electrolysis

Question 1

Define electrolysis

Answer 1

The splitting of an ionic compound using electricity.

Question 2

What is an electrolyte ?

Answer 2

a liquid or solution that can conduct electricity.

Question 3

What kind of compounds undergo electrolysis ?

Answer 3

ionic compounds

Question 4

What conditions does an ionic compound have to be in order to undergo electrolysis ?

Answer 4

molten or aqueous

Question 5

Define what an electrode is.

Answer 5

a solid that conducts electricity and is submerged in the electrolyte.

Question 6

During electrolysis what is passed through an electrolyte ?

Answer 6

an electric current
Since electrolytes can conduct electricity it means the ionic compound can be split up with the electricity conducted.

Question 7

What happens when the ions from the ionic compound move towards the electrodes ?

Answer 7

they react and the compound decomposes.

Question 8

Explain how electrolysis works.

Answer 8

1) place the electrodes in an electrolyte.
2) The positive ions in the electrolyte will move towards the cathode (-ve electrode) gaining electrons and being reduced.
3) the negative ions will move towards the anode (+ve electrode) losing electrons and being oxidised.

Question 9

What is the anode ?

Answer 9

+ve electrode

Question 10

What is the cathode ?

Answer 10

-ve electrode

Question 11

Why do the negative ions in the electrolyte move towards the anode and vise versa ?

Answer 11

Because opposite charges attract creating a flow of charge through the electrolyte for electrolysis to happen.

Question 12

What happens as the ions gain or lose electrons through oxidation and reduction ?

Answer 12

They form the unchanged element and are discharged from the electrolyte.

Question 13

Why can an ionic solid not be electrolysed ?

Answer 13

This is because the ions are in fixed positions and can’t move.

Question 14

Why can molten ionic compounds be electrolysed ?

Answer 14

Because the ions can move freely and therefore conduct electricity.

Question 15

Why should the electrodes be made of an inert material for electrolysis ?

Answer 15

so they don’t react with the electrolyte.

Question 16

When is electrolysis used to extract metals from their ores ?

Answer 16

If a metal is too reactive on the reactivity series to be reduced with carbon and released form its core or reacts with carbon electrolysis is used.

Question 17

Why is extracting metals from their ores expensive ?

Answer 17

This is because it requires a lot of energy to melt the ore and produce the required current.

Question 18

What is Aluminium’s ore called ?

Answer 18

Bauxite

Question 19

What is the chemical formula for aluminium oxide that bauxite contains ?

Answer 19

Al₂O₃

Question 20

Give one problem faced when extracting Aluminium oxide from its core.

Answer 20

the compound has a very high mpt (around 2072 °C) so requires a lot of energy to melt the core.

Question 21

How is the problem faced with Aluminium oxides high mpt solved for extraction from bauxite ?

Answer 21

The ionic compound is mixed with cryolite to lower the mpt to around 100°C.

Question 22

When electrolysing What electrode are the positive Al³⁺ attracted to ?

Answer 22

the negative electrode. ( cathode )

Question 23

What happens once the Al³⁺ reach the negative electrode ?

Answer 23

They each pick up three electrons and are reduced to turn into neutral aluminium atoms.

Question 24

What happens to the negative O²⁻ ions when they reach the positive electrode during the electrolysis of Aluminium from its core ?

Answer 24

They each lose 2 electrons and are oxidised to form neutral oxygen atoms to combine to form O₂ molecules.

Question 25

When electrolysing what electrode are the negative O²⁻ ions attracted to ?

Answer 25

the positive electrode ( anode )

Question 26

What material is the anode made from and why is this a problem ?

Answer 26

made from graphite (containing carbon) - Therefore when the O²⁻ ions reach the anode since opposite charges attract the ion reacts to form CO₂. This results in the anode corroding meaning we have to replace it regularly when working with oxygen compounds.

Question 27

state the symbol half equation of the electrolysis of Aluminium oxide at the cathode.

Answer 27

Al³⁺ + 3e⁻ –> Al

( reduction )

Question 28

State the symbol half equation of the electrolysis of Aluminium oxide at the anode.

Answer 28

2O²⁻ –> O₂ + 4e⁻

( oxidation )

Question 29

Why do the ions in a molten ionic compound lose electrons if they are positively charged in electrolysis and vice versa ?

Answer 29

to become a neutral atom

Question 30

In the electrolysis of aqueous solutions, what other ions apart from the ionic compound are there ?

Answer 30

hydrogen ions ( H⁺ )
and
Hydroxide ions ( OH⁻ )

Question 31

The ions that are discharged from the electrodes when the electrolysis of an aqueous solution occurs depends on what ?

Answer 31

depends on the relative reactivity of all the ions in the solution.

Question 32

When electrolysing aqueous solutions what rules will determine which ion will be produced at the negative elctrode ?

Answer 32

1) In the ionic compound if the metals ions from an elemental metal less reactive than hydrogen the metal’s ions will be discharged instead of the H⁺ ions from the water.
2) If the metal ions from an elemental metal more reactive than hydrogen H⁺ ions will be discharged instead.

Question 33

When electrolysing aqueous solutions if the metal ion in the ionic compound is produced at the cathode what happens to the cathode ?

Answer 33

A solid layer of pure metal will be produced, which will coat the cathode.

Question 34

When electrolysing aqueous solutions what rules determine which ion will be produced at the anode ?

Answer 34

1) If there is a group 7 non metal element in the ionic compound then molecules of the halide ions will be formed, instead of the OH⁻
2) If there are no halide ions present then the OH⁻ ions from the water will be discharged and oxygen gas and water will be formed.

Question 35

Which electrode are OH⁻ ions attracted to during the electrolysis of aqueous solutions ?

Answer 35

anode

Question 36

Which electrode are H⁺ ions attracted to during the electrolysis of aqueous solutions ?

Answer 36

cathode

Question 37

Describe the practical of the electrolysis of an Aqueous solution of NaCl.

Answer 37

1) Place 50cm³ of NaCl solution into a beaker
2) turn on a low voltage power supply
3) When you look at the anode, bubbles of pale green chlorine gas are produced since it is a halide ion.
4) This can tested by seeing if the gaseous product bleaches damp litmus paper turning it white in which case it is chlorine gas that has been produced.

5) When you look at the cathode gas bubbles appear.
6) hydrogen gas is produced since sodium metal is more reactive than hydrogen.
7) This can be proved by lighting a splint and if the gaseous product produces a squeaky pop it is hydrogen gas.

Question 38

What is represented by half equations when electrolysing aqueous solutions.

Answer 38

they show ions or atoms gaining or losing electrons.

Question 39

For the practical of the electrolysis of the aqueous solution of NaCl give the half equation at the anode.

Answer 39

2Cl⁻ - 2e⁻ –> Cl₂

( Chlorine is a diatomic element so that’s why there are 2 chloride ions in the start of the equation ).

Question 40

For the practical of the electrolysis of the aqueous solution of NaCl give the half equation at the cathode.

Answer 40

2H⁺ + 2e⁻ –> H₂

( Hydrogen is a diatomic element so that’s why there are 2 hydrogen ions at the start of the equation)

Question 41

What is the main difference in the electrolysis of a molten ionic compound and the electrolysis of a an aqueous solution ?

Answer 41

In the electrolysis of a molten ionic compound there are only the negative and positive ions from the compound,
whereas, in the electrolysis of a an aqueous solution there are both OH⁻ ions and H⁺ ions as well as the non metal and metal ions ( + and - ions)