Electrolysis - 5th

Question 1

what is electrolysis?

Answer 1

• the breaking down of a compound using electricity
• the substance to be broken down is connected into a circuit with a cell or a battery

Question 2

what connects the substance to the circuit?

Answer 2

• two electrodes
• the positive electrode is called the anode
• the negative electrode is called the cathode

Question 3

what must the electrodes be made out of?

Answer 3

• the electrodes must be made from a solid that conducts electricity but is inert (unreactive)
• usually, one of graphite (cheap) or platinum (expensive) are used

Question 4

what is the electrolyte?

Answer 4

• the substance to be broken down, and it needs to conduct electricity

Question 5

what are the only compounds that conduct electricity?

Answer 5

• the ionic compounds that are molten or dissolved so the ions are free to move

Question 6

why wouldn’t solid ionic compounds conduct electricity?

Answer 6

• because the ions cannot move
• covalent compounds won’t work because the molecules aren’t even charged

Question 7

what are the ions in the compound called?

Answer 7

• positive ions are called cations - they are attracted to the cathode
• negative ions are called anions - they are attracted to the anode

Question 8

what does electrolysis of a molten compound produce?

Answer 8

• its elements
• compound -> elements

Question 9

how is electrolysis thought of?

Answer 9

• in two halves
• the anode half and the cathode half

Question 10

what happens at the anode?

Answer 10

• non-metal ions lose electrons to form molecules
• the Cl- ions are attracted to the anode and they lose electrons to form neutral Cl atoms
• each Cl- ion has a 1- charge, they lose one electron each to become neutral

Question 11

what are non-metals made of?

Answer 11

• diatomic molecules
• Cl atoms pair up to form Cl2 molecules

Question 12

how do we write out what happens at the anode?

Answer 12

2Cl- -> Cl2 + 2e-

Question 13

what happens to the Cl- ions?

Answer 13

• they are oxidised - have lost electrons

Question 14

what will likely be seen at the anode?

Answer 14

• a pale green gas (Cl2)
• bubbles of pale green gas

Question 15

what happens at the cathode?

Answer 15

• metal ions gain electrons to form atoms
• the Mg2+ ions are attracted to the cathode and they gain electrons to form neutral Mg atoms
• as each Mg2+ ion has a 2+ charge, they gain two electrons to become neutral

Question 16

why don’t atoms pair up at the cathode?

Answer 16

• because metals aren’t made up of atons

Question 17

how do we write out what happens at the cathode?

Answer 17

• as a half-equation, showing the Mg2+ ions gaining electrons and becoming atoms
• Mg2+ + 2e- -> Mg

Question 18

what happens to the Mg2+ ions?

Answer 18

• they have been reduced
• they have gained electrons

Question 19

what would be seen at the cathode?

Answer 19

• Mg is a grey solid, so some grey solid would form on the surface of the cathode

Question 20

describe what happens in the electrolysis of molten Al2O3. (word equation)

Answer 20

• overall: aluminium oxide -> aluminium + oxygen

Question 21

what happens at the anode in this reaction?

Answer 21

• 2O2 -> O2 + 4e-
• O2- ions are oxidised because they lost electrons
• bubbles of colourless gas would be observed at the anode

Question 22

what happens at the cathode in this reaction?

Answer 22

• Al3+ + 3e- -> Al
• Al3+ ions are reduced because they gain electrons
• a shiny grey solid would be observed at the cathode

Question 23

what can water molecules break apart to form?

Answer 23

• to form H+ and OH- ions
• H2O (l) -> H+ (aq) + OH- (aq)

Question 24

what do the extra ions do?

Answer 24

• complicate electrolysis

Question 25

how do you know which ions react in aqueous electrolysis at the anode and which don't?

Answer 25

- at the anode: if a halide ion (Cl-/Br-/I-) is present, that element is produced as per normal - 2Br- -> Br2 + 2e-

Question 26

how do you know which ions react in aqueous electrolysis at the cathode and which don't?

Answer 26

- whichever ions did not react remain leftover in the solution as a third product

Question 27

what does NaCl (aq) contain?

Answer 27

- NaCl = Na+ and Cl- ions - H2O = H+ and OH- ions

Question 28

what happens at the anode for NaCl?

Answer 28

- chloride (Cl-) is a halide ion, so it reacts at anode : - 2Cl- -> Cl2 + 2e- - bubbles of pale green gas would be observed

Question 29

what happens at the cathode for NaCl?

Answer 29

- sodium (Na) is more reactive than hydrogen (H2), so we get hydrogen instead : - 2H+ + 2e- -> H2 - bubbles of colourless gas would be observed

Question 30

what is leftover in the solution for NaCl?

Answer 30

- the Na+ (from NaCl) and OH- ions (from H2O) didn't react - sodium hydroxide (NaOH) is left over

Question 31

what does CuSO4 (aq) contains?

Answer 31

- CuSO4 = Cu2+ and SO4 2- ions - H2O = H+ and OH- ions

Question 32

what happens at the anode for CuSO4 (aq) ?

Answer 32

- sulfate (SO4 2-) is not a halide ion, so instead we get O2 and H2O - 4OH - -> O2 + 2HO + 4e- - bubbles of colourless gas would observed

Question 33

what happens at the cathode for CuSO4 (aq)?

Answer 33

- copper (Cu) is less reactive than hydrogen (H2), so we get copper: - Cu 2+ + 2e- -> Cu - A pink orange solid would be observed

Question 34

what is leftover in the solution for CuSO4 (aq)?

Answer 34

- the SO4 2- (from CuSO4) and H+ ions (from H2O) didn't react - sulfuric acid (H2SO4) is leftover