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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 electrolysing a molten mixture of aluminium oxide and cryolite. 7.1 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
7.1 Explain why a mixture is used as the electrolyte instead of using only aluminium oxide.
Answer
A mixture is used as the electrolyte because it has a lower melting point than pure aluminium oxide. This makes the electrolysis process more efficient, as less energy is required to maintain the electrolytic process.
Step 2
Step 3
Step 4
7.4 Explain why the positive electrode must be continually replaced.
Answer
The positive electrode, often made of carbon, reacts with oxygen during electrolysis, leading to its consumption. This reaction produces carbon dioxide, resulting in the need for continuous replacement to maintain the process.
Step 5
7.5 Calculate the mass of oxygen produced when 2000 kg of aluminium oxide is completely electrolysed.
Answer
From the equation 2 Al₂O₃ → 4 Al + 3 O₂, we see that 2 moles of Al₂O₃ produce 3 moles of O₂. The molar mass of Al₂O₃ is 102 g/mol, thus:
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Calculate moles of Al₂O₃:
ext{moles of Al₂O₃} = rac{2000 ext{ kg}}{102 ext{ g/mol}} = 19608 ext{ mol}
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Calculate moles of O₂ produced:
ext{moles of O₂} = rac{3}{2} imes 19608 = 29412 ext{ mol}
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Calculate mass of O₂ produced:
ext{mass of O₂} = 29412 ext{ mol} imes 32 ext{ g/mol} = 941 kg.
Step 6
7.6 Explain why sodium chloride solution cannot be used as the electrolyte to produce sodium metal.
Answer
Sodium chloride solution cannot be used to produce sodium metal because, during electrolysis, hydrogen gas would be produced instead of sodium. This is because sodium is more reactive than hydrogen.
Step 7
7.7 Calculate the volume of 150 kg of chlorine gas at room temperature and pressure.
Answer
First, calculate moles of Cl₂:
ext{moles of Cl₂} = rac{150000 ext{ g}}{71 ext{ g/mol}} ext{ (Molar mass of Cl₂)} = 2113.0 ext{ mol}
Next, calculate the volume:
ext{Volume} = ext{moles} imes 24.0 ext{ dm³/mol} = 2113.0 ext{ mol} imes 24.0 ext{ dm³/mol} = 50700 ext{ dm³}.