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.
07.1 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.
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Answer
A mixture is used because it has a lower melting point than pure aluminium oxide, which reduces energy requirements during electrolysis. Additionally, the presence of cryolite enhances the efficiency of the process.
Step 2
What happens at the negative electrode during the production of aluminium?
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Answer
Aluminium ions gain electrons.
Step 3
Complete the balanced half-equation for the process at the positive electrode.
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Answer
4OH⁻ → O2 + 2H2O + 4e⁻
Step 4
Explain why the positive electrode must be continually replaced.
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Answer
The positive electrode must be continually replaced because it reacts with oxygen to form carbon dioxide and is gradually consumed during the process.
Step 5
Calculate the mass of oxygen produced when 2000 kg of aluminium oxide is completely electrolysed.
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Answer
First, determine the number of moles of Al2O3:
n=102 g/mol2000 kg=19608 mol
For every 2 moles of Al2O3, 3 moles of O2 are produced:
O2=19608×23=29412 mol
Next, calculate the mass of O2:
mass=29412 mol×32 g/mol=941kg
Step 6
Explain why sodium chloride solution cannot be used as the electrolyte to produce sodium metal.
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Answer
Sodium chloride solution cannot be used because, during electrolysis, hydrogen gas would be produced instead of sodium due to sodium's higher reactivity compared to hydrogen.
Step 7
Calculate the volume of 150 kg of chlorine gas at room temperature and pressure.
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Answer
To find the volume, first convert the mass of Cl2 into moles:
n=71extg/mol150000extg≈2113extmol
Using the ideal gas law, the volume at RTP is:
V=n×24.0extdm3=2113×24.0=50700extdm3
