Electrochemistry Flashcards
Galvanic Cell
What controls the current produced?
3 points
- reaction at either electrode
- movement of ions through solution
- salt bridge
3 electrode
Describe the potential at the reference electrode
It is fixed
3 electrode
What is the purpose of the porous fit at the bottom of the reference electrode?
Acts as a salt bridge between reference electrode and reaction solution
3 electrode
What is the point of having an electrolyte solution?
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)
3 electrode
What is the point of the reference electrode
Keep a constant reference potential
3 electrode
What happens to the potential of the working electode?
Changes with applied potential
3 electrode
Only reactivity of one electode can be controlled. Which one?
Working
3 electrode
What is the purpose of counter electrode
Makes sure that reaction at working electrode controls current (can forget about reactionat counter electrode)
3 electrode
Why is it bad to have an insulating solution?
Electrode transfer occurs by electrons tunnelling between reagent and electrode
3 electrode
What happens when inert electrolyte is added to the solution?
2 points
- All of potential dropped at W.E. to control electrode reaction
- There is not field in bulk solution to attract/repel ions.
3 electrode
What is meant by the inert ions in the working solution?
- Less reactive than solvent over a wide potential range
- Allows redox reactions to be studied within these potential limits
What is Hermholtz double layer model?
3 points
- 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
What are the problems with Hermholtz model?
- 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
Gouy and Chapman double layer
What are the assumptions of the model?
- Ions have zero size
- Ions can approach infinitely close to the electode
How is Gouy and Chapman model different from Hermholtz?
2 points
- Potential dropped over much larger distance (bad for electrochemistry)
- Diffuse double layer becomes thinner as increases
Double layer
How was Stern’s approach different
- Ions approach surface to distance of closest approach of one ionic radius
- Not complete charge compensation
CV
How is rerversibility of CV shown?
3 points
- Peak to peak separation = 57/n mV
- Current peak heights are equal
- Adherence to the Randles-Sevcik Equation
CV
What happens if scan rate is increased?
Greater chance of re-oxidising the electrode product arising from the reduction (before decomposition occurs) and therefore ipa will increase
CV
Why is TBAPF6 commonly used in voltammetric measurements on transition metal complexes?
Non-coordinating electrolyte, soluble in many non-aqueous solvents used for voltammetry.
RDS
What is steady state current?
Constant current independent of sweep rate and direciton of sweep
RDS
When is steady state current achieved?
When rotation speed is <= 10 mV/s
What is E1/2
Half wave potential. Midway between the anodic and cathodic peak potentials.
RDE
What is mass transport controlled by?
Varying W (frequency of rotation)
RDE
What is the electrochemical reaction term controlled by?
Varying E (potential?)
Micro
What is steady state current
- Reproducible currents without external convection
- Mass transport parameters of reaction can be analysed as with RDE
Mass transfer limiting current
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
