The Lead Storage Battery

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Positive plates lead grids filled with Pb02 Negative plates lead grids filled with spongy lead 

O Six of these cells are connected together in series, although only three are shown, so that their voltages add to make a 12-volt battery. Although not shown, the cathodes are all connected In series, as are the anodes. 

The lead ions then react with hydrogen sulfate ions from the sulfuric acid to form insoluble lead(n) sulfate. This coats the lead electrode. 

The net cell reaction for discharge and its standard potential are obtained by adding the net anode and cathode half-reactions and their tabulated potentials. The tabulated E° value for the anode half-reaction is reversed in sign because it occurs as oxidation during discharge. 

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The lead storage battery, the largest single user of lead and its compounds, is made possible by the high degree of reversibiUty, both chemical and physical, in the fundamental chemical reaction... 

Values taken from S. Glasstone. Thermodynamics for Chemists. D. Van Nostrand Company Inc., Toronto, p. 443 (1947). The values tabulated in this reference were taken from D. N. Craig and G. W. Vinal, J. Res. Natl. Bur. Stand.. Thermodynamic Properties of Sulfuric Acid Solutions and Their Relation to the Electromotive Force and Heat of Reaction of the Lead Storage Battery", 24, 475-490 (1940). More recent values at the higher molality can be found in W. F. Giauque. E. W. Hornung. J. E. Kunzler and T. R. Rubin, The Thermodynamic Properties of Aqueous Sulfuric Acid Solutions and Hydrates from 15 to 300° K", J. Am. Chem. Soc.. 82, 62-70 (1960). 

In an electrochemical cell, electrical work is obtained from an oxidation-reduction reaction. For example, consider the process that occurs during the discharge of the lead storage battery (cell). Figure 9.3 shows a schematic drawing of this cell. One of the electrodes (anode)q is Pb metal and the other (cathode) is Pb02 coated on a conducting metal (Pb is usually used). The two electrodes are immersed in an aqueous sulfuric acid solution. 

Figure 9.3 The lead storage battery. The key to obtaining electrical energy from a redox chemical reaction is to physically separate the two half-cell reactions so that electrons are transferred from the anode through an external circuit to the cathode. In the process, electrical work is accomplished.

To apply the features that characterize galvanic cells, Example describes the lead storage battery. 

The lead storage battery provides electrical power in automobiles. It is well suited for this use because it supplies the large current needed to drive starter motors and headlights and can be recharged easily. Figure 19-20 shows the lead storage cell in a schematic view. The half-reactions are the subject of Example ... 

C19-0090. Explain why the lead storage battery, despite being the battery of choice for automobiles, is not suitable for space flights. 

Because of these damaging effects, most uses of lead that involve direct exposure for humans are being phased out. Unleaded gasoline and lead-free paints have replaced two former major commercial uses of lead. Lead has proved to be Indispensable, however, in the lead storage battery, which now provides the major use of this metal. Although leakage from damaged batteries is still a potential hazard, contemporary batteries are manufactured in such a way that human exposure to battery contents is minimized. 

The lead storage cell (six of which make up the lead storage battery commonly used in automobiles) will be discussed as an example of a practical cell. The cell, pictured in Fig. 14-2, consists of a lead electrode and a lead dioxide electrode immersed in relatively concentrated H2S04 in a single container. When the cell delivers power (when it is used), the electrodes react as follows ... 

Pb02 is the oxidizing agent and lead is the reducing agent in the lead storage battery. The chemistry of the battery can be summarized by the following equation ... 

In the lead storage battery, insoluble lead sulfate, PbS04(s), is produced at both the anode and cathode during cell discharge. 

Antimony alloys have many commercial applications. The metal makes its alloys hard and stiff and imparts resistance to corrosion. Such alloys are used in battery grids and parts, tank linings, pipes and pumps. The lead plates in the lead storage batteries constitute 94% lead and 6% antimony. Babbit metal, an alloy of antimony, tin, and copper is used to make antifriction machine bearings. Alloys made from very high purity grade antimony with indium, gallium and bismuth are used as infrared detectors, diodes, hall effect devices and thermoelectric coolers. 

