Electrolysis Flashcards
Define Electrolysis
A separation and purification technique used to extract metals from their metal ores.
The process of Molten electrolysis.
- Electric current is passed through an electrolyte (ionic compound)
- +ve ions move to cathode , where they gain electrons ( reduction)
- -ve ions move to anode where they lose electrons (oxidation)
- this creates a flow of charge through the electrolyte.
- As ions lose or gain electrons, they form the uncharged element and are discharged from the electrolyte.
Why can molten ionic compounds be electrolysed?
Because an ionic solid cannot conduct electricity as it doesn’t have any free ions, when molten, free ions conduct electricity.
Uses of Aluminium
- Air craft
- Electricity
- drink cans
Why do the graphite electrodes need to be replaced?
Because they are made of graphite which contains carbon, and the carbon reacts with oxygen to form CO2.
What is an ore?
An ore is a type of rock that contains metal compounds.
Oxidation
Gain of oxygen
Reduction
Loss of oxygen
What is reduction with carbon ?
If metal is less reactive than carbon the oxygen is reduced from metal's ore so the ore is reduced the carbon gains that oxygen becomes oxidised forms CO2
How is iron extracted from an iron oxide ore?
Iron oxide is reduced in a blast furnace to make iron.
What happens in a blast furnace?
Blast furnace reduces iron oxide using heat, turns it into liquid ‘hot iron’.
What is a redox reaction?
Give an example.
Reduction and Oxidation of electrons happens at the same time.
Displacement reactions are redox reactions.
Define Oxidation and Reduction in terms of electrons.
Oxidation is loss of electrons.
Reduction is gain of electrons.
What is a displacement reaction?
When a more reactive metal displaces a less reactive metal from its compound,
In a displacement reaction, which one gets oxidised and reduced, metal ion or metal atom.
Metal ion always gains electrons to become an atom ( reduction)
Metal atom loses electrons to become a metal ion (oxidation)
Rule to remember oxidation and reduction for half equations.
Electron on the right of the arrow = oxidation
Electron on the left of the arrow = reduction
What is an aluminium ore called?
Bauxite - contains aluminium oxide.
Explain the molten electrolysis of Aluminium from Bauxite.
Aluminium oxide has very high melting point which means it would require too much energy —> expensive.
So AL203 is mixed with cryolite to lower its melting point.
It’s molten, so it conducts electricity.
+ve AL3+ ions are attracted to cathode , they gain 3 electrons (reduction)
-ve O2- ions are attracted to the anode, they lose 2 electrons (oxidation)
Write the two half equations for the electrolysis of aluminium oxide.
Al3+ + 3e- —-> Al (reduction)
2O2- —–> O2 + 4e- (oxidation)
What are the rules for aqueous electrolysis at cathode?
1st = Transition metal 2nd = H+ ions 3rd = any other metal
What are the rules for aqueous electrolysis at anode?
1st = Halide 2nd = OH- ions, hydroxides 3rd = any other negative ion
Cathode
Reduction
Anode
Oxidation
Half equation for reduction of Hydrogen
2H+ + 2e- —-> H2
Half equation for oxidation of Oxygen
2O2- —–> O2 + 4e-
Half equation for oxidation of chlorine
2Cl- ——> Cl2 + 2e-
Half equation for oxidation of hydroxide OH-
4OH- —-> O2 + 2H2O + 4e-
Why not use K ,Na, Li, Mg, for extraction, why use carbon?
Because the ‘very reactive’ metals are expensive, carbon is cheap.
Why was iron able to be extracted much earlier than aluminium?
- Iron is less reactive than carbon so it was extracted by reduction with carbon.
- Aluminium is more reactive than carbon so it was only extracted when the process of electrolysis was discovered.
Why is there a diff between the annual production of aluminium and iron.
Al costs more
Al is harder to extract.
Why are some metals found as uncombined elements in the earth?
E.g gold and silver and copper
they are very unreactive, so they can underground without reactive with any oxygen or water.
Why is graphite used for electrodes?
Because it can conduct electricity.
