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This question is about electrolysis - AQA - GCSE Chemistry - Question 7 - 2019 - Paper 1
Question 7
This question is about electrolysis. Aluminium is produced by electrolysis of a molten mixture of aluminium oxide and cryolite. 1. Explain why a mixture is used as... show full transcript
Worked Solution & Example Answer:This question is about electrolysis - AQA - GCSE Chemistry - Question 7 - 2019 - Paper 1
Step 1
Explain why a mixture is used as the electrolyte instead of using only aluminium oxide.
Answer
A mixture, specifically one that includes cryolite, is used as the electrolyte because it has a lower melting point than pure aluminium oxide. This reduction in melting point decreases the amount of energy needed to maintain the liquid state during electrolysis, making the process more efficient and cost-effective.
Step 2
Step 3
Step 4
Explain why the positive electrode must be continually replaced.
Answer
The positive electrode, usually made of carbon, must be continually replaced because it reacts with oxygen produced during electrolysis. This process leads to the degradation of the electrode as it is consumed, often converting into carbon dioxide.
Step 5
Calculate the mass of oxygen produced when 2000 kg of aluminium oxide is completely electrolysed.
Answer
First, calculate the moles of Al₂O₃:
o{Al₂O₃} = rac{2000 ext{ kg}}{102 ext{ g/mol}} imes 1000 = 19608 ext{ mol Al₂O₃}
From the equation, 2 moles of Al₂O₃ produce 3 moles of O₂:
o{O₂} = 19608 ext{ mol Al₂O₃} imes rac{3 ext{ mol O₂}}{2 ext{ mol Al₂O₃}} = 29412 ext{ mol O₂}
Now, calculate the mass of oxygen produced:
ext{Mass of O₂} = 29412 ext{ mol O₂} imes 32 ext{ g/mol} = 941 ext{ kg O₂}.
Step 6
Explain why sodium chloride solution cannot be used as the electrolyte to produce sodium metal.
Answer
Sodium chloride solution cannot be used as the electrolyte because it would produce hydrogen gas instead of sodium. This occurs because sodium is more reactive than hydrogen, leading to the preferential discharge of hydrogen ions during electrolysis.
Step 7
Calculate the volume of 150 kg of chlorine gas at room temperature and pressure.
Answer
To find the moles of Cl₂:
o{Cl₂} = rac{150 ext{ kg}}{71 ext{ g/mol}} imes 1000 = 2113 ext{ mol Cl₂}
Now, calculate the volume:
ext{Volume} = 2113 ext{ mol Cl₂} imes 24.0 ext{ dm³/mol} = 50700 ext{ dm³}.