Secondary cells are voltaic cells that can be recharged repeatedly. The lead storage battery and nickel-cadmium cell are examples of secondary cells. The lead storage battery consists of six voltaic cells. Its electrodes are lead alloy plates, which take the form of a grill, filled with spongy lead metal. The cathode consists of another group of plates filled with lead (IV) oxide, P6O2. Dilute sulfuric acid is the electrolyte of the cell. When the battery delivers a current, the lead is oxidized to lead ions, which combine with sulfate fS0 7 ions of the electrolyte to cover the lead electrode. 

The lead storage battery can be recharged by passing a direct current through the cell in the reverse direction. The electrical energy required to make this reaction happen is furnished by an alternator to convert alternating current to direct current The reverse reaction is as follows ... 

Another type of rechargeable battery is the nickel-cadmium, Ni-Cd, battery, cadmium acts as an anode, and nickel (IV) oxide is reduced to nickel (II) hydroxide, Ni(0H)2, at the cathode. As in the lead storage battery, the nickel-cadmium type can be recharged indefinitely. 

As important as batteries are to modern civilization, a number of inherent problems are associated with them. For example, the lead-storage battery present in all modern motor vehicles is very heavy (it contains one of the densest of common metals, lead) and it must be continually recharged, eventually wears out, and presents serious environmental problems during its manufacture and disposal. After the discovery of conductive polymers, many scientists hoped and dreamed that these materials could he used to make efficient, lightweight plastic batteries. 

Pb02 is the oxidizing agent in the lead storage battery where the reducing agent is metallic lead. The overall reaction for the battery is... 

The lead storage cell, six of which constitute the lead storage battery, is familiar for its use in most automobiles. It will be described in some detail because it has several features that are different from those of the Daniell cell, described earlier. The lead storage cell (Figure 17.2) consists of a lead electrode coated with a paste of lead(II) sulfate and another electrode which has lead(IV) oxide as the active oxidizing agent, also coated with lead(II) sulfate. The electrolyte is concentrated sulfuric acid, in which PbS04 is insoluble. Both electrodes are situated in the same solution. The half-reactions are... 

Since about 1912 when self-starters were first used in automobiles, the lead storage battery has been a major factor in making the automobile a practical means of transportation. This type of battery can function for several years under temperature extremes from —30°F to 100°F and under incessant punishment from rough roads. 

As in the lead storage battery, the products adhere to the electrodes. Therefore, a nickel-cadmium battery can be recharged an indefinite number of times. 

Yellow lead(II) oxide, known as litharge, is widely used to glaze ceramic ware. Lead(IV) oxide does not exist in nature, but a substance with the formula PbOj.9 can be produced in the laboratory by oxidation of lead(II) compounds in basic solution. The nonstoichiometric nature of this compound is caused by defects in the crystal structure. The crystal has some vacancies in positions where there should be oxide ions. These imperfections in the crystal (called lattice defects) make lead(IV) oxide an electrical conductor, since the oxide ions jump from hole to hole. This makes possible the use of lead(IV) oxide as an electrode (the cathode) in the lead storage battery. 

Lead dioxide, PbOg, is a brown substance made by oxidizing a solution of sodium plumbite, NaoPb(OH)4, with hypochlorite ion, or by anodic oxidation of lead sulfate. It is soluble in sodium "hydroxide and potassium hydroxide, forming the hexahydroxyplumbate ion, Pb(OH)g—. The principal use of lead dioxide is in the lead storage battery (Chap. 14). 

A new vanadium pentoxide battery produces more electrical energy per pound than the lead storage batteries in cars today. They are also likely to cause fewer environmental disposal problems. 

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