Definitive Study Guide on Primary Batteries and Electrochemical Cells

A battery is defined as a device that serves as a source of electrical energy, consisting of one or more than one cell connected in series.
In essence, a battery or cell is a galvanic cell where the chemical energy derived from a redox reaction is converted directly into electrical energy.
For a battery to be suitable for practical use, it must fulfill several criteria: - It should be reasonably light in weight. - It should be compact in size. - Its voltage should not vary appreciably during its operational use.

Primary batteries are characterized by chemical reactions that occur only once.
After a specific period of time, the chemicals are exhausted, and the battery becomes "dead."
These batteries cannot be reused or recharged.
The most familiar example of a primary battery is the dry cell, also known as the Leclanché cell, named after its discoverer.
Typical applications for dry cells include transistors and clocks.

Components and Structure (Fig. 2.8): - Anode: A zinc container ( $Zn$ ) which acts as the anode cup. - Cathode: A carbon (graphite) rod placed at the center. - Cathode Surroundings: The graphite rod is surrounded by carbon black and powdered manganese dioxide ( $MnO_2$ ). - Electrolytic Paste: The space between the anode and cathode is filled with a moist paste of ammonium chloride ( $NHCl$ ) and zinc chloride ( $ZnCl_2$ ).
Electrode Reactions: - Anode: $Zn(s) \rightarrow Zn^{2+} + 2e^-$ - Cathode: $MnO_2 + NH_1^+ + e^- \rightarrow\rightarrow MnO(OH) + NH$
Oxidation State Changes: In the cathode reaction, manganese is reduced from the $+4$ oxidation state to the $+3$ oxidation state in the form of $MnO(OH)$ .
Coordination Chemistry: The ammonia ( $NH$ ) produced during the cathode reaction forms a complex with the $Zn^{2+}$ ions to produce the complex ion $[Zn(NH_3)_2]^+$ (Note: documented based on transcript text).
Electrical Potential: The cell provides a constant potential of nearly $1.5\,V$ .

Suitability: The mercury cell is specifically suitable for low current devices such as hearing aids and watches.
Cell Components (Fig. 2.9): - Anode: Zinc-mercury amalgam ( $Zn(Hg)$ ). - Cathode: A paste composed of mercury(II) oxide ( $HgO$ ) and carbon. - Electrolyte: A paste made of potassium hydroxide ( $KOH$ ) and zinc oxide ( $ZnO$ ).
Electrode Reactions: - Anode Reaction: $Zn(Hg) + 2OH^- \rightarrow ZnO(s) + H_2O + 2e^-$ - Cathode Reaction: $HgO + H_2O + 2e^- \rightarrow Hg(l) + 2OH^-$