C6 Electrolysis Past Papers Flashcards
Products at anode and cathode of electrolysing potassium iodide solution
Hydrogen at cathode
Oxygen at anode
Why is oxygen formed at the anode of electrolysis potassium iodide
Hydroxide ions are more easily oxidized than iodide ions as they have a lower oxidation potential
Why is hydrogen formed at the cathode of electrolysis potassium iodide
If metal element formed during electrolysis is more reactive than hydrogen, then hydrogen will be produced at the cathode
What happens if you place a growing splint in the tube of the gas
Will burn more brightly as oxygen is supporter of combustion
Relights
How to test the gas to show it’s hydrogen
Hydrogen ignites in air so if hydrogen is present in a test tube
A lighted splint held near its mouth ignites with a squeaky pop
why is hydrogen not magnesium produced at anode during electrolysis
It is less reactive than magnesium
Give 1 reason why metal spoons are sometimes silver plated
Chemical resistance
Expensive appearnce
The student found that the metal spoon eroded and a thin layer of copper formed on the pure silver electrode
Suggest two changes that the student must make to his apparatus to be able to silver plate the metal spoon. Give a reason for each change.
Solution must be silver nitrate or contain silver ions or silver will not be deposited
Spoon must be the cathode as silver ions have positive charge
Why is it difficult to electroplate plastic spoons
Plastic is not a conductor of electricity
Why can aluminium not be extracted by heating aluminium oxide with carbon
Aluminium is more reactive than carbon
Explain why aluminium forms at the negative electrode during electrolysis
Has positive ions that are attracted to the negative electrode where they can gain electrons and be reduced
Explain how carbon dioxide forms at the positive electrodes during electrolysis.
Molten aluminium sinks to the bottom of the cell, where it is tapped off.
This oxygen reacts with the carbon of the positive electrodes, forming carbon dioxide
How could the students separate silver from silver nitrate solution
Filtration
Determine mass of silver produced after 12 min
0.14
Observation at cathode of copper sulfate
Copper plating
Pink, red, orange
Observation at cathode sodium chloride
Bubbles and fizzing
A teacher demonstrates the electrolysis of molten lead bromide
The products at the electrodes are lead and bromine
Why should the teacher do the demonstration in a fume cupboard
Toxic/poisonous fumes
Electrolysis and chemical cells both involve chemical reactions and electricity
Explain the difference between the processes in electrolysis and in a chemical cell
Voltage for electricity
Current for electricity
Balance the equation
___Br- ———> _________ + ________
2Br- ———> Br2 + 2e-
What is formed at anode in copper nitrate electrolysis
Oxygen
Why is oxygen formed at anode in copper nitrate electrolysis
No halogen present
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
Filter mixture
Wash and dry the copper / residue
Weigh copper collected
Add to the increase in mass of the electrode
A student correctly concluded that the total mass of copper produced is directly proportional both to the time and to the current
How do the results in Figure 2 support the conclusion that the total mass of copper produced is directly proportional to the time?
When time doubles, mass doubles
How do the results in Figure 2 support the conclusion that the total mass of copper produced is directly proportional to the current
When current doubles, mass doubles
Why blue colour of copper nitrate solution fades during electrolysis
Solution becomes less concentrated
Determine the number of atoms of copper produced when copper nitrate solution is electrolysed for 20 minutes at a current of 0.6 A
-3
0.24 ➗ 63.5 = 3.78 x 10 or 0.00378
23 21
0.00378 x 6.02 x 10 = 2.28 x 10
Observation if metal produced at negative electrode
Solid produced
Observation if gas produced at positive electrode
Bubbles
Which salt solution in table above does not match the student’s hypothesis
Give one reason why
Potassium nitrate
Hydrogen is not a metal
2 reasons why graphite is used for electrodes
Conducts electricity
Inert
The purple crystal dissolved in the electrolyte solution
What happens to the purple coloured ions
Ions move towards the positive electrode
Opposite charges attract
Why aluminium has high melting point
Strong metallic bonds
Alloy
Mixture of metals
Bauxite contains between 15% and 25% aluminium
Aluminium oxide always contains 53% aluminium
How does this show that bauxite is a mixture and not a compound
Bauxite contains variable percentage of aluminium
Suggest two possible problems with storing the waste material in lakes of mud
Danger from dam bursting
Damage to habitats
How do you know when oxygen is produced at anode
Oxygen is produced unless halide
As halide ions have single negative charge
What is positive electrode made of in molten aluminium oxide
Carbon
Why is anode usually made of carbon
Good conductor
Cheap
A sample of bauxite contains 25% aluminium
Calculate maximum mass of aluminium that can be extracted from 300000 kg of the sample of bauxite
25
______ x 300,000
100
Why colour changed at positive electrode copper chromate solution
2-
CrO4 Chromate ions moved to positive electrode as opposite charges attract
Name the substance in the solution that provides the hydroxide ions
Water
How the solid forms at negative electrode
Copper ions gain 2 electrons to form copper
Explain why a mixture is used as the electrolyte instead of using only aluminium oxide
Mixture has lower melting point so less energy needed
What happens at the negative electrode during the production of aluminium
Aluminium ions gain electrons
