Electrolysis - 5th Flashcards
1
Q
what is electrolysis?
A
- 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
2
Q
what connects the substance to the circuit?
A
- two electrodes
- the positive electrode is called the anode
- the negative electrode is called the cathode
3
Q
what must the electrodes be made out of?
A
- 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
4
Q
what is the electrolyte?
A
- the substance to be broken down, and it needs to conduct electricity
5
Q
what are the only compounds that conduct electricity?
A
- the ionic compounds that are molten or dissolved so the ions are free to move
6
Q
why wouldn’t solid ionic compounds conduct electricity?
A
- because the ions cannot move
- covalent compounds won’t work because the molecules aren’t even charged
7
Q
what are the ions in the compound called?
A
- positive ions are called cations - they are attracted to the cathode
- negative ions are called anions - they are attracted to the anode
8
Q
what does electrolysis of a molten compound produce?
A
- its elements
- compound -> elements
9
Q
how is electrolysis thought of?
A
- in two halves
- the anode half and the cathode half
10
Q
what happens at the anode?
A
- 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
11
Q
what are non-metals made of?
A
- diatomic molecules
- Cl atoms pair up to form Cl2 molecules
12
Q
how do we write out what happens at the anode?
A
2Cl- -> Cl2 + 2e-
13
Q
what happens to the Cl- ions?
A
- they are oxidised - have lost electrons
14
Q
what will likely be seen at the anode?
A
- a pale green gas (Cl2)
- bubbles of pale green gas
15
Q
what happens at the cathode?
A
- 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
16
Q
why don’t atoms pair up at the cathode?
A
- because metals aren’t made up of atons
17
Q
how do we write out what happens at the cathode?
A
- as a half-equation, showing the Mg2+ ions gaining electrons and becoming atoms
- Mg2+ + 2e- -> Mg
18
Q
what happens to the Mg2+ ions?
A
- they have been reduced
- they have gained electrons
19
Q
what would be seen at the cathode?
A
- Mg is a grey solid, so some grey solid would form on the surface of the cathode
20
Q
describe what happens in the electrolysis of molten Al2O3. (word equation)
A
- overall: aluminium oxide -> aluminium + oxygen
21
Q
what happens at the anode in this reaction?
A
- 2O2 -> O2 + 4e-
- O2- ions are oxidised because they lost electrons
- bubbles of colourless gas would be observed at the anode
22
Q
what happens at the cathode in this reaction?
A
- Al3+ + 3e- -> Al
- Al3+ ions are reduced because they gain electrons
- a shiny grey solid would be observed at the cathode
23
Q
what can water molecules break apart to form?
A
- to form H+ and OH- ions
- H2O (l) -> H+ (aq) + OH- (aq)
24
Q
what do the extra ions do?
A
- complicate electrolysis
25
how do you know which ions react in aqueous electrolysis at the anode and which don't?
- at the anode: if a halide ion (Cl-/Br-/I-) is present, that element is produced as per normal - 2Br- -> Br2 + 2e-
26
how do you know which ions react in aqueous electrolysis at the cathode and which don't?
- whichever ions did not react remain leftover in the solution as a third product
27
what does NaCl (aq) contain?
- NaCl = Na+ and Cl- ions
- H2O = H+ and OH- ions
28
what happens at the anode for NaCl?
- chloride (Cl-) is a halide ion, so it reacts at anode :
- 2Cl- -> Cl2 + 2e-
- bubbles of pale green gas would be observed
29
what happens at the cathode for NaCl?
- sodium (Na) is more reactive than hydrogen (H2), so we get hydrogen instead :
- 2H+ + 2e- -> H2
- bubbles of colourless gas would be observed
30
what is leftover in the solution for NaCl?
- the Na+ (from NaCl) and OH- ions (from H2O) didn't react
- sodium hydroxide (NaOH) is left over
31
what does CuSO4 (aq) contains?
- CuSO4 = Cu2+ and SO4 2- ions
- H2O = H+ and OH- ions
32
what happens at the anode for CuSO4 (aq) ?
- 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
33
what happens at the cathode for CuSO4 (aq)?
- copper (Cu) is less reactive than hydrogen (H2), so we get copper:
- Cu 2+ + 2e- -> Cu
- A pink orange solid would be observed
34
what is leftover in the solution for CuSO4 (aq)?
- the SO4 2- (from CuSO4) and H+ ions (from H2O) didn't react
- sulfuric acid (H2SO4) is leftover
