18. Electrochem

Question 1

Def standard electrode potential

Answer 1

potential diff btw electrode of half-cell and SHE (standard hydrogen elect) measured under standard conditions

Question 2

def standard cell potential

Answer 2

max potential diff btw 2 elect of an electrochem cell measured under standard conditions

(conc is 1mol dm⁻³, pressure gas 1 bar, temp 25⁰C)

Question 3

Standard ans for exp why addition of __ will change E cell

Answer 3

1. [change]
2. POE shift __ to [counteract change]
3. E more (+/-)
4. E cell more (+/-)

Question 4

Limitations of predictions using E values

Answer 4

• changing the conditions (eg conc) would change the E values
• cannot predict reaction rates (catalyst, high Ea)

Question 5

eqn for rxn btw gibbs free energy and electrode potential

Answer 5

ΔG = − zF E cell

z = num e transferred divided by no. moles of __

Question 6

eqn linking faraday’s constant and charge on e

Answer 6

F = Le
(e is charge on e)

Question 7

what is brine made of

Answer 7

conc NaCl

Cl⁻ preferentially oxi when conc (not aq)

Question 7

Exp anodising of Al

Answer 7

• use Al anode, Pt cathode, dilute H₂SO₄
• 4Al + 6H₂O –> 2 Al₂O₃ + 6H₂
  (@ anode: H₂O to O₂, then react w Al)

Question 8

What happens during purification of Cu

Answer 8

1. Anode
   * Eө(Ni²⁺/Ni) more (-) than Eө(Cu²⁺/Cu), Ni will also be oxi
   * Ni²⁺ and Cu²⁺ dissolve and migrate to cathode
   * Eө(Ag⁺/Ag) more (+), wont oxi, anode sludge
2. Cathode
   * Eө(Ni²⁺/Ni) less (+) than Eө(Cu²⁺/Cu), Cu preferentially reduced
   * Ni²⁺ remain in sol

Question 9

Purpose of salt bridge in electrochemical cell

Answer 9

• electron neutrality
• prevent mixing of sol by diffusion, else direct chem change destroy cell