[23.4] electrode potentials

Question 1

how do you produce electrical energy in a redox reaction?

Answer 1

1. separate the oxidation reaction from the reduction reaction
2. allow each to occur in isolation in a half cell
3. make a complete electrochemical cell by joining two half cells together and allowing electrons to flow through an external circuit from electron donor to electron acceptor

Question 2

what does a half cell contain?

Answer 2

• chemical species present in a redox reaction
• eg. rod of metal dipped into a solution of its own ions

Question 3

what is an ion / ion half cell?

Answer 3

• solution contains same element in different oxidation states
• there is no metal to transport electrons in or out of the half-cell so an inert metal electrode is used
• eg. iron (II) and iron (III) sulfate solution with platinum electrode

Question 4

what is a voltaic acell?

Answer 4

made by connecting 2 different half cells together, allowing electrons to flow between them

Question 5

what does the direction of electron flow in a voltaic cell depend on?

Answer 5

the relative tendency of each electrode to release electrons

Question 6

how is the equilibrium in a half cell written by convention?

Answer 6

forward reaction shows reduction, reverse reaction shows oxidation

Question 7

in an operating cell, how do you know which electrode has the greater tendency to release electrons?

Answer 7

• electrode with more reactive metal loses electrons and is oxidised (negative electrode)
• electrode with less reactive metal gains electrons and is reduced (positive electrode)

Question 8

how and why have standard electrode potentials been measured?

Answer 8

• actual potential of half cell cannot be measured direction
• can only measure a potential difference between 2 half cells
• one half cell has been chosen and assigned the potential of 0.00V
• all others are measured compared to it
• standard is the standard hydrogen electrode (SHE)

Question 9

what is the standard electrode potential?

Answer 9

emf of a half cell compared with the standard hydrogen half cell measured at standard conditions

Question 10

what are the standard conditions for the SEP?

Answer 10

• concentration = 1.0 mol⁻³ (for ions in half eq)
• temperature = 298 K
• pressure = 100 kPa (if gases involved)
• current = zero (use high resistance voltmeter)

Question 11

what is the purpose of the wire and salt bridge when measuring SEPs?

Answer 11

• wire allows electrons to flow
• salt bridge (KNO₃) allows ions to flow

Question 12

what is the anode and cathode in the cell?

Answer 12

• anode = negative electrode, oxidised, more reactive metal
• cathode = positive electrode, reduced, less reactive metal

Question 13

how do you calculate the standard cell potential?

Answer 13

E (positive electrode) - E (negative electrode)