NCSU – Dept. of Chemistry – Lecture Demonstrations
Electrochemistry
Dry Cell (Pencil Battery)
Description: A dry-cell battery is constructed using MnO2 and zinc.
Materials:
MnO2
NH4Cl
Zn strip
Pencil (sharpened on both ends)
Deionized water
Voltmeter
Beaker
Connecting wires
Alligator clips
Procedure:
For large lecture halls, project demonstration using a document camera.
1. The electrolyte paste for this dry cell is generated in a beaker by mixing
86.0 g MnO2 and 53.5 g NH4Cl in 36 mL of deionized water.
2. Using the pencil and zinc strips as electrodes, connect the positive
terminal (red wire) to the sharpened pencil tip and the negative terminal
(black wire) to the zinc strip. Once the electrodes are connected, a
voltage close to 1.5 V should be produced.
Discussion:
A dry-cell is composed of a cathode (metal or graphite) surrounded by a moist
electrolyte paste. The paste is encased in the anode and the whole cell is
typically cased in another material to prevent leakage as shown in the figure
below. In this experiment, the chemistry of a standard dry-cell battery is
demonstrated inside a glass beaker. The sharpened pencil serves as the
graphite rod (an inactive cathode) and is immersed in an acidic (NH41+)
electrolyte paste containing the oxidizing agent, MnO2. The cell is completed by
the presence of the zinc electrode which serves as the anode. The two half
reactions taking place in this cell are shown below, although the reaction
pathway for the reduction is still unclear (one possible pathway is shown). A
reference discussing possible pathways is given. When constructed properly,
this dry cell produces around 1.5 V and can be used to power electronic devices
requiring small voltages.
Anode
Cathode
Zn → Zn2+ + 2 e12 MnO2 + 2 NH41+ + 2 e1- → Mn2O3 + 2 NH3 + H2O
NCSU – Dept. of Chemistry – Lecture Demonstrations
Electrochemistry
Safety: Wear proper protective equipment including gloves and safety glasses
when preparing and performing this demonstration. MnO2 is a strong oxidizing
agent.
Disposal: Electrolyte paste should be disposed of in a solid waste container.
Electrodes should be rinsed, dried and cleaned by scrubbing with steel wool.
References:
Shakhashiri, B. Z. In Chemical Demonstrations: A Handbook for Teachers of
Chemistry; The University of Wisconsin Press: 1992; Vol. 4, p 111-114.
Kozawa, A.; Powers, R. A. J. Chem. Educ. 1972, 49, 587. (discussion on MnO2
reduction pathway)