Electrochem II

Question 1

What is electrolytic cell?

Answer 1

The process in which electrical energy is used to bring about non-spontaneous redox reaction

Question 2

Where does electrolysis take place?

Answer 2

• in electrolytic cell which consists of a pair of electrodes to carry an electric current into and out of the cell
• with an electrolyte, a compound in aqueous or molten state which conducts an electric current due to presence of mobile ions

Question 3

Is anode positive or negative in an electrolytic cell?

Answer 3

Positive (rmb: PANIC: Positive Anode, Negative Is Cathode)

Question 4

What does anode in an electrolytic cell attract?

Answer 4

Anions (rmb: Anode attracts anions)

Question 5

What reaction takes place at the anode for an electrolytic cell?

Answer 5

Anions are oxidised because they lose electrodes (rmb: an-ox)

Question 6

Is cathode positive or negative in an electrolytic cell?

Answer 6

Negative (rmb: PANIC: Positive Anode, Negative Is Cathode)

Question 7

What does cathode attract in an electrolytic cell?

Answer 7

Cations (rmb: cathode attracts cations)

Question 8

What reaction takes place at the cathode for an electrolytic cell?

Answer 8

Cations are reduced because they gain electrons (rmb: red - cat)

Question 9

What is the flow of electrons and why?

Answer 9

From cathode (-) anode (+)
Electrons move from lower potential to higher potential (think physics circuit, opposite of current)

Question 10

What are the factors affecting the identity of substances liberated?

Answer 10

1. State of electrolyte (molten or aqueous)
2. Position of species in the redox series
3. Concentration of species in the electrolyte
4. Nature of electrode

Question 11

How does the state of electrolyte affect the identity of substance liberated?

Answer 11

• molten electrolyte: only consider cations at cathode and anions at anode
• aqueous electrolyte: need consider water at both cathode and anode (water is present at large quantities)

Question 12

What is the equation for the reduction of water?

Answer 12

2H2O (l) + 2e- –> H2 (g) + 2OH- (aq)

Question 13

What is the equation of the oxidation of water?

Answer 13

2H2O (l) –> 4H+ (aq) + O2 (g) + 4e-

Question 14

How does the position of species in the redox series affect the identity of substance liberated?

Answer 14

• species with more positive E naught value is more readily reduced
• species with less positive E naught value is more readily oxidised

Question 15

Since water can auto-ionise, why does reduction of H+ and oxidation of OH- not occur?

Answer 15

[H+] and [OH-] are too low compared to concentration of H2O and hence not considered

Question 16

How does concentration of species in the electrolyte affect the identity of substance liberated?

Answer 16

The high [M-] concentration causes position of equilibrium to lie on left/right, this causes E(M/M-) to be more of less positive than E(K/K-), thus M is preferentially oxidised or reduced over K

Question 17

What anions do not oxidise even at high concentrations?

Answer 17

NO3- and SO4 2-

Question 18

How to choose equations, is the species supposed to be on right or left and why?

Answer 18

Oxidation: species on right
Reduction: species on left

• Anode , oxidation, loses electron –> choose the equation where species is at right
• Cathode, reduction, gains electrons –> choose equation where species at left

Question 19

How does the nature of electrodes affect the identity of the substance liberated?

Answer 19

• Inert electrodes: do not participate in reactions (except if O2 liberated and at high temperature, graphite electrode may burn and produce CO2 or CO)
• Active electrodes: Participate in reactions occurring at electrodes

Question 20

What are the equations for quantitative electrolysis?

Answer 20

I = Q/t
Q = It = nF = nLe
F = Le

n = amount of electrons
F = Faraday’s constant
L = (Avogadro’s constant
e = charge on 1 electron

Question 21

What is anodising?

Answer 21

Increase the thickness of the corrosion resistant oxide layer by electrolysis

Question 22

What are the advantages of anodising?

Answer 22

-Oxygen produced at the anode reacts with any exposed aluminum, forming a thicker oxide layer which is corrosion resistant
- allow dye pigment to be absorbed onto the porous oxide layer during anodising

Question 23

What must be done to the oxide layer during anodising?

Answer 23

The oxide layer is initially porous and must be sealed by immersing the aluminum object into boiling hot deionised water

Question 24

What is the main thing about anodising?

Answer 24

Reaction at anode:
2H2O (l) –> O2 (g) + 4H+ (aq) + 4e-
2Al (s) + 3/2O2 (g) –> Al2O3 (s)

O2 produced reacts with Al electrode

Question 25

What is the main thing about the purification of copper?

Answer 25

Impure copper has various other metals which also have their reduction potentials

At anode, metals that have E value less positive than E(Cu) are oxidised into their cations and dissolve into the electrolyte. Metals that have E value more positive than E(Cu) are reduced into their anions and fall off the anode and collects to the bottom of cell as ‘anode sludge’

At cathode, metal ions that is less positive than E(Cu) are not reduced and remain in solution

Question 26

What happens to the concentration during purification of copper?

Answer 26

Remains relatively constant throughout the process

Question 27

What happens to the mass of anode and cathode during purification of copper?

Answer 27

Mass of anode decreases (dissolves), mas of cathode increases (deposits of pure Cu(s))