C2.7 Electrolysis

Question 1

What is electrolysis?

Answer 1

The breakdown of an ionic substance that’s molten or in solution by passing an electric current through it

Question 2

What is an electrolyte?

Answer 2

The liquid which is required in electrolysis - they’re usually molten or dissolved ionic substances

Question 3

What are the products of electrolysis?

Answer 3

Pure elements

Question 4

Where are electrons lost/gained in electrolysis?

Answer 4

They are lost at the positive electrode and gained at the negative electrode

Question 5

What reactions does electrolysis have?

Answer 5

Oxidation and reduction reactions

Question 6

In a reduction reaction, are electrons lost or gained?

Answer 6

Gained

Question 7

In an oxidation reaction, are electrons lost or gained?

Answer 7

Lost

Question 8

What is the acronym for oxidation and reduction?

Answer 8

O.I.L.R.I.G - Oxidation is Lose, Reduction is Gain

Question 9

In electrolysis, where would an oxidation reaction take place?

Answer 9

At the positive electrode

Question 10

In electrolysis, where would a reduction reaction take place?

Answer 10

At the negative electrode

Question 11

What do half equations show?

Answer 11

The reactions at the electrodes

Question 12

What are the products made at the negative electrode?

Answer 12

The element of the positive ion in the electrolyte

Question 13

What are the products made at the positive electrode?

Answer 13

The element of the negative ion in the electrolyte and the amount of electrons which were lost in order to turn this ion into an element

Question 14

If hydrogen ions are present in an electrolyte along with a metal more reactive than hydrogen, what will form at the negative electrode? Why?

Answer 14

Hydrogen will be formed at the negative electrode, this is because more reactive elements are keener to stay as ions

Question 15

If hydroxide ions are present in an electrolyte along with halide ions, what will form at the positive electrode?

Answer 15

Molecules of chlorine, bromine or iodine - if no halide is present, then oxygen will form at the positive electrode

Question 16

What is produced from the electrolysis of sodium chloride dissolved in water? (3)

Answer 16

Hydrogen, chlorine and sodium hydroxide

Question 17

Hydrogen, chlorine and sodium hydroxide are all products from the electrolysis of sodium chloride, what can they be used for? (3)

Answer 17

Chlorine is used for the production of bleach and plastics, and sodium hydroxide is a strong alkali which is in the chemical industry e.g. to make soap

Question 18

In the electrolysis of sodium chloride, what happens at the negative electrode?

Answer 18

The positive hydrogen ions move to the negative electrode where they gain an electron and combine to become hydrogen molecules (H2*) - this is a reduction reaction

Question 19

In the electrolysis of sodium chloride, what happens at the positive electrode?

Answer 19

The negative chloride ions are attracted to positive electrode where they lose an electron and combine to form chlorine molecules (Cl2*) - this is an oxidation reaction

Question 20

In the electrolysis of sodium chloride, what ions remain in the solution?

Answer 20

The sodium ions stay in the solution because they’re more reactive than hydrogen, and hydroxide ions from water are also left behind - this combine to form sodium hydroxide

Question 21

What is aluminium’s main ore?

Answer 21

Bauxite

Question 22

After mining and purifying Bauxite, what is left behind?

Answer 22

A white powder, aluminium oxide - the aluminium has to be extracted from this using electrolysis

Question 23

Why is aluminium oxide not just melted?

Answer 23

Because it has a very high melting point which would be expensive

Question 24

Why is bauxite dissolved in molten cryolite before electrolysis?

Answer 24

Because it brings down the melting point drastically, which saves energy making the process cheaper and easier

Question 25

What are the electrodes made of in the electrolysis of aluminium? Why?

Answer 25

They are made of graphite (carbon) because it is a good conductor of electricity

Question 26

What is formed at the negative electrode in the electrolysis of aluminium oxide?

Answer 26

Aluminium

Question 27

What is formed at the positive electrode in the electrolysis of aluminium oxide? What’s bad about this?

Answer 27

Oxygen is produced - this combines with carbon from the electrode to form carbon dioxide, this not only contributes to global warming, but also means that the + electrode gets eaten away and needs to be replaced

Question 28

What is electroplating?

Answer 28

The process of using electrolysis to coat the surface of one metal with another metal

Question 29

What is the negative electrode in electroplating?

Answer 29

The metal object you want to plate

Question 30

What is the positive electrode in electroplating?

Answer 30

The pure metal you want the object to be plated with

Question 31

What must be in the electrolyte for electroplating to work?

Answer 31

Ions of the of the plating metal

Question 32

How does electroplating work?

Answer 32

The ions in the solution plate the metal, but the positive electrode replaces these ions back into the solution

Question 33

Why would you want to electroplate something? (2)

Answer 33

Decoration e.g. silver is attractive but expensive, so ugly brass cups are coated in silver - they look the same but are cheaper, and conduction e.g. copper is used to plate metal for electronic circuits and computers