Electrolysis

Question 1

Why can’t covalent compounds conduct electricity?

Answer 1

• in covalent bonds, all the electrons on the outer shell are used to form it
• this means that there are no mobile electrons that can carry a current as they are all used up
• no mobile charge carriers means that no current can flow
• so covalent compounds cannot conduct electricity

Question 2

Why can graphite conduct electricity despite having a giant covalent lattice structure?

Answer 2

• in graphite, each carbon atoms forms 3 bonds to other carbons instead of 4
• this means that layers are produced
• this leaves one free delocalised electron, which become mobile charge carriers and are free to move around the layers
• this allows them to conducts electricity as a current can flow

Question 3

Why can ionic compounds only conduct electricity when they are molten or in an aqueous solution?

Answer 3

• when solid, ionic compounds cannot conduct electricity as their ions are vibrating in fixed positions within the lattice
• however, when molten or in aqueous solutions, the ions dissociate from each other and become free to move
• this makes them mobile charge carriers as they are no longer in fixed positions so can carry a current
• this means that ionic compounds can conduct electricity in molten or aqueous form

Question 4

What is the positive ion called?

Answer 4

Cation

Question 5

What is the negative ion called?

Answer 5

Anion

Question 6

What is electrolysis?

Answer 6

The process of splitting up a compound in either molten or aqueous form, into two new substances (elements) using electricity

Question 7

What is an electrode?

Answer 7

• The conductor through which electricity is passed into and out of an electrolyte
• often made out of carbon/graphite

Question 8

What is an electrolyte?

Answer 8

The liquid that undergoes electrolysis; this is either molten of an aqueous solution containing mobile ions

Question 9

What state do insoluble ionic compounds have to be in for electrolysis?

Answer 9

Insoluble Ionic compounds have to be melted into a molten state in order to undergo electrolysis

Question 10

What do some industrial companies do to save money when doing electrolysis on molten ionic compound?

Answer 10

as lots of energy is needs to melt an insoluble ionic compounds (as they have high melting points), substances such as cryolite can be added to the ionic compound to reduce its melting point. This means less energy is required to melt it = cheaper

Question 11

LEO says GER

Answer 11

• Loss of Electrons = Oxidation
• Gain of Electrons = Reduction

Question 12

What two reactions occur simultaneously in electrolysis?

Answer 12

Reduction and oxidation (so they are often called redox reactions)

Question 13

What is the positive electrode called?

Answer 13

Anode

Question 14

What is the negative electrode called?

Answer 14

Cathode

Question 15

Describe what will be observed in the electrolysis of lead(II)bromide

Answer 15

• once molten, if a lightlbulb is in circuit, it will light up as electrons start flowing
• bubbling will occur around the anode due to formation of brown bromine gas, which may or may not be visible depending upon the scale of the experiment
• no obvious change will be seen near cathode, but a lump of silver metal will have formed under once heating stops

Question 16

What are the ions present in lead(II) bromide?

Answer 16

• anion = bromide Br- (goes to anode)
• cation = lead(II) Pb2+ (goes to cathode)

Question 17

Which electrode do each ion go to?

Answer 17

• cation = cathode
• anion = anode

Question 18

Explain what happens when lead(II) bromide undergoes electrolysis

Answer 18

• molten state = ions move around freely
• negative bromide ions attracted to anode (as opposite charges attract)
• bromide ions become discharged and lose an electron to become bromine
• they form bromine diatomic molecules to be stable
• given off as bromine gas
• electrons from bromide ions travel to cathode and are given to lead(II) ions
• lead(II) ions become lead atoms by gaining 2 electrons
• lead metal will deposit underneath cathode

Question 19

Lead half equation

Answer 19

Pb²⁺ (l) + 2e^- => Pb (l)

Question 20

Bromine half equation

Answer 20

2Br^- (l) => Br₂ (g) + 2e^-

Question 21

What happens to water during electrolysis?

Answer 21

The ions in water (H⁺ and OH^-) naturally dissociate from each other

Question 22

What ions are present during the electrolysis of sodium chloride (NaCl) solution?

Answer 22

• Na⁺ (from ionic compound)
• Cl^- (from ionic compound)
• H⁺ (from water)
• OH^- (from water)

Question 23

Describe what would be observed in the electrolysis of sodium chloride solution

Answer 23

a gas would collect in the test tube over the cathode and the same would happen for the anode

Question 24

Explain what would happen during the electrolysis of sodium chloride solution

Answer 24

• hydrogen is less reactive than sodium, so it is easier to add an electron onto it for it to be discharged
• this means that hydrogen ions are attracted to the cathode and are given an electron
• hydrogen ions become hydrogen molecules and hydrogen gas is given off
• gas is collected in the inverted test tube above the cathode
• halide ions (in this case Cl^-) are preferentially attracted to anode instead of OH^- ions
• an electron is removed from chloride ion and become chlorine molecules + chlorine gas is given off and collected in the inverted test tube above the anode
• the remaining ions (Na ⁺ and OH^-) together form sodium hydroxide solution

Question 25

What ions are present during the electrolysis of dilute sulfuric acid?

