NCSU – Dept. of Chemistry – Lecture Demonstrations Electrochemistry Dry Cell (Pencil Battery) Description: A dry-cell battery is constructed using MnO 2 and zinc. Materials: MnO 2 Voltmeter NH 4Cl Beaker Zn strip Connecting wires Pencil (sharpened on both ends) Alligator clips Deionized water 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 MnO 2 and 53.5 g NH 4Cl 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 1+ graphite rod (an inactive cathode) and is immersed in an acidic (NH 4 ) electrolyte paste containing the oxidizing agent, MnO 2. 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 Zn → Zn 2+ + 2 e 1- 1+ 1- Cathode 2 MnO 2 + 2 NH 4 + 2 e → Mn 2O3 + 2 NH 3 + H 2O NCSU – Dept. of Chemistry – Lecture Demonstrations Electrochemistry Safety: Wear proper protective equipment including gloves and safety glasses when preparing and performing this demonstration. MnO 2 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 MnO 2 reduction pathway) .