Alkaline dry cell

If too large a current is drawn from a Ledanche cell, the ammonia forms a gaseous insulating layer around the carbon cathode. When this happens, the voltage drops sharply and then returns slowly to its normal value of 1.5 V. This problem can be avoided by using an alkaline dry cell, in which the paste between the electrodes contains KOH rather than NH4CL In this case the overall cell reaction is simply... 

No gas is produced. The alkaline dry cell, although more expensive than the Leclanche cell, has a longer shelf life and provides more current. 

Schematic diagram and examples of alkaline dry cells. These batteries provide electrical current for many portable devices.

The alkaline dry cell is a modified version of the Leclanche cell in which the acidic NH4C1 electrolyte of the Leclanche cell is replaced by a basic electrolyte, either NaOH or KOH. As in the Leclanche cell, the electrode reactions involve oxidation of zinc and reduction of manganese dioxide, but the oxidation product is zinc oxide, as is appropriate to the basic conditions ... 

Closely related to the alkaline dry cell is the mercury battery, often used in watches, heart pacemakers, and other devices where a battery of small size is required (Figure 18.10). The anode of the mercury battery is zinc, as in the alkaline dry cell, but the cathode is steel in contact with mercury(II) oxide (HgO) in an alkaline medium of KOH and Zn(OH)2. Zinc is oxidized at the anode, and HgO is reduced at the cathode ... 

The alkaline dry cell lasts longer than the acidic cell mainly because the zinc anode corrodes less rapidly under basic conditions than under acidic conditions. 

What is reduced and what is oxidized in the ordinary zinc-carbon dry cell battery What features make the alkaline dry cell an improvement over the earlier type of dry cell battery ... 

The Teclanche cell has the disadvantage that its concentrations change with time, and thus the voltage of the battery falls as it is used. In an alkaline dry cell, the ammonium chloride is replaced by potassium hydroxide, and the half-cell reactions become... 

A third primary dry cell is the zinc-mercuric oxide cell depicted in Figure 17.7. It is commonly given the shape of a small button and is used in automatic cameras, hearing aids, digital calculators, and quartz-electric watches. This battery has an anode that is a mixture of mercury and zinc and a steel cathode in contact with solid mercury(II) oxide (HgO). The electrolyte is a 45% KOH solution that saturates an absorbent material. The anode half-reaction is the same as that in an alkaline dry cell,... 

Figure 21-15 (a) The Leclanche cell is a dry cell that generates a potential difference of about 1.6 volts, (b) Some commercial alkaline dry cells. 

Alkaline dry cells are similar to Leclanche dry cells except that (1) the electrolyte is basic (alkaline) because it contains KOH, and (2) the interior surface of the Zn container is rough this gives a larger surface area. Alkaline cells have a longer shelf life than ordinary dry cells, and they stand up better under heavy use. The voltage of an alkaline cell is about 1.5 volts. During discharge, the alkaline dry cell reactions are... 

With regard to cylindrical manganese and alkaline dry cells, which account for some 98% (on a weight basis) of all primary cells sold in Japan, no mercury has been used in the production since 1992. Furthermore, as the production of mercury cells was stopped at the end of 1995, the amount of mercury contained in the dry cells sold in Japan was equal to 1.4 tons in 1997, i.e. only 3% with respect to the 1985 s total (45 tons). In addition, as button type cells (alkaline, silver oxide and zinc-air cells) still contain a small amount of mercury, BAJ will continue to reduce it and promote the collection and recycling of spent dry cells. 

Why does an alkaline dry cell battery typically last longer than a normal dry cell Write the chemical equation for the overall cell reaction in an alkaline dry cell. 

Alkaline batteries A more efficient alkaline dry cell, shown in Figure 20.9, is replacing the standard zinc-carbon dry cell in many applications. In the alkaline cell, the zinc is in a powdered form, which provides more surface area for reaction. The zinc is mixed in a paste with potassium hydroxide, a strong base, and the paste is contained in a steel case. The cathode mixture is manganese(IV) oxide, also mixed with potassium hydroxide. The anode half-cell reaction is as follows. 

SECTION 20.7 A battery is a self-contained electrochemical power source that contains one or more voltaic cells. Batteries are based on a variety of different redox reactions. Several common batteries were discussed. The lead-acid battery, the nickel-cadmium battery, the nickel-metal-hydride battery, and the lithium-ion battery are examples of rechargeable batteries. The common alkaline dry cell is not rechargeable. Fuel cells are voltaic cells that utilize redox reactions in which reactants such as H2 have to be continuously supplied to the cell to generate voltage. 

The only power sources feasible for all these portables are electrochemical batteries. Disposable batteries were the classical power source for flashlights, and still hold a very strong position (salt Leclanche later, alkaline dry cells of zinc-manganese dioxide type). Rechargeable batteries became ever more important first, nickel-cadmium, and more recently, nickel-hydride and lithium ion batteries. For convenient handling, a power source is usually placed somewhere inside the device, so it should respect certain limitations as to weight and volume. As a rule of thumb, a power source should not exceed 30-40% by mass and volume of the device powered by it. A similar upper limit 30-40% applies to the cost. 

Other primary cells are also used as batteries for certain applications. In some instances, the battery must be quite small. For a medical device, such as a heart pacemaker, a battery should not only be small, but long lasting. Mercury batteries have come to fill this role. In the mercury battery, shown in Figure 13.14, zinc is the anode as in the alkaline dry cell ... 

In the alkaline dry cell (alkaline battery), the acidic ammonium chloride of the regular dry cell is replaced by basic (alkaline) potassium hydroxide. With this chemical, corrosion of the zinc case is greatly reduced. 

An alkaline dry cell (Figure 20.10) is similar to the Leclanche cell, but it has potassium hydroxide in place of ammonium chloride. This cell performs better under current drain and in cold weather. The half-reactions are... 

Nernst equation (20.7) zinc-carbon (Leclanche) dry cell (20.8) alkaline dry cell (20.8) lithium-iodine battery (20.8) lead storage cell (20.8) nickel-cadmium cell (20.8)... 

Dry cell see Alkaline dry cell and Zinc-carbon dry cell. 

The batteries found in a portable compact disc player or other small electronic device are frequently alkaline dry cells. These cells do not have a carbon rod cathode, as in the zinc-carbon cell. The absence of the carbon rod allows them to be smaller. Figure 2.3b shows a model of an alkaline battery. This cell uses a paste of Zn metal and potassium hydroxide instead of a solid metal anode. The half-reaction at the anode is as follows. 

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