5 Electricity and Chemistry Flashcards
Define electrolysis
The breakdown of an ionic compound, molten or in aqueous solution, by the passage of electricity
What are electrolytes
Compounds that can conduct electricity
has to be molten or dissolved in water (ions must be free to move around)
What is the anode
positive terminal of the power supply
What is the cathode
negative terminal of the power supply
Explain the electrolysis of molten lead(II) bromide (5)
Lead(II) bromide (PbBr2) is ionically bonded and contains Pb2+ ions and Br- ions.
When the solid is melted and voltage is applied, the ions are free to move
The positive lead ions move to the cathode and the bromide ions move to the anode
The products are lead and bromine
silvery deposits of lead form near the bottom of the dish and brown bromine vapour is found need the anode
What happens when an electric current passes through an electrolyte
new substance is formed
Why doesnt the electrodes undergo any chemical change
electrodes are usually made of carbon, which is inert
Explain electrolysis of concentrated sodium chloride solution
When concentrated sodium chloride solution is electrolysed, hydrogen ions (from the water solvent) form hydrogen molecules at the cathode and chloride ions form chlorine molecules at the anode
Explain electrolysis of copper(II) sulfate solution using carbon electrodes
Products: copper and oxygen
copper forms as a red-brown coating on the cathode (Cu2+ and H+ move to cathod and only Cu2+ ions are discharged)
bubbles of colourless gas, oxygen seen next to the anode
(SO4 2- and OH- move to the anode, but only OH- ions are discharged)
Explain electrolysis of dilute sulfuric acid (5)
hydrogen at cathode
oxygen at anode
Colourless gases at both electrodes
Cathode gas makes a lighted splint go ‘pop’
Anode gas relights a glowing splint
How to predict products of electrolysis of simple MOLTEN ionic compounds
metal form at cathode
non-metal form at anode
EG: electrolysis of aluminium oxide
aluminium —> cathode
oxygen —> anode
How to predict products of electrolysis for AQUEOUS solutions
cathode: metal below hydrogen in reactivity series —> deposited on the cathod
metal above hydrogen —> hydrogen gas deposited on cathode
Anode: depends on concentration of solution
What is electroplating
When a piece of one metal is coated in a thin layer of another metal
Why are metal objects electroplated (2)
prevent corrosion
improve appearance
Basic principles of electroplating
The object to be electrolplated is the cathode
The anode is composed of the metal to be plated onto the object
The electrolyte solution is a salt containing ions of the metal to be plated onto the objcet
Explain electrolysis of copper(II) sulfate solution using copper solution (2)
Copper deposited as a red coating at the cathode(Cu2+ ions gain two electrons and are discharged)
The anode slowly dissolves and loses mass (copper atom loses two electrons and Cu2+ ions are formed)
How to refine copper (3)
At the anode (+), copper atoms lose electrons and go into solution as copper(II) ions:
Cu(s) → Cu 2+ (aq) + 2e –
The Cu 2+ ions are attracted to the cathode (–) where they gain electrons, forming copper atoms:
Cu 2+ (aq) + 2e – → Cu(s)
A layer of pure copper builds up on the cathode.
Precious metals such as platinum, silver and gold, that might have been present in the impure copper anode, drop to the bottom. This mixture is known as anode slime
How is aluminium extracted
Impurities are removed from bauxite to produce pure aluminium oxide.
The aluminium oxide is dissolved in molten cryolite (Na 3 AlF 6 ), another ore of aluminium. This is because the melting point of Al 2 O 3 is very high, at around 2000 °C. A mixture of Al 2 O 3 dissolved in molten cryolite has a lower melting point, which saves heat energy, and also has a higher conductivity.
The molten Al 2 O 3 undergoes electrolysis in a tank where both the anode and the cathode are made of carbon:
Al 3+ ions move to the lining of the tank (the cathode (–)) where they gain three electrons to form aluminium atoms:
Al 3+ (l) + 3e – → Al(l) reduction
The liquid aluminium is run off the bottom of the tank.
O 2– ions move to the carbon anodes (+) where they lose 4 electrons to form oxygen:
2O 2– (l) → O 2 (g) + 4e – oxidation
The oxygen forming at the anodes reacts with the carbon anode, forming carbon dioxide, CO 2 :
C(s) + O 2 (g) → CO 2 (g)
This means that the anodes wear away and have to be replaced regularly.
Properties of aluminium(3)
impermeable
resistant to corrosion
high strength to weight ratio
Uses of aluminium (3)
packaging drinks can etc
transport - aeroplanes
building material
Manufacturing sodium hydroxide,, hydrogen and chlorine
