Electrochemistry

Question 1

Galvanic Cell

What controls the current produced?

3 points

Answer 1

• reaction at either electrode
• movement of ions through solution
• salt bridge

Question 2

3 electrode

Describe the potential at the reference electrode

Answer 2

It is fixed

Question 3

3 electrode

What is the purpose of the porous fit at the bottom of the reference electrode?

Answer 3

Acts as a salt bridge between reference electrode and reaction solution

Question 4

3 electrode

What is the point of having an electrolyte solution?

Answer 4

To ensure that a suitably thin electric double-layer, so that the potential is dropped very close to the working electrode (maximises the potential difference between the electrons in the metal and the redox species in solution)

Question 5

3 electrode

What is the point of the reference electrode

Answer 5

Keep a constant reference potential

Question 6

3 electrode

What happens to the potential of the working electode?

Answer 6

Changes with applied potential

Question 7

3 electrode

Only reactivity of one electode can be controlled. Which one?

Answer 7

Working

Question 8

3 electrode

What is the purpose of counter electrode

Answer 8

Makes sure that reaction at working electrode controls current (can forget about reactionat counter electrode)

Question 9

3 electrode

Why is it bad to have an insulating solution?

Answer 9

Electrode transfer occurs by electrons tunnelling between reagent and electrode

Question 10

3 electrode

What happens when inert electrolyte is added to the solution?

2 points

Answer 10

• All of potential dropped at W.E. to control electrode reaction
• There is not field in bulk solution to attract/repel ions.

Question 11

3 electrode

What is meant by the inert ions in the working solution?

Answer 11

• Less reactive than solvent over a wide potential range
• Allows redox reactions to be studied within these potential limits

Question 12

What is Hermholtz double layer model?

3 points

Answer 12

• Ions are assumed to be of a fixed size
• When electrode charge is negative, positive ions form a layer at the electrode
• Gives required potential profile for reaction to occur asno potential outside of ion layer

Question 13

What are the problems with Hermholtz model?

Answer 13

• Does not allow for randomising effect of thermal motion of solvent
• If those effects are large, then the force holding the electrons to electrode will be overcome
• Residual charge outside of the Hermholtz layer

Question 14

Gouy and Chapman double layer

What are the assumptions of the model?

Answer 14

• Ions have zero size
• Ions can approach infinitely close to the electode

Question 15

How is Gouy and Chapman model different from Hermholtz?

2 points

Answer 15

1. Potential dropped over much larger distance (bad for electrochemistry)
2. Diffuse double layer becomes thinner as increases

Question 16

Double layer

How was Stern’s approach different

Answer 16

• Ions approach surface to distance of closest approach of one ionic radius
• Not complete charge compensation

Question 17

CV

How is rerversibility of CV shown?

3 points

Answer 17

• Peak to peak separation = 57/n mV
• Current peak heights are equal
• Adherence to the Randles-Sevcik Equation

Question 18

CV

What happens if scan rate is increased?

Answer 18

Greater chance of re-oxidising the electrode product arising from the reduction (before decomposition occurs) and therefore ipa will increase

Question 19

CV

Why is TBAPF6 commonly used in voltammetric measurements on transition metal complexes?

Answer 19

Non-coordinating electrolyte, soluble in many non-aqueous solvents used for voltammetry.

Question 20

RDS

What is steady state current?

Answer 20

Constant current independent of sweep rate and direciton of sweep

Question 21

RDS

When is steady state current achieved?

Answer 21

When rotation speed is <= 10 mV/s

Question 22

What is E1/2

Answer 22

Half wave potential. Midway between the anodic and cathodic peak potentials.

Question 23

RDE

What is mass transport controlled by?

Answer 23

Varying W (frequency of rotation)

Question 24

RDE

What is the electrochemical reaction term controlled by?

Answer 24

Varying E (potential?)

Question 25

Micro

What is steady state current

Answer 25

• Reproducible currents without external convection
• Mass transport parameters of reaction can be analysed as with RDE

Question 26

Mass transfer limiting current

Answer 26

Current at which the reaction is completely governed by mass transfer (diffusion). At mass transfer limiting current the concentration of the electroactive ion at the
surface of the electrode can approach zero