Purification, Extraction And Manufacture Of Useful Substances

Question 1

What is the formula for aluminium oxide and is it amphoteric?

Answer 1

Al2O3
Yes, its amphoteric = acts as an acid or base

Question 2

What are the uses of aluminium oxide?

Answer 2

• desicant
• catalyst
• refractory materials like furnaces as theyre physically strong at a high temp + unreactive

Question 3

Where is aluminium oxide found and how can it be extracted for its use?

Answer 3

Found in bauxite ore, extracted using bayer process

Question 4

Aluminium oxide is a refractory material, what does this mean?

Answer 4

Material that is stable under high temperatures

Question 5

What are the steps of the bayer process to extract Al2O3 from bauxite (aluminium ore)?

Answer 5

• crush bauxite into grains to increase ROR
• react with hot concentrated NaOH to form a slurry, alumina is amphoteric so reacts with this alkali
• filter to remove undissolved impurities, alumina is acidic so dissolves in alkali
• Add solid Al(OH)3 and cool
• filter to remove NaOH salt to have pure Al(OH)3
• solid seeds of Al2O3 is added to encourage crystallisation
• heat to dry Al(OH)3 in oven to form alumina

Al(OH)3 = aluminium hydroxide

Question 6

To get pure aluminium from alumina, what process is used?

Answer 6

Hall - Heroult Electrolysis

Question 7

Why does aluminium have to be extracted by electrolysis?

Answer 7

Its very reactive and aluminium oxide (bauxite) has a very high mp so it is dissolved in cryolite to lower mp, reducing energy costs instead of melting it.

Question 8

Why is extracting aluminium by electrolysis expensive?

Answer 8

• large amount of electricity is required
• electrodes have to be replaced frequently

Question 9

Are aluminium ions oxidised or reduced at cathode, describe process

Answer 9

• Al3+ ions gain electrons at cathode so are reduced to aluminium atoms
• molten aluminium sinks to bottom of cell where it is tapped off

Al3+ + 3e- = Al

Question 10

Are oxide ions oxidised or reduced at anode, describe process

Answer 10

• oxide ions lose electrons at anode so are oxidised to produce oxygen gas
• this reacts with positive electrode made of carbon, forming carbon dioxide
• this causes it to wear away, has to be replaced frequently

2O2- = O2 + 4e-

Question 11

why are refractory materials like furnaces a use of aluminium oxide?

Answer 11

as theyre physically strong at a high temp + unreactive

Question 12

How is aluminum extracted by electrolysis?

Answer 12

• aluminium has to be extracted from its ore (bauxite - Al2O3) by electrolysis as its very reactive
• ore is mixed with cryolite to lower its mp (reduces energy costs as aluminium oxide has a very high mp, expensive to melt - cheaper to dissolve)
• ore is then melted so ions can move
• positively charged ions gather at cathode (Al3+)
• negatively charged ions gather at anode (O2-)
• this causes anode to wear away, so has to be changed frequently (expensive)