Primary Batteries

Primary Batteries

Primary batteries are single-use batteries because they cannot be recharged. A common primary battery is the dry cell (see the figure below). The dry cell is a zinc-carbon battery. The zinc can serves as both a container and the negative electrode. The positive electrode is a rod made of carbon that is surrounded by a paste of manganese(IV) oxide, zinc chloride, ammonium chloride, carbon powder, and a small amount of water. The reaction at the anode can be represented as the ordinary oxidation of zinc:

\(\begin{array}{ll}\text{Zn}(s)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{Zn}}^{2+}(aq)+2{\text{e}}^{\text{−}}\hfill & \phantom{\rule{3em}{0ex}}{E}_{{\text{Zn}}^{2+}\text{/Zn}}^{°}=-0.76\text{18 V}\hfill \end{array}\)

The reaction at the cathode is more complicated, in part because more than one reaction occurs. The series of reactions that occurs at the cathode is approximately

\(2{\text{MnO}}_{2}(s)+2{\text{NH}}_{4}\text{Cl}(aq)+2\phantom{\rule{0.2em}{0ex}}{\text{e}}^{\text{−}}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{Mn}}_{2}{\text{O}}_{3}(s)+2{\text{NH}}_{3}(aq)+{\text{H}}_{2}\text{O}(l)+2{\text{Cl}}^{\text{−}}\)

The overall reaction for the zinc–carbon battery can be represented as

\(2{\text{MnO}}_{2}(s)+2{\text{NH}}_{4}\text{Cl}(aq)+\text{Zn}(s)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{Zn}}^{2+}(aq)+{\text{Mn}}_{2}{\text{O}}_{3}(s)+2{\text{NH}}_{3}(aq)+{\text{H}}_{2}\text{O}(l)+2{\text{Cl}}^{\text{−}}\)

with an overall cell potential which is initially about 1.5 V, but decreases as the battery is used.

It is important to remember that the voltage delivered by a battery is the same regardless of the size of a battery. For this reason, D, C, A, AA, and AAA batteries all have the same voltage rating. However, larger batteries can deliver more moles of electrons. As the zinc container oxidizes, its contents eventually leak out, so this type of battery should not be left in any electrical device for extended periods.

The diagram shows a cross section of a flashlight battery, a zinc-carbon dry cell.

Optional Video:

Watch the video below to learn more about zinc-carbon batteries.

Alkaline batteries (see the figure below) were developed in the 1950s partly to address some of the performance issues with zinc–carbon dry cells. They are manufactured to be exact replacements for zinc-carbon dry cells. As their name suggests, these types of batteries use alkaline electrolytes, often potassium hydroxide. The reactions are

\(\begin{array}{l}\underset{¯}{\begin{array}{l}\text{anode:}\phantom{\rule{4.5em}{0ex}}\text{Zn}(s)+2{\text{OH}}^{\text{−}}(aq)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}\text{ZnO}(s)+{\text{H}}_{2}\text{O}(l)+{\text{2e}}^{\text{−}}\phantom{\rule{4em}{0ex}}{E}_{\text{anode}}^{°}=\text{−1.28 V}\\ \text{cathode:}\phantom{\rule{0.2em}{0ex}}2{\text{MnO}}_{2}(s)+{\text{H}}_{2}\text{O}(l)+{\text{2e}}^{\text{−}}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{Mn}}_{2}{\text{O}}_{3}(s)+{\text{2OH}}^{\text{−}}(aq)\phantom{\rule{4em}{0ex}}{E}_{\text{cathode}}^{°}=\text{+0.15 V}\end{array}}\\ \hline \text{overall:}\phantom{\rule{4.2em}{0ex}}\text{Zn}(s)+{\text{2MnO}}_{2}(s)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}\text{ZnO}(s)+{\text{Mn}}_{2}{\text{O}}_{3}(s)\phantom{\rule{4em}{0ex}}{E}_{\text{cell}}^{°}=\text{+1.43 V}\end{array}\)

An alkaline battery can deliver about three to five times the energy of a zinc-carbon dry cell of similar size. Alkaline batteries are prone to leaking potassium hydroxide, so these should also be removed from devices for long-term storage. While some alkaline batteries are rechargeable, most are not. Attempts to recharge an alkaline battery that is not rechargeable often leads to rupture of the battery and leakage of the potassium hydroxide electrolyte.

Alkaline batteries were designed as direct replacements for zinc-carbon (dry cell) batteries.