Intro, cell potentials and thermodynamics Flashcards
Name the 4 energy storage categories
Potential, kinetic, thermal and electrochemical
True or False: Fuel cells store energy
False - they convert it
Which reaction occurs at the cathode?
Reduction
What are the units of current?
Ampere
What are the units of potential?
Volt
What are the units of resistance?
ohm
What are the units of electrical conductance?
siemens
What are the units for electric charge?
coulomb
What does Faraday’s law describe?
The relationship between the amount of current that flows through the external circuit and the amount of material that is either consumed or produced in a half-cell reaction.
Write the equation for total charge passed (Q) at a constant current
Q = current x time
Define ‘faradaic efficiency’. What does it help to determine?
The fraction of the total current that drives the desired reaction. It helps to identify the desired reaction from the multiple reactions that take place at a single electrode.
What is current density? What does it help to identify.
The current divided by the area of the electrode. It helps to understand the reactions that occur on the electrode surface.
Define electrostatic potential
The work required to move a unit charge from infinity to a position in the metal or solution in Volt
Write the equation relating current density and electrostatic potential. What does this characterise?
How well the material moves charge from a high potential to low potential
By what transport mechanism does current transfer from high to low potential?
Convection
How is the potential drop/potential difference across the electrolyte calculated?
Modified ohms law - integration
What are the two categories of electrochemical cells?
Galvanic and electrolytic
What is the difference between a galvanic and electrolytic cell?
Electrical work/energy is the output of a galvanic cell whereas it is the input of an electrolytic cell.
Is the chlor-alkali process galvanic or electrolytic?
Electrolytic - DC supplied electrical work
True or false: You can control both potential and current.
False - you cannot control both simultaneously
What happens to the current (rate of production) when potential is increased? What is this mode of operation called?
Current (rate of production) also increases. This is potentiostatic mode.
What is the mode of operation when the current (rate of production) is set? What happens to the cell potential?
Galvanostatic.
Picturing an I-V curve, what happens to the cell potential in galvanic mode vs electrolytic mode? Why?
Cell potential is below the equilibrium potential in galvanic mode whereas it is above in electrolytic mode. This is due to irreversible losses when chemical energy is converted to electrical energy in galvanic mode (i.e. discharging)
In electrolytic mode, the applied potential must be greater than the equilibrium voltage to generate and move current.
What does equilibrium mean in an electrochemical system? What is this condition called?
No net current flow. This is called open circuit
Define equilibrium potential
zero external current + general dynamic equilibrium within cell
Describe the reactions that occur at each of the electrodes and whether an electron is being accepted or gained.
** DURING DISCHARGING **
Anode - oxidation - electrons are lost - ionic charge becomes more positive
Cathode - reduction - electrons are gained - ionic charge becomes more negative
How do you calculate the thermodynamic potential?
- If no external voltage is being supplied *
The difference in potential, subtracting the more negative potential from the more positive.
This is usually cathode minus anode.
True or false: During discharging, electrons flow from low to high potential. This is natural due to its negative charge.
True
What are half-cell reaction potentials measured against?
A reference electrode called the standard hydrogen electrode (SHE)
What is the Faraday constant?
The conversion factor between the mole and the coulomb. The Faraday constant is the quotient of avogadros constant divided by one coulomb equivalent (1/e). = 96485 C/eq
How do you calculate the maximum amount of work available in a closed reversible system at a constant temperature and pressure?
W (electrical) = G = - n F U
How do you calculate thermodynamic potential at standard conditions using Gibbs energy data?
Gibbs for the reaction = sum ( stoic x Gibbs for component i)
** Only use either the ionic species data OR neutral aqueous, NOT both **
U = - (Gibbs reaction)/ nF
What is approximated value for activity of pure solid components, solvents (water) and electrons?
1
Why is a simplified activity correlation used in the thermodynamic potential equation?
To correct the cell potential with the products on the top and reactants on the bottom.
What is the value of the thermodynamic potential (and Gibbs) at equilibrium?
0
What is the purpose of the reference electrode?
Provides a known and stable potential which other potentials can be measured against.
True or false: Current passes through a reference electrode.
False, it doesn’t. This is so that it remains at its equilibrium potential.
What are the 4 characteristics of a reference electrode?
- Reversible reactions
- Stable and well-defined potential
- Ion(s) in the reference electrode reaction is present in solution
- No offset in potential caused by liquid junctions
Give 3 examples of reference electrodes.
Hydrogen electrode, calomel electrode and silver-silver chloride electrode.
Draw a Daniel Cell with copper and Pb(II)
image
