Electrochemistry Flashcards
Which electrode is the cathode?
The electrode where reduction takes place.
Which electrode is the anode?
The electrode where oxidation takes place.
What is the function of electrolyte?
To transport ions (reacting species) from/to the reduction site.
What are the characteristics of an electrolyte?
Ionic conductivity
Poor electrical conductivity
Electrochemical stability
What is a Galvanic Cell?
An electrochemical cell that spontaneously produces work.
What is an Electrolytic Cell?
An electrochemical cell that requires work.
What is a potentiostat?
An instrument able to control the potential of an electrode with respect to another one.
What is a galvanostat?
An instrument able to control the current flowing from one electrode to another one.
Equation for Faraday constant
F = qNA
q is fundamental charge
NA is avagadro number
Faraday’s Law equation
ni = Q/((n/v)F)
n number of moles of electrons for half equation
v is number of moles of species for half equation
What is a Faradaic reaction?
A reaction where electron transfer is directly related to the consumption of reactants and formation of products
Faradaic efficiency equation.
η = amount of desired material produced/amount that could be produced if all charge is supplied
Equations for current density.
i = I/A
current per unit area
or
i = FΣzN
N is molar flux (vector)
z is electrical charge of species
When does current flow?
When there is a potential gradient.
Ohm’s law for 1D conduction
ix = -κ dφ/dx
κ is conductivity
φ is potential
If current density is constant, what happens when you integrate the potential?
Δφ = iL/κ
L is distance between two electrodes.
Equation relating G and E at chemical equilibrium in an electrochemical cell.
ΔG = -nFΔE
What is the current density at chemical equilibrium in an electrochemical cell?
0
Calculating ΔE from half cell potentials
E positve - E negative
Derive the Nernst Equation
Check derivation
Equation for ΔGr
ΔGr = ΣvΔGf
Derive an expression linking temperature to standard potential for small temperature ranges.
Check derivation
Derive an expression linking temperature and standard potential over larger temperature ranges where ΔS and ΔH are not constant.
Check derivation
Desired characteristics of a reference electrode.
Reversible reaction
Stable and well defined potential
Ions that participate in reference electrode are present in solution
No liquid junctions that cause an offset in potential
What are typical reference electrodes?
Hydrogen Electrode
Calomel Electrode
Silver-Silver Chloride Electrode
What is a liquid junction potential?
A potential difference at open circuit associated with the junction of two liquids of different composition
Why do liquid junctions form?
At an open cirvuit there is a diffusion driving force from high to low concentration.
Cations and anions will typically have different diffusivities.
Ion with larger diffusivity will move more quickly causing an imbalance in charge.
Potential gradient resulting from charge imbalance means there is a potential difference across the junction.
Is the formation of a liquid junction thermodyamic in its origin?
No it results from transport.
What is activity of a species?
The “effective concentration” of a species in a mixture
In a real solution, other than temperature, what does chemical potential depend on?
Activity of the real solution, for an ideal solution it would be concentration.
What is molality?
Number of moles per unit mass of solvent.
Why is molality more useful than concentration?
It does not depend on density which depends on temperature causing the introduction of error.
How are mean ionic activity and activity coefficent defined for salt solutions?
a(±) = a(+)a(-) γ(±) = γ(+)γ(-)
What assumption can we make about salts in solution that simplifies molality and activities of electrlytes?
Complete dissociation of salt
Express equilibrium constant with potential.
Check expression
Steps for calculating cell potential
Write two half cell reactions and standard potentials for each
Put the most positive reaction on the right and set as the cathode
Write full reaction making sure electrons are balanced
Take the differmev between standard electrode potentials to get standard potential of full cell
Use activity (or conc if a is not known) to correct the cell potential.
Use full cell reaction to get stoichiometric coefficient for each speices and the total number of electrons for the reaction.
What is the chloro-alkali cell used for in industry?
Production of hydrogen, chlorine and sodium hydroxide.
What is the conventional concentration for a Pourbaix diagram?
10^-6 mol
How to find lines for a Pourbaix Diagram.
Use the Nernst euqtaion.
What are the electrodes in a lead acid battery?
Pb/PbSO4 and PbO2/PbSO4
What is the electrolyte in a lead acid battery?
H2SO4
What ways are there of measuring energy density?
Gravimetric and Volumetric
What is the equation for Gravimetric energy density and the units?
=ΔG/m in Whkg^-1
What is the equation for Volumetric energy density and the units?
=ΔG/V in Whl^-1
Why is the concentration of H2SO4 high (5M) in a lead acid battery?
The overcharge of the battery ≈ 2.39V whereas the stability window of water is 1.23V. Thi causes H2 and O2 to be produced resulting in loss of water and builid up of gas.
At equilibrium what does ΔG equal for the half cell?
∆Gr = 0
Derive an expression for electrochemical potential of each species that takes part in the reaction in a cell at constant T and P.
