practical
conditions, the actual output is no more than 1.5 V.
Uses: remote controls, portable radios, toys, torches, smoke detectors
Advantages: inexpensive, available in many sizes, negligible environmental impact (non-hazardous)
Disadvantages: lower energy density, at high currents, NH3 builds up causing drop in voltage, and short
shelf life due to zinc anode reacting with the acidic NH4+ ions
Alkaline dry cell
As in the dry cell, zinc is used as the reducing agent and manganese
dioxide as the oxidising agent. Instead of an ammonium chloride
electrolyte it uses potassium hydroxide in the electrolyte paste. This
considerably improves its shelf life by illuminating the effect of
ammonium ions that dissolve the zinc anode of a dry cell. The zinc
anode is in the form of powdered zinc instead of zinc foil as in the
dry cell. Alkaline cells contain a greater mass of reductant an oxidant
than a dry cell of similar size.
Anode half reaction: Zn(s) + 2OH-(aq) → ZnO(s) + H2O(l) + 2e- E° = 1.28 V
Cathode half reaction: 2MnO2(s) + H2O(l) + 2e- → Mn2O3(s) + 2OH(aq) E° = 0.15 V
Overall reaction: Zn(s) + 2MnO2(s) → ZnO(s) + Mn2O3(s) E° = 1.43 V
Uses: toys, portable radios, CD players, electronic games, torches
Advantages: longer shelf life by eliminating ammonium ions that dissolve zinc anode, more constant
voltage than other dry cells, higher energy density, negligible environmental impact (non-hazardous)
Disadvantages: expensive
