electrolysis

Question 1

2 formula

calculations involving Q (quantity of charge)

Answer 1

• Q = nF
  
  • n is no of moles of electrons
  • F is Faraday constant
• Q = It
  
  • I is current (A)
  • t is time (s)

Question 2

calculations involving F (Faraday constant)

Answer 2

F = Le
- L is Avogadro constant
- e is the charge on 1 electron

Question 3

relationship between standard cell potential (Eº) and Gibbs free energy (∆G)

Answer 3

ΔG = -nFE°cell
- n is no of moles transferred per mole of cell rxn

Question 4

Electrolytic purification of copper

Answer 4

1.
- Cu is oxidised (a) at the anode
(+ (d) oxidation half-eqn)
- Metal impurities with E° less positive (more negative) than E° (Cu2+/Cu)
will be (b) more readily oxidised than Cu
=> (Metal impurity) will also be oxidised (a) at anode and
(c) dissolves into electrolyte as (metal ion)
(+ (d) oxidation half-eqn)
- Metal impurities with E° more positive (less negative) than E° (Cu2+/Cu)
is (b) less readily oxidised than Cu
=> (metal impurity) will not be oxidised (a) at anode and
(c) sinks to the bottom to be collected as “anode sludge”

2. • Only Cu2+ ions will be preferentially reduced and (b) deposited (a) at cathode
     (+ (d) oxidation half-eqn)
   • Metal impurities ions with E° less positive than E° (Cu2+/Cu)
     is (b) less readily reduced than Cu
     => (Metal impurity ions) will not be reduced (a) at the cathode and
     (c) will remain in the electrolyte

Question 5

functions of anodising of aluminium

Answer 5

background: Al objects are usually resistant to corrosion when exposed to air as Al forms a layer of Al2O3 which acts as a protective layer which seals off Al beneath it from further reaction
- Anodised aluminium can be used to make window frames or drink cans,
and anodising it increases the thickness of corrosion resistant Al2O3 layer,
thus making objects more durable
- Al2O3 layer is porous and can accept and hold dyes that would not otherwise stick to Al, thus allowing anodised Al object to be dyed

Question 6

half-eqns of purification of aluminium from Al2O3

Answer 6

• Cathode: Al3+ + 3e -> Al (Reduction)
• Anode: 2 O2- -> O2 + 4e (Oxidation)

Question 7

half-eqns for the reactions at the electrodes

purification of aluminium from Al2O3

Answer 7

• cathode: Al3+ + 3e- -> Al [R]
• anode: 2 O2- -> O2 + 4e- [O]

Question 8

requirements for a fully labelled diagram of an electrochemical cell

Answer 8

• electrodes
  
  • Pt (platinum) unless stated otherwise
• reactants
  
  • if gas, write …(gas)…, 1 bar, 298K
  • if aq, write 1 mol dm-3 …(reactant)…, 298K
• voltmeter
• salt bridge