Dry Cells

Dry cells are the things that most of us commonly call batteries. Specifically those tubes with the bump on the end that come in sizes ranging from AAA to D. What follows is a rough description of the structure and function of a dry cell. It is worth noting that companies are always researching and improving these cells, so what is inside your flashlight may be somewhat different from what is shown here. 

Making things even more complicated, there have been a number of different types of dry cells produced and each type is built and functions slightly differently. We will look at two of the common types here.

The Zinc-Carbon Dry Cell

The Reactions

The reactions that occur in a zinc-carbon cell are the following:

\(Zn \rightarrow Zn^{+2} + 2~e^{-1}\)

\(2~MnO_2 + 2~NH_4Cl + H_2O + 2~e^{-1} \rightarrow Mn_2O_3 + 2~NH_4OH + 2~Cl^{-1}\)

The second reaction mixture is a paste with just enough water to allow ions to move, but not enough to make it "wet" or to risk spilling.

In addition, there is a secondary reaction that the zinc undergoes which is explained below.

The Construction

The basic construction of the zinc-carbon dry cell is shown below.

Electrons are pulled in through the bump on the top and are passed down through the graphite electrode where the \(MnO_2\) takes them.

Those electrons are taken (eventually) from the zinc can that comprises the outer shell.

Two Problems

There are two problems with this that need to be solved. The first is that we need to make sure that the electron theft reaction ONLY occurs by passing electrons through our flashlight (or calculator, etc) and not through direct contact. Up until now we have managed that by putting the half-reactions in separate containers. Here, however, the two half-reactions are in the same container.

The second problem is that we have a charge issue. Remember that in our first galvanic cell, we needed a salt bridge to allow the current to continue flowing. The same issue occurs here. As the zinc reacts, \(Zn^{+2}\) ions collect at the outside. This will make it less and less likely that electrons will move away from this positive charge. At the same time, as electrons enter the middle of the cell, the paste becomes negative which will discourage additional electrons from entering.

Solving Those Problems

The solution to both problems is the porous separator.

The porous separator is a “can” of heavy duty paper that fits just inside the router zinc can. The \(MnO_2\) paste is held inside this paper can and prevented from touching the zinc directly. That solves the first problem. The charge problem is solved because the \(Zn^{+2}\) ions move in through the pores in the paper. They do this because they are attracted to the increasing negative charge inside. When they move, they solve both charge problems. The outer can stops being positive and the positive ions balance the increasing negative charge inside.

Once inside, the \(Zn^{+2}\) ions react with chloride ions (\(Cl^{-1}\)) to form \(ZnCl_2\). This is the secondary Zinc reaction mentioned above and it stabilizes the zinc ions inside the paste.

The Alkaline Dry Cell

Alkaline cells use similar chemistry to the classic zinc-carbon cell, but the paste is basic (alkaline) rather than acidic.

The Reactions

The half-reactions that occur in an alkaline cell are these:

\(Zn + 2~OH^{-1} \rightarrow ZnO + H_2O + 2~e^{-1}\)

\(2~MnO_2 + H_2O + 2~e^{-1}\rightarrow Mn_2O_3 + 2~OH^{-1}\)

In this construction, the Zn is in powder form and mixed in a paste that contains hydroxide. Since both reactants are now in paste form, the structure of the cell is somewhat different.

Here is a diagram (from Wikipedia) showing the structure of an alkaline dry cell.

Notice that, even though the zinc is no longer on the outside, it is still electrically connected to the flat bottom of the cell.

Alkaline cells are generally preferred to zinc-carbon cells these days because they have a higher energy density (you can get more life out of them) and a longer shelf life.