Topic 1: Revision Flashcards
1
Q
- What are the three microelectrode (<100 µm characteristic dimension) current equations?
A
- ilim = 4naFCc*
- ilim = 2πnaFDc*
- ilim = 4πnaFDc*
2
Q
- What is the macroelectrode (≥1 nm) current equation and what conditions does it describe?
A
- ip = 2.69E+05*n3/2*A*D1/2*v1/2*c*
- (diffusion limited current, i.e. reversible electron transfer at T = 298K)
3
Q
- What is the relationship describing kt for each electrode?
A
- kt = expression for characteristic current /nAFc
- kt = D/d (in SECM positive feedback mode)
4
Q
- 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
A
- 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
5
Q
- What is the electrical double layer equivalent to in a circuit?
A
- An electrochemical capacitor/ with a potential difference across it
6
Q
The … … forms at any surface in … solution
A
- The double layer forms at any surface in electrolyte solution
7
Q
- 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
A
- 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
8
Q
- Why is it more likely for an inner Helmholtz plane to form at a positively charged electrode?
A
- 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.
9
Q
- Describe the relationship between the size of the double layer as a function of the concentration of electrolyte in water at 298K
A
- Inverse relationship
- More ions leads to better packing over shorter distances
10
Q
- Describe the four different kinds of electrodes
A
- 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- ⇌ H2(g) E = 0.00
11
Q
- Describe the general from of redox potentials?
A
- 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
12
Q
- What is Eo and what does its value indicate?
A
- 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
13
Q
- How is Eo related to K?
A
- ΔGo = -RTlnK
- ΔGo = -nFEo
14
Q
- What is the Nerst equation?
A
- Can be used to predict E of a cell in non-equilibrium concentration of reactant/product
- ΔG = ΔGo
- E = Eo – (RT/nF)ln(prod/rec)
15
Q
- Give an example of an outer sphere redox couple to describe its electron transfer
A
- Species do not interact with surface in order for electrons to transfer
16
Q
- Give an example of an inner sphere redox couple to describe its electron transfer
A
- 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
17
Q
- Describe the experimental setup of a dynamic electrochemistry experiment
A
- 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
18
Q
- Using a sketch of a chemical example, describe the electrochemical setup and reaction at the working and counter electrode. What is the energy applied
A
- Apply a voltage and measure an induced current flow due to either oxidation or reduction
- Fe3+ + e- –> Fe2+ reduction (potential more -ve than Eo)
- Fe2+ - e- –> Fe3+ oxidation (potential more +ve than Eo)
- 2 electrode setup
20
Q
- 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
A
- Need a reference electrode
23
Q
- Describe two different reference electrodes, highlighting their requirements as electrodes
A
- 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
24
Q
- Using simple energy level diagrams, describe the movement of electrons between an electrode and a species in solution in a reduction process
A
- 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
