4. Electrolysis

Question 1

Electrolysis

Answer 1

The breakdown of an ionic compound, molten or in aqueous solution, by the passage of electricity.

Question 2

Electrolytic cell

Answer 2

In which electrolysis is carried out.

An electrochemical cell in which electrical energy is transferred into chemical energy.

Question 3

3 main components of an electrolytic cell

Answer 3

• Battery
• 2 Electrodes (cathode + anode)
• Electrolyte

Question 4

Cathode

Answer 4

The negative electrode of an electrolytic cell.

It gets charged up with electrons which flow from the negative terminal from the battery.

(Attracts cations)

Question 5

Anode

Answer 5

The positive electrode of an electrolytic cell.

‘Takes’ electrons from anions.

(Attracts anions)

Question 6

Electrolyte

Answer 6

Usually an ionic compound that when molten or aqueous is ionised and so conducts electricity.

Essential for electrolysis.

Question 7

Why are electrodes often made of carbon or platinum?

Answer 7

They are inert metals + good conductors of electricity.

Question 8

General stages of electrolysis

Answer 8

1. Electrolyte made dissolved in water/made molten
2. Electric current pass through the cell
3. Cations move towards (negatively charged) cathode.
4. Cations gain electrons (cathode ‘gives them’)
5. Anions move towards (positively charged) anode.
6. Anions lose electrons (anode ‘takes them’)

Question 9

In what form does an electrolyte have to be?

Answer 9

Molten or aqueous

–> So the ions can move to the electrodes

Question 10

Redox - Electrolysis (main stages)

Answer 10

Cathode attract cations –> Cations gain electrons –> Reduction

Anode attracts anions –> Anions lose electrons –> Oxidation

Question 11

What is molten electrolysis?

Answer 11

In molten electrolysis a solid ionic compound is melted using heat.

The ionic lattice then breaks apart, forming cations and anions free to move around.

–> The ions can then be oxidised or reduced depending on their charge.

Metal/Hydrogen (cation) forms at cathode.

Non-metal (anion) forms at anode.

Question 12

Molten electrolysis - Prodcts at cathode/ anode (sodium choride)

Answer 12

Cathode = Sodium (liquid form of metal)

Anode = Chlorine gas

Question 13

Which particles are present in aqueous electrolysis?

Answer 13

Metal ion
Non-metal ion
Water molecules.

(Water can ionise / dissociate into H⁺ ions, OH⁻ ions)

Therefore it is not certain which particles form at the electrodes during electrolysis.

Question 14

Rules of aqueuos electrolysis

Answer 14

1. Unreactive metals like copper always form at the cathode (instead of hydrogen)
2. If the compound is a sulfate or a nitrate, oxygen gas will form at the anode (oxygen formed through breakdown of water)
3. Hydrogen (from breakdown of water) forms at the cathode when the alternative is a reactive metal.
4. If the compoudn is a halide, the halogen will form at the anode.

Question 15

Aqueous electrolysis –> Results of oxygen or hydrogen forming.

Answer 15

When oxygen forms from the breakdown of water –> H⁺ ions are always left in solution.

When hydrogen forms from the breakdown of water –> OH⁻ ions are always left in solution

Question 16

Predicting the products of aqueous electrolysis - (General overview)

Answer 16

The less reactive particle will form at the electrodes.

This is becuase it is more easily oxidised/ reduced.

More specific explanation why this works….

Eg. displacement reaction –> more reactive element replaces less reactive element

Therefore the less reactive element is oxidised / reduced.

Question 17

Predicting the product formed at the cathode during electrolysis…

Answer 17

Metals or Hydrogen is formed at the cathode during electrolysis.

Potassium
Sodium
lithium
Calcium
Magnesium
Aluminium
Water
Zinc
Iron
Lead
Hydrogen
Copper
Silver
Gold

• Anything below water is less reactive and will form at the cathode.
• Anything above water is more reactive, therefore water (less reactive), will form at the cathode.

Question 18

Predicting the product formed at the anode during electrolysis… (Halogens)

Answer 18

Flourine
Chlorine
Water
Bromine
Iodine

• Anything above water is more reactive, so water forms at the anode
• Anything below water is less reactive, so it will form at the anode.

Question 19

Concentrated vs Dilute Sodium Chloride during electrolysis

Answer 19

Normally during sodium chloride, hydrogen and oxygen form at the cathodes and anodes, because both chlorine and sodium is more reactive than water.

Concentrated NaCl - Sodium + Chlorine form at electrodes bc there are more particles of NaCl surrounding the electrodes.

Dilute NaCl - Hydrogen and Oxygen form at electrodes.

Question 20

Forming ionic half equations during electrolysis –> eg. molten lead(II) bromide

Answer 20

Pb²⁺ (l) + 2e⁻ –> Pb (s)

2Br⁻(aq) –> Br² (g) +2e⁻

Question 21

2 main applications of electrolysis

Answer 21

1. Electroplating
2. Purification of copper/ refining copper

Question 22

Electroplating

Answer 22

Coating one metal with a thin layer of another metal either to improve the appearance of an object or to prevent corrosion.

Question 23

How does electroplating work?

Answer 23

The object which will be electroplated is made the cathode (connected to negative terminal)

The anode is made from the metal providing the coating layer.

The electrolyte contains metal ions of the metal providing the electroplating layer.

The anode is connected to the positive terminal. It needs to gain electrons therefore. It does so by oxidising atoms in the electrode and ‘taking’ the electrons. (Thus creating cations)

These cations are then attracted to the negatively charged cathode where they gain electrons and become atoms, plating the cathode.

The reason why atoms in the anode are oxidised instead of anions in the solution, is because they are ‘closer’ to the charged electrode. (This doesn’t happen in normal electrolysis because the electrodes are inert.

Question 24

Why does the electrolyte used in electroplating have to include ions of the metal used to plate?

Answer 24

If the electrolyte contains ions of a lower reactivity than the cations being created, then those cations could be reduced –> Causing different metal to electroplate.

Also more efficient. (Anode of same metal is just replenshing the level of cations in the solution)

Question 25

Why do the anodes need to be replaced in electroplating?

Answer 25

The atoms in the anodes get oxidised into cations to replace teh diminishing levels of cations in teh electrolyte.

Eventually the anode gets worn down as it continues to lose atoms (being converted into cations)

Question 26

ADD ELECTROLYSIS OF SODIUM CHLORIDE

Question 27

Hydrogen - Oxygen fuel cells

Answer 27

2H2(g) + O2(g) → 2H2O(g)

This reaction is exothermic → Gives out energy.
It only creates water, meaning it has no CO2 emissions (clean fuel).

At the anode, hydrogen is being pumped in.
At the cathode, air (containing oxygen) is being pumped in.

At the anode, hydrogen loses electrons and forms anions.

At the cathode, the hydrogen cations gain electrons and reacts with oxygen to create water.