Check derivation
Derive an expression for potential difference across the interface for Cu2+ + 2e- –> Cu
Check derivation
What happens when the interface potential difference is greater than the equilibrium interface potential difference?
Anodic reaction
What happens when the interface potential difference is less than the equilibrium interface potential difference?
Cathodic reaction
State equation for Debye length
Check expression
Expression for potential distribution including Debye length
Check expression
Using Debye Hückel theory derive an expression for activity coefficient.
Check derivation
What is the Debye Hückel limiting law?
I–>0, lnγ± = z(+)z(-)α√I
What concentrations is Debye Hückel model valid for?
<0.1M
What parts make up a double layer around an electrode?
Inner Helmholtz Plane (IHP)
Outer Helmholtz Plane (OHP)
Diffuse
What happens in the IHP?
The position of the centres of ions or molecules adsorbed onto the electrode surface.
Adsorbtion causes partial sheding of waters of hydration.
Why are anions typically adsorbed in IHP?
They hydrate less strongly than cations so are adsorbed more easily.
What is the OHP?
The plane of closest approach to the IHP for solvated ions.
How is the position of the OHP defined?
By the radius, including waters of hydration, of the largest solvated ions.
What is the order of magnitude of the OHP?
≈0.2nm
What forces are present in IHP?
VdW along with Coulombic
What is thinkness of diffuse double layer characterised by?
Debye length.
How does charge vary in diffuse double layer?
The excess charge decreases gradually from OHP to the solution side of the diffuse double layer where the solution is electrically neutral.
Order of magnitude of thickness of diffusion layer.
≈1mm
What are lectric field strengths like around diffuse double layer?
Very high
=∆φ/λ
λ is Debye length
How are diffuse double layers exploited in electrochemical double layer capacitors (EDLC)?
Since C=A.ε/d and d is very small, increasing A by making rough surface causes C of the order of 10^2 F
What is a Ragone Plot?
A plot of Power density against energy density of electrochemical devices.
Increasing potential does what to the energy of electrons in the electrode?
Lowers the potential energy promoting oxidation.
Decreasing potential does what to the energy of electrons in the electrode?
Increases the potential energy promoting reduction.
Give an expression for rate of reaction with activation energy.
Check expression
Derive an expression for reaction rates at the cathode and anode with a potential.
Check derivation
Using exchange current density at equilibrium, derive the Butler Volmer equation
Check derivation
What is surface roughness ratio?
Ratio of true surface area and apparent surface area.
How can exchange current density at equilibrium vary?
Widely varied with reaction and between the metal of electrode that a reaction takes place on.
Reactions with a higher exchange current density occur…
…more readily.
Reactions with a hugher exchange current density result in what kind of current denisities for a given over potential?
Higher.
Determine the concentration dependence of current density for a single electron elementary reaction.
Check derivation.
What are the two types of electron transfer reactions at an electrode?
Outer sphere reactions and inner sphere reactions.
What happens in an outer sphere reaction?
Reactants and products do not interact with electrode surface (no adsoption).
1+ solvent layer away from surface.
Electron transferred from OHP to electrode.
What happens in an inner sphere reaction?
Strong interaction with electrode surface (adsorption).
At IHP.
How can water be split for energy storage?
An electrolytic cell is set up to split water to hydrogen and oxygen at different electrodes where gases can be collected and stored.
Galvanic cell then set up with stored gases to produce electricity.
Derive the Tafel equation for large positive overpotential.
Check derivation
Derive the Tafel equation for a large negative overpotential.
Check derivation
When is the Tafel equation valid?
For overpotential > 100mV
How can BV equation be approximated at low overpotentials?
By expanding the full BV equation about 0 overpotential.
Derive the linear approximation of the BV equation,
Check derivation
What types of transport mechanisms act in solutions?
Migration
Diffusion
Convection
How does migration occur?
If electric field is applied to electrolyte, charged species will react.
How does diffusion occur?
Caused by concentration gradient in solution.
How does convection occur?
If the solution moves, ions are transported with it.
State formula for migratoion and electrical conductivity.
Check expressions
State molecular conductivity formulae.
Check expressions.
State Kohlrausch’s limiting law,
Check expression
Derive an expression for current transported by each ion in solution,
Check expression
State transport number equation
Check expression
What is transport number?
The fraction of current transported by one kind of ion with respect to the total current.
What laws govern diffusion?
Fick’s first and second laws.
What does Fick’s first law correlate?
It correlates the transport of a species in solution with its gradient of concentration in the phase.
What does Fick’s second law correlate?
It correlates the change of concentration of a species with the gradient of flux of the species.
Convection equation.
J=cv
J is flux
v is speed vector of convection
State Nernst-Plank equation.
Check expression.
What does the Nernst-Plank equation do?
Gives an expression for flux by summing flux expressions from migration, diffusion and convection
From the Nernst-Plank equation, derive an expression for current density resulting from transport.
Check derivation.
