Chemistry Kenya
Effect of an Electric Current on Substances
Effect of an Electric Current on Substances
Effect of an Electric Current on Substances
Conduction of electricity
Conductors and non-conductors
Test for conduction of electricity by; Solids, metals and non-metals;( wood, aluminium foil, sodium chloride, sugar and lead (II) bromide), aqueous solutions of sugar, urea, copper (II) chloride, sodium chloride and mineral acids, melts: Sulphur, lead (II)bromide or lead (II)iodide and sugar
Electrolytes and non-electrolytes
The conductivity of ionic compounds is explained by the fact that ions move in a particular direction in an electric field. This can be shown in experiments with coloured salts. For example, copper (II) chromate (VI) (CuCrO4) dissolves in water to give a green solution. This solution is placed in a U-tube. A colourless solution of dilute hydrochloric acid (HCl) is then layered on top of the salt solution in each arm. Graphite rods are fitted as shown in figure 13.3. These rods (electrodes) carry the current into and out of the solution.
After passing the current for a short time, the solution around the cathode becomes blue. Around the anode, the solution becomes yellow. These colours are produced by the movement (migration) of the ions in the salt. The positive copper ions (Cu2+) are blue in solution. They are attracted to the cathode (negative electrode). The negative chromate ions (CrO42-) are yellow in solution. They are attracted to the anode (the positive electrode). The use of coloured ions in solution has shown the direction that positive and negative ions move in an electric field. Always positive ions (cations) move to the cathode and negative ions (anions) move to the anode.
Ions as the particles in electrolyte solutions and melts
Molecules as the particles i non-electrolyte solutions and melts
Passage of a direct electric current through an electrolyte (electrolysis of molten lead (II)bromide or lead(II)iodide
Anode and cathode
Applications of electrolysis: - electroplating, production and purification of metals. NOTE: Details of the processes not required at this point.
Some metals can be purified by means of electrolysis. This process is used in industry to purify copper, which must be very pure 99.9% for electrical wiring. Copper made by roasting the sulphide ore is about 99.5% pure (so it has an impurity level of 0.5%). This level of impurity cuts down electrical conductivity significantly.
This is how the electrolytic purification (refining) process is carried out:The anode is made of a large block of impure copper. The cathode is a thin sheet of pure copper. The electrolyte is copper (II) sulphate solution.During the refining process, the copper atoms of the impure block become ions (the anode dissolves).Cu → Cu2+ + 2e-
The ions from the solution become atoms.
Cu2+ + 2e- → Cu(s)
They stick onto the cathode. A layer of pure copper builds up on the cathode. As electrolysis takes place, the cathode gains mass as copper is deposited on it. As a result, the cathode gets smaller while the cathode gets bigger as electrolysis proceeds. Eventually the whole cathode dissolves.
Only pure copper sticks to the cathode. Most impurities fall to the bottom of the electrolytic cell. They form a solid material (anode sludge or slime) which contains small quantities of precious metals such as silver, gold and platinum. The precious metals recovered from the slime are purified and sold.
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