7 This question is about electrolysis.
Aluminium is produced by electrolysing a molten mixture of aluminium oxide and cryolite.
7-1 Explain why a mixture is used as the electrolyte instead of using only aluminium oxide.
[2 marks]
ANSWER
Unfortunately, aluminium Oxide has very high melting point of 2050oC. However, chemists have found way of save energy. They mix the aluminium Oxide with cryolite. Cryolite is another ionic compound. The molten mixture can be electrolysed at about 850oC. The large amount of electrical energy that is transferred to the electrolysis cells keeps the mixture molten.
7-2 What happens at the negative electrode during the production of aluminium?
ANSWER
Aluminium ions gain electrons
7-3 Oxygen is produced at the positive electrode.
Complete the balanced half-equation for the process at the positive electrode.
[2 marks]
ANSWER
7-4 Explain why the positive electrode must be continually replaced.
[3 marks]
ANSWER
The oxygen reacts with the hot carbon anodes, making carbon dioxide gas i.e.
C (s) + O2
So the carbon anodes gradually burn away and need to be replaced regularly.
7-5 The overall equation for the electrolysis of aluminium oxide is:
2Al2O3 → 4Al + 3O2
Calculate the mass of oxygen produced when 2000 kg of aluminium oxide is completely electrolysed.
Relative atomic masses (Ar): O = 16 Al = 27
[4 marks]
ANSWER
Mr of Al2O3
Moles of Al2O3
Now by using unitary method, 2 moles of Al2O3 produces 3 moles of Oxygen
therefore,
1 moles of Al2O3 produces
Hence, 19608 moles of Al2O3 produces
Now,
Mass in gram
7-5 Explain why sodium chloride solution cannot be used as the electrolyte to produce sodium metal.
[2 marks]
ANSWER
When electrolyte is sodium chloride solution, so the ipositive ions that are available to travel towards cathode are Na+ from NaCl and H+ from aqueous (water). Now, among these two positive ions i.e. Na+ and H+, only H+ will go and get discharge at cathode and gain electron i.e. get reduced. It is so because hydrogen is less reactive than hydrogen and so less reactive gets discharge. Therefore, the Na+ goes into the solution rather being going to discharge at cathode. Hence, no soudium metal at cathode.
7-6 Calculate the volume of 150 kg of chlorine gas at room temperature and pressure.
The volume of one mole of any gas at room temperature and pressure is 24.0 dm3
Relative formula mass (Mr): Cl2 = 71
[2 marks]
ANSWER
Moles of Cl2
Volume in dm3
