Everything Science Grade 12

5.5 CHAPTER 5. THE CHEMICAL INDUSTRY

• A battery consists of one or more voltaic cells, each of which is madeup of two
  half cells that are connected in series by a conductive electrolyte. Eachhalf cell
  has a net electromotiveforce (emf) or voltage.The net voltage of the battery is
  the difference betweenthe voltages of the half-cells. This potential difference
  between the two half cells is what causes an electric current to flow.


• A primary battery cannot be recharged, but a secondary battery can be recharged.


• The capacity of a battery depends on the chemical reactions in the cells, the
  quantity of electrolyteand electrode material in the cell, and the discharge
  conditions of the battery.


• The relationship between the current, discharge time and capacity of abattery is
  expressed by Peukert’s law:

Cp= Ikt

In the equation, ’Cp’ represents the battery’s capacity (Ah), I is the discharge

current (A), k is the Peukert constant and t is thetime of discharge (hours).

• Two common types ofbatteries are lead-acid batteries and the zinc-carbon dry
  cell.


• In a lead-acid battery, each cell consists of electrodes of lead (Pb) and lead (IV)
  oxide (PbO 2 ) in an electrolyte of sulfuric acid (H 2 SO 4 ). When the battery dis-
  charges, both electrodes turn into lead (II) sulphate (PbSO 4 ) and the electrolyte
  loses sulfuric acid to become mostly water.


• A zinc-carbon cell is made up of an outer zinc container, which acts as the anode.
  The cathode is the central carbon rod, surrounded by a mixture of carbon and
  manganese (IV) oxide (MnO 2 ). The electrolyte is a paste of ammonium chloride
  (NH 4 Cl). A fibrous fabric separates the two electrodes, and a brass pin in the
  centre of the cell conducts electricity to the outside circuit.


• Despite their many advantages, batteries are made of potentially toxicmaterials
  and can be damaging tothe environment.

Chapter 5 End of Chapter Exercises

1. Give one word or term for each of the following descriptions:
   (a) A solid organic compound that can be usedto produce liquid
   fuels.
   (b) The process used toconvert heavy hydrocarbons into light hydro-
   carbons.
   (c) The process of separating nitrogen from liquid air.
   (d) The main raw material in the chloralkali industry.