Topic 1: Revision

Question 1

• What are the three microelectrode (<100 µm characteristic dimension) current equations?

Answer 1

• i_lim = 4naFCc*
• i_lim = 2πnaFDc*
• i_lim = 4πnaFDc*

Question 2

• What is the macroelectrode (≥1 nm) current equation and what conditions does it describe?

Answer 2

• i_p = 2.69E+05*n^3/2*A*D^1/2*v^1/2*c*
• (diffusion limited current, i.e. reversible electron transfer at T = 298K)

Question 3

• What is the relationship describing k_t for each electrode?

Answer 3

• k_t = expression for characteristic current /nAFc
• k_t = D/d (in SECM positive feedback mode)

Question 4

• Describe the interfacial region close to an electrode surface when the electrode is at a potential which is negative of the potential of zero charge. Use a sketch to aid you answer

Answer 4

• A voltage applied to the electrode-electrolyte causes rearrangement at that interface.
• The Goug-chapman- diffuse double layer forms from the ordering of opposite charged ions
• Distance of closest approach of solvated cations forms the Outer Helmholtz plane

Question 5

• What is the electrical double layer equivalent to in a circuit?

Answer 5

• An electrochemical capacitor/ with a potential difference across it

Question 6

The … … forms at any surface in … solution

Answer 6

• The double layer forms at any surface in electrolyte solution

Question 7

• How would the interfacial region close to an electrode surface change when the electrode is at a potential is positive of the potential of zero charge. Use a sketch to support your answer

Answer 7

• An outer Helmholtz plane forms, which is the distance of closest approach of an anion to the, now positive, electrode surface.
• An inner Helholtz plane is the distance of closest approach for an unsolvated ion

Question 8

• Why is it more likely for an inner Helmholtz plane to form at a positively charged electrode?

Answer 8

• More likely to form anions on a cationic surface as they are generally larger in size and have a lower charge density than less stable cations which will stabilise through solvation.

Question 9

• Describe the relationship between the size of the double layer as a function of the concentration of electrolyte in water at 298K

Answer 9

• Inverse relationship
• More ions leads to better packing over shorter distances

Question 10

• Describe the four different kinds of electrodes

Answer 10

• 1) Redox electrodes: 2 species is solution transferring electrons at the electrode interface, platinum supports this transfer (supply or receive). E.g. Fe³⁺ + e^- ⇌ Fe²⁺ E = 0.771
• 2) Metal/Metal Ion: e.g. Cd²⁺ + 2e^- ⇌ Cd_(s) E = -0.403
• 3) Metal/Insoluble-Salt: e.g AgCl_(s) + e^- ⇌ Ag_(s) + Cl^- E = +0.22. used a lot in terms of reference electrodes in dynamic electrochemistry. Electron transfer occurring at metal salt interface
• 4) Gas Electrode: e.g. 2H⁺ + 2e^- ⇌ H_2(g) E = 0.00

Question 11

• Describe the general from of redox potentials?

Answer 11

• Used to measure position of equilibrium in a redox reaction
• Ox + ne^- ⇌ Red
• This is a coupling of the following two reactions
• A + e^- –> B – reduction
• A –> B + e^- - oxidation
• Redox potentials are given in to terms of the reduction

Question 12

• What is E^o and what does its value indicate?

Answer 12

• Electrode potential that reflects the position of equilibrium of a reaction
• Says nothing about rate of electron transfer
• A more positive value indicates forward reaction (reduction) is favoured
• A more negative value indicates backward (oxidation) is favoured

Question 13

• How is E^o related to K?

Answer 13

• ΔG^o = -RTlnK
• ΔG^o = -nFE^o

Question 14

• What is the Nerst equation?

Answer 14

• Can be used to predict E of a cell in non-equilibrium concentration of reactant/product
• ΔG = ΔG^o
• E = E^o – (RT/nF)ln(prod/rec)

Question 15

• Give an example of an outer sphere redox couple to describe its electron transfer

Answer 15

• Species do not interact with surface in order for electrons to transfer

Question 16

• Give an example of an inner sphere redox couple to describe its electron transfer

Answer 16

• Species do interact (i.e. bind) with surface for electrons to transfer
• Metal will matter in reaction process, and binding will vary from one metal to another

Question 17

• Describe the experimental setup of a dynamic electrochemistry experiment

Answer 17

• Potentiostat used to apply potential, current is measured in a 3-electrode set-up via current flowing from WE to CE, V measured with respect to RE, potentiostat corrects automatically
• No current flows through reference electrode
• Reaction is driven in direction desired via one half of a redox couple in solution
• Uses a feedback loop to correct for ohmic drop

Question 18

• Using a sketch of a chemical example, describe the electrochemical setup and reaction at the working and counter electrode. What is the energy applied

Answer 18

• Apply a voltage and measure an induced current flow due to either oxidation or reduction
• Fe³⁺ + e^- –> Fe²⁺ reduction (potential more -ve than E^o)
• Fe²⁺ - e^- –> Fe³⁺ oxidation (potential more +ve than E^o)
• 2 electrode setup

Question 19

• How can we minimise Ohmic drop – iR_solution

Answer 19

• Keep current small as i ∝ electrode area (use micro/nano electrodes <50 µm)

Keep R_soln as low as possible by increasing conductivity of solution via a supporting electrolyte (0.1M KNO₃)

Question 20

• If Ohmic drop is reduced via using a small electrode (<50 µm), how must we change our experimental setup? Give a sketch to support you answer

Answer 20

• Need a reference electrode

Question 21

• What experimental setup is used for large electrodes (macroelectrodes > 1nm)

Answer 21

• Must use 3 electrodes
• WE/REF/CE
• CE corrects for Ohmic drop

Question 22

• What material are working electrodes made from

Answer 22

• Pt
• Au
• Glassy carbon
• Graphite

Question 23

• Describe two different reference electrodes, highlighting their requirements as electrodes

Answer 23

• Must maintain a constant potential i.e. potential determining ions must keep a constant activity
• Both only depend on Cl^- in solution
• Potential kept constant with saturated KCl(s) crystals

Question 24

• Using simple energy level diagrams, describe the movement of electrons between an electrode and a species in solution in a reduction process

Answer 24

• When sweeping the voltage, the position of the fermi level changes
• Filled states transfer charge to LUMO of species via path of least resistance
• Opposite is an oxidation with a positive energy
• Neither process occurs thermodynamically as system hits equilibrium where levels are aligned