(U2) Electrode potentials

Question 1

what is standard electrode potential? (3)

Answer 1

• The potential difference measured when a half-cell
• is connected to the standard hydrogen electrode
• under standard conditions

Question 2

what is e.m.f.?

Answer 2

The potential difference measured when two half-cells are connected

Question 3

how do you write conventional cell notation? (2)

Answer 3

• cathode (-ve) (oxidation half-cell) is written on the left
• the anode (+ve) (reduction half-cell) is written on the right

oxidised | oxidised to ||reduced | reduced to

Question 4

what are the environmental risks associated with electrical cells? (2)

Answer 4

• primary cells can easily leak harmful substances
• primary and secondary cells are sources of waste and have to be disposed of

both primary and secondary degenerate over time and produce reduced current

Question 5

what are the reactions in a lithium ion battery?

Answer 5

• cathode: Li –> Li⁺ + e^-
• anode: Li⁺ + CoO₂ –> Li⁺[CoO₂]^-

Overall: Li + CoO₂ –> Li⁺[CoO₂]^-

CoO₂ is just an example - can be any lithium containing transition metal compound

Question 6

how do you know if a reaction is feasible based on electrode potentials?

Answer 6

if emf is +ve

Question 7

how do you calculate emf of a cell?

Answer 7

(more +ve) - (more -ve)

(electrode potentials of individual half cells)

Question 8

what is the electrode potential of a half cell an indicator of?

Answer 8

direction of equilibrium

Question 9

give an example of a:

• non-rechargable cell
• rechargable cell
• fuel cell

Answer 9

• NRC: zinc-carbon cell
• RC: lithium-ion cell
• FC: hydrogen-oxygen cell

Question 10

what is a:

• non-rechargable cell
• rechargable cell
• fuel cell

Answer 10

• NRC: cell reaction is irreversible and cell can’t be recharged
• RC: cell reaction is reversible and cell can be recharged by operating the cell in reverse
• FC: reaction of a fuel with oxygen to generate a voltage (chemical to electrical energy)

Question 11

describe the standard hydrogen electrode (4)

Answer 11

• gas bubbled over a platinum electrode
• hydrogen at 100 kPa
• surrounded by H⁺ ions in solution at 1M
• 298K

Question 12

what is the equation for the overall process at the standard hydrogen electrode?

Answer 12

2H⁺_(aq) + 2e^- ⇌ H_{2 (g)}

Question 13

what are the purposes of a salt bridge? (3)

Answer 13

• acts as an electrical connection between 2 half cells
• maintains balances of anions and cations
• completes circuit without introducing a metal

Question 14

if the reactant and product in a half cell are in the same state, what material is the electrode?

Answer 14

inert and conductive metal, typically platinum

Question 15

why might a reaction not occur even in feasible

Answer 15

high activation energy

Question 16

whats usually in a salt bridge?

why

Answer 16

• aqueous KNO₃ in filter paper
• doesn’t react with electrodes