7 This question is about chemical reactions and electricity.
Electrolysis and chemical cells both involve chemical reactions and electricity.
7-1 Explain the difference between the processes in electrolysis and in a chemical cell. [2 marks]
ANSWER
Electrolysis uses electricity to produce a chemical reaction whereas cells use a chemical reaction to produce electricity.
7-2 A teacher demonstrates the electrolysis of molten lead bromide. Bromine is produced at the positive electrode.
Complete the half equation for the production of bromine.
You should balance the half equation. [2 marks]
ANSWER
2Br– → Br2 + 2 e–
7-3 Two aqueous salt solutions are electrolyzed using inert electrodes. Complete Table 4 to show the product at each electrode. [3 marks]
Table 4
ANSWER
Some students investigated the electrolysis of copper nitrate solution using inert electrodes.
Figure 4 shows the apparatus.
Figure 4
The students investigated how the mass of copper produced at the negative electrode varied with:
Time
Current.
This is the method used.
Weigh the negative electrode.
Set up the apparatus shown in Figure 4.
Adjust the power supply until the ammeter shows a current of 0.3 A
Switch off the power supply after 5 minutes.
Rinse the negative electrode with water and allow to dry.
Reweigh the negative electrode.
Repeat steps 1 to 6 for different times.
Repeat steps 1 to 7 at different currents.
7-4 Some of the copper produced did not stick to the negative electrode but fell to the bottom of the beaker.
Suggest how the students could find the total mass of copper produced. [4 marks]
ANSWER
Step1: Filter the mixture;
Step 2: Dry the copper / residue;
Step 3: Weigh the copper collected;
Step 4: Add the weigh of step 3 to the in mass of the electrode (already calculated);
Step 5: Total mass of copper produced will be the calculated value in step 4.
The students plotted their results on a graph.
Figure 5 shows the graph
Figure 5
A student correctly concluded that the total mass of copper produced is directly proportional both to the time and to the current.
7-5 How do the results in Figure 5 support the conclusion that the total mass of copper produced is directly proportional to the time? [1 mark]
ANSWER
For given current, straight line (linear graph) passes through the origin.
7-6 How do the results in Figure 5 support the conclusion that the total mass of copper produced is directly proportional to the current?
Use data from Figure 5 in your answer. [1 mark]
ANSWER
For given time; when current doubles, mass doubles as seen in supporting data.
7-7 Copper nitrate solution is blue.
Suggest why the blue color of the copper nitrate solution fades during the electrolysis. [1 mark]
ANSWER
Copper ions are discharged from the solution.
7-8 Determine the number of atoms of copper produced when copper nitrate solution is electrolysed for 20 minutes at a current of 0.6 A
Give your answer to 3 significant figures.
Use Figure 5
Relative atomic mass (Ar): Cu = 63.5
The Avogadro constant = 6.02 × 1023 per mole [3 marks]
ANSWER
From the graph, by following the current line grapg (0.6A), we get 0.24 gram for 20 minutes.
Therefore,
Avogadro constant: The number of atoms, molecules or ions in one mole of a given substance (the value of the Avogadro constant is 6.02 × 1023).
