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Water, acidified with sulfuric acid, is decomposed by electrolysis - Edexcel - GCSE Chemistry Combined Science - Question 6 - 2019 - Paper 1
Question 6
Water, acidified with sulfuric acid, is decomposed by electrolysis. The water is decomposed to produce hydrogen and oxygen. (i) A sample of hydrogen is mixed with a... show full transcript
Worked Solution & Example Answer:Water, acidified with sulfuric acid, is decomposed by electrolysis - Edexcel - GCSE Chemistry Combined Science - Question 6 - 2019 - Paper 1
Step 1
A sample of hydrogen is mixed with air and ignited. State what would happen.
Answer
When a sample of hydrogen mixed with air is ignited, it reacts explosively with oxygen in the air, producing water vapor. This reaction releases energy in the form of heat and light, and can be described by the equation:
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Step 2
Describe, using the data in Figure 6, what the results show about the volumes of hydrogen and of oxygen produced in this experiment.
Answer
The data in Figure 6 indicates that the volume of hydrogen produced increases at a consistent rate, while the volume of oxygen is produced at half the rate of hydrogen. This relationship suggests that for every 2 volumes of hydrogen produced, 1 volume of oxygen is produced, which aligns with the stoichiometric equation for the decomposition of water during electrolysis. This consistent ratio of volumes supports the law of conservation of mass and indicates that the gases are being produced in a fixed ratio.
Step 3
Explain, in terms of solubility and movement of ions, this difference in behaviour.
Answer
Calcium nitrate is soluble in water, dissociating into its component ions, Ca²⁺ and NO₃⁻, which enables electrical conductivity and allows it to behave as an electrolyte. Conversely, calcium carbonate has low solubility in water, resulting in minimal ion dissociation and consequently, it does not conduct electricity, behaving as a non-electrolyte.
Step 4
Explain all three observations.
Answer
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The sizes of both the anode and the cathode change: During electrolysis, copper ions from the anode dissolve into the solution, leading to a decrease in anode size, while copper ions in the solution gain electrons at the cathode and deposit there, causing the cathode size to increase.
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A solid appears directly beneath the anode: As copper dissolves from the anode, impurities are left behind; these impurities can aggregate as a solid beneath the anode, which is a collection of non-copper materials.
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The colour of the copper sulfate solution does not change: The concentration of copper ions remains relatively constant as copper is being dissolved at the anode and deposited at the cathode, thus the overall colour of the solution remains unchanged.