Answer 25

- H⁺ (from the sulfuric acid) - SO₄^2- (from the sulfuric acid) - H⁺ (from the water) - OH^- (from the water)

Question 26

Describe what would be observed during the electrolysis of sulfuric acid

Answer 26

Twice as much gas would be collected in the test/measuring tube inverted above the cathode than the anode

Question 27

Explain what would happen during the electrolysis of dilute sulfuric acid

Answer 27

- as the only positive ion present is H⁺ from both the water and dilute acid, it is attracted to the cathode and is discharged by being given an electron onto it - hydrogen ions become hydrogen molecules (H₂ molecules) to be stable and gas is given off (seen by effervescence) at the cathode and gas is collected - OH^- is preferentially discharged at any given time so OH^- become oxygen molecules and water molecules - effervescence occurs by the anode and gas is collected - remaining ions (H⁺ and SO₄^2-) become slightly more concentrated sulfuric acid

Question 28

Half equation of hydrogen

Answer 28

2H⁺ (aq) + 2e^- => H₂ (g)

Question 29

Half equation of chlorine

Answer 29

2Cl^- (aq) => Cl₂ (g) + 2e^-

Question 30

Half equation of hydroxide ions

Answer 30

4OH^- (aq) => 2H₂O (l) + O₂ + 4e^-

Question 31

What ions are present during the electrolysis of copper(II) sulfate?

Answer 31

- Cu²⁺ (from the ionic compound) - SO₄^2- (from the ionic compound) - H⁺ (from the water) - OH^- (from the water)

Question 32

Describe what happens during the electrolysis of copper(II) sulfate

Answer 32

- the electrolyte changes colour from blue to colourless - bubbling occurs at the anode - shiny pink metal will form around the cathode

Question 33

Explain what would happen during the electrolysis of copper(II) sulfate

Answer 33

- as copper is less reactive than hydrogen, it would be attracted to the cathode and would gain two electrons to become copper atoms - copper will buildup on the cathode - OH^- ions are preferentially attracted to the anode and lose an electron to produce oxygen and water molecules - effervescence occurs by oxygen production at the anode - remaining ions (H⁺ and SO₄) become sulfuric acid

Question 34

Why does the electrolyte change colour from blue to colourless during the electrolysis of copper(II) sulfate?

Answer 34

- copper ions are discharged to the cathode electrode to form copper metal as less reactive ions are discharged first - copper(II) ions, when removed from the solution, cause discolouring of copper(II) sulfate solution

Question 35

What does sodium chloride solution produce at each electrode during electrolysis? What remains?

Answer 35

- cathode = hydrogen gas - anode = chlorine gas - remains = sodium hydroxide solution

Question 36

What does dilute sulfuric acid produce at each electrode during electrolysis? What remains?

Answer 36

- cathode = hydrogen - anode = oxygen - remains = more concentrated sulfuric acid

Question 37

What does copper(II) sulfate produce at each electrode during electrolysis? What remains?

Answer 37

- cathode = copper metal - anode = oxygen - remains = sulfuric acid

Question 38

What reaction always happens where in electrolysis?

Answer 38

- reduction always occurs at the cathode - oxidation always occurs at the anode

Question 39

What are the 3 main rules to compare the reactivity of ions during electrolysis?

Answer 39

- if the metal is high in the reactivity series, hydrogen is produced at the cathode instead of the metal - if the metal is below hydrogen in the reactivity series, the metal is produced at the cathode - solutions containing halides always obtain the halogen at the anode

Question 40

Rules for what happens at cathode (negative electrode) in electrolysis of aqueous solutions

Answer 40

- if there is a non-reactive metal ion present, that metal will be produced at cathode - if the cation is above hydrogen, hydrogen is produced at cathode - if below hydrogen then that metal is produced - reduction always happens at cathode

Question 41

Rules for anode for aqueous electrolysis

Answer 41

- oxidation happens at the anode - oxygen is always produced at anode unless halides are present, in which the halogen gas is produced

Question 42

Half equation formula for cathode and anode

Answer 42

- negative cathode: cation + electron(s) —> element - this is due to reduCtion which happens at the Cathode = gain of electrons - positive anode: anion —> element + electron(s) - this is due to Oxidation which happens at the anOde = loss of electrons