Electrochemistry

Question 1

Electrolysis

Answer 1

Electrolysis is the decomposition of a liquid electrolyte using a direct current of electricity

Question 2

PANIC

Answer 2

Positive 
Anode
Negative 
Is
Cathode

Question 3

Electrolyte

Answer 3

Ionic compound that is molten or dissolved in water

Question 4

Cathode

Answer 4

Negative electrode

Question 5

Anode

Answer 5

Positive electrode

Question 6

Cation

Answer 6

Positive ion

Question 7

Anion

Answer 7

Negative ion

Question 8

Inert electrode

Answer 8

Metal strips or graphite rods placed into the liquid and connected to a supply of electricity

Question 9

Give one use of electrolysis in industry

Answer 9

Extracting aluminium from its ore

Question 10

How does electrolysis work?

Answer 10

• All electrolytes conduct electricity because they have free ions that can move and carry charge
• ions are attracted to the oppositely charged electrode
• Positive cations at the negative electrode (cathode) gain electrons to become atoms
• When electrons are gained this is reduction
• the negative anions At the positive electrode (anode) lose electrons to become atoms, which may combine to form diatomic molecules for elements e.g chlorine
• When electrons are lost this is oxidation

Question 11

What are electrodes made from ?

Answer 11

Carbon

Question 12

Why graphite is used for electrodes

Answer 12

• it is a good conductors of electricity

- It is unreactive

Question 13

Describe what is observed at the cathode with molten lead (ii) bromide

Answer 13

• Positive ion Pb2+ attracted to negative cathode
• Half equation: Pb2+ + 2e- —> Pb
  Observations: silvery grey liquid formed, which sinks to the bottom

Question 14

Describe what is observed at the anode with molten lead (ii) bromide

Answer 14

Negative ion Br- attracted to positive anode
Half equation - 2Br- —> Br2 + 2e-
Observations - red-brown pungent has evolved

Question 15

Describe what is observed at the cathode with molten lithium chloride

Answer 15

Positive ion Li+ attracted to negative cathode
Half equation - Li+ + e- —> Li
Observations - molten silvery grey liquid formed

Question 16

Describe what is observed at the anode with molten lithium chloride

Answer 16

Negative ion Cl- attracted to anode
Half equation - 2Cl- —> Cl2 + 2e-
Observations - pale green pungent gas evolved. Oxidation reaction

Question 17

Describe the products of electrolysis of dilute sulfuric acid and half equations for the reactions at the anode and cathode

Answer 17

Cathode: Positive ion H+ attracted to cathode
2H+ + 2e- —> H2
Observations - Colourless odourless gas evolved

Anode: Negative ions OH- and SO4 2- attracted to anode
4OH- —> O2 + 2H2O + 4e-
Colourless, odourless gas evolved

Question 18

State what is observed at the anode during electrolysis of dilute sulfuric acid

Answer 18

The negative ions OH- and SO4 2- are attracted to the anode. The PH- ion is preferentially discharged instead of the SO4 2- ion
It takes four electrons to be removed at the anode to produce one O2 molecule

Question 19

Why is the volume of hydrogen gas twice the volume of oxygen gas when using electrolysis of dilute sulfuric acid (2)

Answer 19

• because two electrons given up at the cathode produce one H2 molecule, whereas it takes four electrons to be removed at the anode to produce one O2 molecule
• Four electrons lost and gained will produce two H2 molecules for every one O2 molecule

Question 20

Name aluminium ore

Answer 20

Bauxite

Question 21

Reaction at the anode and cathode of extracting aluminium

Answer 21

Cathode
Al3+ + 3e- —> Al
Anode
2O2- —> O2 + 4e-
Reaction at cathode is reduction because aluminium ions are gaining electrons
Reaction at anode is oxidation because oxide ions are losing electrons

Question 22

Why does carbon have to be replaced periodically ?

Answer 22

It wears away as it reacts with oxygen

C + O2 —> CO2

Question 23

Describe the industrial extraction of aluminium (3)

Answer 23

• Bauxite is purified by mixing it with sodium hydroxide solution
• The alumina (aluminium oxide) in the ore dissolves in molten cryolite to reduce the operating temperature and increase its conductivity
• Crust of aluminium oxide keeps the heat in. Operating temperature is between 900 and 1000 degrees

Question 24

Disadvantage of electrolysis of aluminium

Answer 24

• Extraction of aluminium is expensive because the cost of electricity is high and a high temperature is needed to keep the aluminium oxide molten

Question 25

Advantages of recycling aluminium (3)

Answer 25

- Aluminium oxide crust keeps some heat in, saving money - the expense of recycling aluminium is only a fraction of the cost of producing new aluminium from bauxite - This is why it’s important to recycle materials such as aluminium, it saves resources, saves energy, prevents waste going to landfill and costs less

Question 26

Why is cryolite used ?

Answer 26

The use of cryolite increases the conductivity and reduces the operating temperature, saving money

Question 27

Draw the working out f the products of the electrolysis of dilute sulfuric acid solution

Answer 27

-

Question 29

Definition of electrolyte

Answer 29

The electrolyte is the liquid or solution that conducts electrify and is decomposed by it

Question 30

Inert electrodes definition

Answer 30

Electrodes that do not take part in the reactions

Question 31

What are common electrolytes

Answer 31

Molten ionic compounds and aqueous ionic compounds

Question 32

What happens if a molten metal oxide is electrolysed

Answer 32

Oxygen gas is produced at the anode | Observations include a colourless, odourless gas being evolved

Question 33

Observations of sodium and iodine at the cathode/anode

Answer 33

Silvery grey liquid formed | Purple, pungent gas