Page 30 - Curriculum Visions Dynamic Book

Sulphuric acid as an electrolyte
Dilute sulphuric acid is used in vehicle batteries, sometimes called “wet batteries”, and more accurately called “secondary batteries”. They are called secondary batteries because the cells inside the battery can be recharged. (Contrast this with a primary cell, like a dry cell, where the chemicals inside the cell react to produce electricity only once). In a vehicle battery six cells are combined to give the twelve volt supply that vehicles use worldwide.
Lead-acid batteries are the most commonly used form of vehicle battery. To work as an electrical battery, each of the cells inside it must have two electrodes made of electrically conducting materials. These must be bathed in a liquid that can conduct electricity as well as help the battery store electricity. This liquid is called an electrolyte.
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 Lead-acid batteries are designed
to give each electrode a large surface area. This allows it to change stored chemical energy into electrical energy quickly. The electrodes are made from a lead alloy; half are covered in a paste of lead sulphate. All the electrodes are bathed in dilute sulphuric acid.
Operation of the battery
Positive electrode made of lead dioxide
Sulphuric acid electrolyte
+
Negative electrode made of lead
As the battery is charged (perhaps from
a vehicle generator), and a current is passed through the battery, a chemical reaction occurs that increases the concentration of the sulphuric acid and forms a coating of lead and lead dioxide on the electrodes. This process is electrolysis (the same process that puts chromium coatings on cutlery, gold and silver-plating on ornaments and tin-plate on steel
food cans).
When the battery is called upon to
discharge electricity, the reaction reverses, the lead coatings are converted to lead sulphate and the sulphuric acid is used up.
This process can be repeated many times, giving the secondary cell a useful life of many years of constant service.
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