Electrolysis

Question 1

Define Electrolysis

Answer 1

A separation and purification technique used to extract metals from their metal ores.

Question 2

The process of Molten electrolysis.

Answer 2

• Electric current is passed through an electrolyte (ionic compound)
• +ve ions move to cathode , where they gain electrons ( reduction)
• -ve ions move to anode where they lose electrons (oxidation)
• this creates a flow of charge through the electrolyte.
• As ions lose or gain electrons, they form the uncharged element and are discharged from the electrolyte.

Question 3

Why can molten ionic compounds be electrolysed?

Answer 3

Because an ionic solid cannot conduct electricity as it doesn’t have any free ions, when molten, free ions conduct electricity.

Question 4

Uses of Aluminium

Answer 4

• Air craft
• Electricity
• drink cans

Question 5

Why do the graphite electrodes need to be replaced?

Answer 5

Because they are made of graphite which contains carbon, and the carbon reacts with oxygen to form CO2.

Question 6

What is an ore?

Answer 6

An ore is a type of rock that contains metal compounds.

Question 7

Oxidation

Answer 7

Gain of oxygen

Question 8

Reduction

Answer 8

Loss of oxygen

Question 9

What is reduction with carbon ?

Answer 9

If metal is less reactive than carbon
the oxygen is reduced from metal's ore
so the ore is reduced
the carbon gains that oxygen
becomes oxidised
forms CO2

Question 10

How is iron extracted from an iron oxide ore?

Answer 10

Iron oxide is reduced in a blast furnace to make iron.

Question 11

What happens in a blast furnace?

Answer 11

Blast furnace reduces iron oxide using heat, turns it into liquid ‘hot iron’.

Question 12

What is a redox reaction?

Give an example.

Answer 12

Reduction and Oxidation of electrons happens at the same time.
Displacement reactions are redox reactions.

Question 13

Define Oxidation and Reduction in terms of electrons.

Answer 13

Oxidation is loss of electrons.

Reduction is gain of electrons.

Question 14

What is a displacement reaction?

Answer 14

When a more reactive metal displaces a less reactive metal from its compound,

Question 15

In a displacement reaction, which one gets oxidised and reduced, metal ion or metal atom.

Answer 15

Metal ion always gains electrons to become an atom ( reduction)

Metal atom loses electrons to become a metal ion (oxidation)

Question 16

Rule to remember oxidation and reduction for half equations.

Answer 16

Electron on the right of the arrow = oxidation

Electron on the left of the arrow = reduction

Question 17

What is an aluminium ore called?

Answer 17

Bauxite - contains aluminium oxide.

Question 18

Explain the molten electrolysis of Aluminium from Bauxite.

Answer 18

Aluminium oxide has very high melting point which means it would require too much energy —> expensive.
So AL203 is mixed with cryolite to lower its melting point.
It’s molten, so it conducts electricity.

+ve AL3+ ions are attracted to cathode , they gain 3 electrons (reduction)

-ve O2- ions are attracted to the anode, they lose 2 electrons (oxidation)

Question 19

Write the two half equations for the electrolysis of aluminium oxide.

Answer 19

Al3+ + 3e- —-> Al (reduction)

2O2- —–> O2 + 4e- (oxidation)

Question 20

What are the rules for aqueous electrolysis at cathode?

Answer 20

1st =   Transition metal 
2nd =   H+ ions
3rd =   any other metal

Question 21

What are the rules for aqueous electrolysis at anode?

Answer 21

1st =  Halide
2nd =  OH- ions, hydroxides
3rd =  any other negative ion

Question 22

Cathode

Answer 22

Reduction

Question 23

Anode

Answer 23

Oxidation

Question 24

Half equation for reduction of Hydrogen

Answer 24

2H+ + 2e- —-> H2

Question 25

Half equation for oxidation of Oxygen

Answer 25

2O2- —–> O2 + 4e-

Question 26

Half equation for oxidation of chlorine

Answer 26

2Cl- ——> Cl2 + 2e-

Question 27

Half equation for oxidation of hydroxide OH-

Answer 27

4OH- —-> O2 + 2H2O + 4e-

Question 28

Why not use K ,Na, Li, Mg, for extraction, why use carbon?

Answer 28

Because the ‘very reactive’ metals are expensive, carbon is cheap.

Question 29

Why was iron able to be extracted much earlier than aluminium?

Answer 29

• Iron is less reactive than carbon so it was extracted by reduction with carbon.
• Aluminium is more reactive than carbon so it was only extracted when the process of electrolysis was discovered.

Question 30

Why is there a diff between the annual production of aluminium and iron.

Answer 30

Al costs more

Al is harder to extract.

Question 31

Why are some metals found as uncombined elements in the earth?

Answer 31

E.g gold and silver and copper

they are very unreactive, so they can underground without reactive with any oxygen or water.

Question 32

Why is graphite used for electrodes?

Answer 32

Because it can conduct electricity.