Electrolysis (4.3) (M)

Question 1

What are ions allowed to do when an ionic compound is melted or dissolved in water? What does this mean?

Answer 1

the ions are free to move about within the liquid or solution

these liquids and solutions are able to conduct electricity and are called electrolytes

as they carry charge

Question 2

What are electrolytes?

Answer 2

substances (e.g. ionic compounds) which, when molten or dissolved in solution, will conduct an electric current

Question 3

Passing an electric current through electrolytes causes what?

Answer 3

causes the ions to move to the electrodes

Question 4

In electrolysis, positively charged ions (cations) move to the (…) where they are (…) and (…) electrons

Answer 4

negative electrode / cathode

reduced

gain

Question 5

In electrolysis, negatively charged ions (anions) move to the (…) where they are (…) and (…) electrons

Answer 5

positive electrode / anode

oxidised

lose

Question 6

What happens at the electrodes once ions get there? What is the name of this process

Answer 6

Ions are discharged at the electrodes producing elements

this process is called electrolysis

Question 7

What are positively ions called?

Answer 7

cations

Question 8

What are negatively charged ions called?

Answer 8

anions

Question 9

Electrolysis uses a (…) supply

Answer 9

dc

Question 10

What is oxidation?

Answer 10

loss of electrons

Question 11

What is reduction?

Answer 11

gain of electrons

Question 12

In electrolysis, the electrodes must be…

Answer 12

inert (chemically inactive)

so are electrically charged

Question 13

When a simple ionic compound is electrolysed in the molten state using inert electrodes, what is produced at the cathode and anode?

Answer 13

Cathode - metal

Anode - non-metal

Question 14

When molten lead bromide is electrolysed, what is produced at the cathode and anode?

Answer 14

Cathode - lead

Anode - bromine

Question 15

What is something useful can be extracted from molten compounds using electrolysis?

Answer 15

metals

Question 16

When is electrolysis used to extract metals?

Answer 16

if the metal is too reactive to be extracted by reduction with carbon (in their oxide form) or if the metal reacts with carbon (i.e. above carbon in the reactivity series)

Question 17

2 things

Large amounts of energy are used in the extraction process of electrolysis to do what?

Answer 17

• to melt the compounds
• to produce the electrical current

Question 18

How is aluminium manufactured via electrolysis?

Answer 18

Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide and cryolite (catalyst) using carbon as the positive electrode (anode)

• alumnium liquifies before it is collected*
• may need to give info on general electrolysis processes relevant to Al*

Question 19

key

What is cryolite’s role in the electrolysis of aluminium?

Answer 19

To act as a catalyst, and reduce energy needed (which are high) by lowering the melting point of the mixture and reducing energy costs

Question 20

Why does a mixture need to be molten for electrolysis to occur?

Answer 20

because current is carried through the electrolyte by the movement of ions - not electrons

Question 21

What is produced at the positive electrode during the extraction of aluminium?

Answer 21

carbon dioxide

Question 22

Why must the positive electrode must be continually replaced?

Answer 22

Because a gas is often formed between the electrode (e..g carbon) and electrolyte (oxygen) and so it [the electrode] burns away

Question 23

The ions discharged when an aqueous solution is electrolysed using inert electrodes depend on what?

Answer 23

the relative reactivity of the elements involved

Question 24

What ions will be present in an aqueous solution before electrolysis?

Answer 24

• metal ion (+)
• non-metal ion (-)
• hydrogen ion (+)
• hydroxide ion (-)

Question 25

What is produced at the negative electrode (cathode) if the metal is more reactive than hydrogen in an aqueous solution?

Answer 25

hydrogen produced

Question 26

What is produced at the negative electrode (cathode) if the metal is less reactive than hydrogen in an aqueous solution?

Answer 26

the metal

Question 27

What is produced at the positive electrode (anode) if the solution does not contain halide ions?

Answer 27

oxygen

Question 28

What is produced at the positive electrode (anode) if the solution contains halide ions?

Answer 28

the halogen

Question 29

Why is hydrogen (if less reactive than metal) produced at the cathode and oxygen (unless halide ions present) at the anode during electrolysis of an aqueous solution?

Answer 29

because in the aqueous solution water molecules break down producing hydrogen ions and hydroxide ions that are discharged

Question 30

Write the half equations for the formation of Na and Cl in molten sodium chloride (NaCl)

Answer 30

Reduction: Na⁺ + e^– ⇢ Na

Oxidation: 2Cl^– ⇢ Cl₂ + 2e^–

a good rule of thumb is that the electron is on the left in reduction, and on the right in oxidation

Question 31

Write the half equations for the electrolysis of sodium sulphate in an aqueous solution?

Answer 31

2H⁺ + 2e^– ⇢ H₂

4OH^– ⇢ O₂ + 2H₂O + 4e^–

For oxidation, it may also look like 4OH^– - 4e^–⇢ O₂ + 2H₂O

Question 32

Write the half equations for the electrolysis of sodium chloride in an aqueous solution?

Answer 32

2H⁺ + 2e^– ⇢ H₂

2Cl^– ⇢ Cl₂ + 2e^–

Question 33

Why would electrolysis not take place in a solid?

Answer 33

Solid compound does not conduct electricity

because the ions cannot move

and carry charge

Question 34

What would be observed if a metal formed at the negative electrode?

Answer 34

Solid metal deposit on the negative electrode

Question 35

3 marker - higher tariff may require more info

Why is carbon dioxide produced at the positive electrode/anode in the electrolysis of aluminium oxide?

Answer 35

The anode/ is made out of carbon (graphite)

Oxygen is produced at the anode

Oxygen reacts with the anode

allow positive electrode for anode throughout