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9 (a) Water, acidified with sulfuric acid, is decomposed by electrolysis - Edexcel - GCSE Chemistry - Question 9 - 2019 - Paper 1
Question 9
9 (a) Water, acidified with sulfuric acid, is decomposed by electrolysis. (i) A sample of hydrogen is mixed with air and ignited. State what would happen. (ii) Thr... show full transcript
Worked Solution & Example Answer:9 (a) Water, acidified with sulfuric acid, is decomposed by electrolysis - Edexcel - GCSE Chemistry - Question 9 - 2019 - Paper 1
Step 1
(i) A sample of hydrogen is mixed with air and ignited.
Answer
When a sample of hydrogen mixed with air is ignited, it would react explosively due to the presence of oxygen in the air. This reaction produces water vapor, and the equation for the reaction can be represented as:
ightarrow 2H_2O$$Step 2
(ii) Describe, using the data in Figure 9, what the results show about the volumes of hydrogen and of oxygen produced in this experiment.
Answer
From the data in Figure 9, we observe that the volume of hydrogen produced is consistently twice that of the oxygen over the entire experiment. At 2 minutes, for every 2 cm³ of hydrogen produced, there is 1 cm³ of oxygen. This relationship aligns with the stoichiometry of water decomposition, which states that 2 volumes of hydrogen combine with 1 volume of oxygen, confirming that the experiment validates the 2:1 ratio of hydrogen to oxygen.
Step 3
Step 4
Explain, in terms of solubility and movement of ions, this difference in behaviour.
Answer
Calcium nitrate is soluble in water, which allows it to dissociate into calcium ions (Ca²⁺) and nitrate ions (NO₃⁻), enabling the movement of ions and thus permitting electrical conductivity. In contrast, calcium carbonate is insoluble in water, meaning it does not dissociate into ions. As a result, no free ions are available to move in solution, which explains why calcium carbonate does not behave as an electrolyte.
Step 5
Explain three observations.
Answer
- Change in size: As copper ions are reduced at the cathode to form pure copper, the cathode will gain mass while the anode, which loses copper to the solution, will decrease in mass.
- Solid formation: The solid that appears beneath the anode is due to the oxidation of copper, which leads to the formation of copper ions in the solution; these ions must travel to the cathode to be deposited as solid copper.
- Colour of solution: The copper sulfate solution retains its color because the concentration of copper sulfate remains constant, even though copper ions are transferred from one electrode to another.