S4 Electrolysis

Question 1

Define electrolysis.

Answer 1

Electrolysis is the process of decomposing compounds by passing through electricity.

Question 2

What are the 3 components of electrolysis?

Answer 2

1) batteries (source of energy)
2) electrode (cathode and anode)
3) electrolyte

Question 3

What is an anode?

Answer 3

It is the electrode connected to the positive terminal of the battery. It is positively charged and attracts anions.

Question 4

What is the cathode?

Answer 4

It is the electrode connected to the negative terminal of the battery. It is negative charged and attracts cations.

Question 5

What is the typical half equation at the cathode?

Answer 5

metal ion (aq) + e (number based on cation charge) —> metal solid

Question 6

What is the typical half equation at the anode?

Answer 6

Anion –> gas/liquid + e (number based on anion charge)

Question 7

What are the 3 inert electrodes?

Answer 7

Carbon, graphite, platinum

Question 8

What are the 3 factors that decide the product discharged during electrolysis?

Answer 8

1) preferential discharge of ions
2) concentration of ions
3) types of electrodes used (inert or reactive)

Question 9

How are the preferential discharge of cations decided?

Answer 9

Based on the electrochemical series (metal reactivity series), the less reactive metal will be most preferentially discharged.

Question 10

Why are ions of very reactive metals unable to discharge in the presence of water?

Answer 10

The metals will immediately react with water to give hydrogen.

Question 11

What is the sequence of anions’ rate of being preferentially discharged? (Descending order)

Answer 11

1) SO42-
2) NO3-
3) Cl-
4) Br-
5) I-
6) OH-

Question 12

Which 3 anions can replace OH- if they are very concentrated?

Answer 12

Cl-, Br-, I-

Question 13

What is the equation of OH- being discharged?

Answer 13

4OH-(aq) –> 2H2O(l) + O2 (g) +4e

Question 14

How to write an overall equation? (4)

Answer 14

1) make number of electrons for both half equations equal
2) add the two half equations together, with reactants at one side and products at the other
3) cancel out electrons
4) balance

Question 15

What happens if the electrodes are reactive?

Answer 15

At the anode, the reactive anode will discharge itself into the water, forming ions. The anions will not be discharged. The reactive anode’s ions will be attracted to the cathode.

Question 16

How is aluminum extracted from Al2O3 (l)

Answer 16

Use carbon electrodes. Hot O2 will corrode the carbon anode to form CO2, which escapes into the air. Aluminum ions are then attracted to the cathode to form solid aluminum.

Question 17

Why does the carbon anode during aluminum extraction need to be replaced frequently?

Answer 17

It is corroded by oxygen in Al2O3 to form carbon dioxide, which escapes into air.

Question 18

Why is Al2O3 liquid during extraction of aluminum?

Answer 18

Liquid is easier to transport than solid. A high temperature is needed to keep it in liquid state.

Question 19

During electroplating, what is anode and what is the cathode? What is the electrolyte?

Answer 19

Anode: pure metal
Cathode: metal object to be copper plated (metal to conduct electricity)
Electrolyte: any solution containing the metal

Question 20

What is the cathode, anode and electrolyte during purification of copper?

Answer 20

Cathode: thin sheet of pure copper
Anode: impure copper
Electrolyte: any solution containing copper

Question 21

Define a simple cell.

Answer 21

A simple cell is a device which converts chemical energy to electrical energy. It is also know as an electric cell.

Question 22

What are 3 differences between a simple cell and an electrolytic cell?

Answer 22

1) batteries are used in electrolytic cells but no batteries are used in simple cells.
2) electrolytic cells convert electrical energy to chemical energy, while simple cells convert chemical energy to electrical energy.
3) electrolytic cells are made up of batteries, electrodes and electrolytes, while simple cells are made up of 2 different metals in contact with an electrolyte.

Question 23

What determines the cathode and anode of a simple cell?

Answer 23

The more reactive metal provides electrons, becoming the negative terminal, which is the anode.
The less reactive metal passively receives electrons from the more reactive metal. It is a cathode.

Question 24

What determines the voltage of simple cells?

Answer 24

The further apart the two metals are in the reactivity series, the higher the voltage produced. No current will flow if both electrodes are made of the same metal.

Question 25

Define fuel cell.

Answer 25

A fuel cell is a chemical cell in which reactants are continuously supplied to produce electricity directly.

Question 26

Name an example of a fuel cell.

Answer 26

Hydrogen-oxygen fuel cell is used as a source of electrical power in space vehicles.

Question 27

How does a hydrogen-oxygen fuel cell work? (3)

Answer 27

1) at the anode, hydrogen is oxidised to form water.
2) at the cathode, oxygen is reduced to form hydroxide ions.
3) overall eqn: O2(g) + 2H2(g) -> 2H2O (l)

Question 28

What is one difference between fuel cell and a electrolysis cell?

Answer 28

The reactants are not contained within the cell, but instead continuously supplied from an external reservoir.

Question 29

Why is hydrogen a good fuel?(2)

Answer 29

1) renewable energy source

2) pollution free: only water is produced when hydrogen reacts with oxygen

Question 30

What are 2 problems with using hydrogen as a fuel?

Answer 30

1) it is difficult to find a cheap source of hydrogen: there are lots of hydrogen but they are mostly combined with oxygen in seawater. One way to obtain hydrogen is by electrolysis of water, which defeats the purpose of a fuel cell.
2) it is difficult to find effective means of storing and transporting the gas as it flammable. (Pressurized tanks are needed)

Question 31

Why does the color of the solution remain the same during electroplating?

Answer 31

At the anode, metal atoms are oxidised and become cations. At the cathode, cations are discharged/reduced to form metal atoms. One cation forms at the anode for each cation that is discharged at the cathode. Hence, the concentration of cations remain the same as there is no net gain or loss of Cations in the solution.

Question 32

Explain why the more reactive metal is the negative electrode (Anode) in simple cells.

Answer 32

More reactive metal loses electrons more readily and electrons flow from negative terminal to positive terminal.