Topic 19: Redox Processes (HL)

Question 1

Electromotive force, EMF

Answer 1

Energy supplied by a source divided by the electric charge transported through the source

Question 2

Cell potential

Answer 2

the potential difference between the cathode and the anode when the cell is operating and is always less than the maximum voltage that can be delivered by the cell

Also depends on the concentrations of the species involved and operating temperature

Question 3

Standard cell potential, Ecell

Answer 3

Cell potential under standard conditions (1 mol dm^3 [reactants] in solution and 100 kPa for gaseous reactants)

Ecell = Erte - Elhe

Erte - electrode potential at the cathode, ie right hand side electrode

Elhe - electrode potential at the anode, ie the left hand side electrode

Question 4

Standard hydrogen electrode, SHE

Answer 4

• universal reference electrode
• gas electrode
• consists of an inert platinium electrode in contact with 1 mol dm^(-3) hydrogen ions and hydrogen gas at 100kPa
• the standard electrode potential of SHE is taken at 0V at all temperatures, thus the potentials of other electrodes are then compared to the SHE reference at the same temperature
• reduction half-equation:

2H+ (aq) + 2e ⇋ H2 (g)

Question 5

Formula for energy

Answer 5

energy (J) = potential (V) x charge (C)

Question 6

Cell potential and GIbbs free energy formula

Answer 6

ΔG = –nFEcell

where:
n = amount, in mol, of electrons transferred in the balanced equation;
F = Faraday’s constant = 96500 C mol 1;
Ecell = standard cell potential (calculated from E cell = E rhe - E lhe)

Question 7

Describe the functions of the platinum electrode in the SHE

Answer 7

• the platinum electrode is actually a platinized platinum to increase the surface area
• doesn’t corrode or ionize, ie will not act as an electrode in the system
• acts as a heterogeneous catalyst, ie. it provides surface for the dissociation of hydrogen molecules
• equilibrium of the absorbed molecules of hydrogen and the hydrogen ions in the solution are established on the platinum surface
• acts as an electrical conductor to the external circuit

Question 8

For (ΔG = –nFE cell):

ΔG negative and E cell positive

Answer 8

spontaneous reactions, favours formation of products

Question 9

For (ΔG = –nFE cell):

ΔG positive and E cell negative

Answer 9

non-spontaneous, favours the formation of reactants

Question 10

For (ΔG = –nFE cell):

ΔG zero and E cell zero

Answer 10

equal formation of both reactants and products

Question 11

Overvoltage

Answer 11

In an electrolytic experiment, the applied potential needed to carry out the electrolysis is always greater than the potential calculated from standard redox potentials - this extra difference in potential or voltage is overvoltage.

Question 12

How to test for colourless hydrogen gas?

Answer 12

By lighting a match in a test tube filled with the gas, the gas will ignite with a small popping sound. If the gas is a mixture of hydrogen and air then there’ll be a much louder pop

Question 13

Electrorefining of copper

Answer 13

purification of copper, e.g. in electrical wires the purity needs to be very high

Question 14

Electroplating

Answer 14

Process which involved the use of active electrodes in electrolysis to deposit a thin layer of one metal onto the cathode of another.

Widely used in jewellery

Question 15

List three factors that affect the amount of product formed at the electrodes during electrolysis.

Answer 15

Q = I x t
1) Current (I)

2) Duration of electrolysis (t)
3) Charge on the ion (z)

Question 16

Faraday’s first law of electrolysis

Answer 16

States that the mass of an element deposited during electrolysis is directly proportional to the quantity of electricity (that is the charge, Q) passing through during the electrolysis.

Question 17

Faraday’s second law of electrolysis

Answer 17

States that the amount, in mol, of e- needed to discharge 1 mol of an ion at an electrode is equal to the charge on the ion, z.