3.1.11 Electrode potentials and electrochemical cells Flashcards
what is meant by ‘standard electrode potential’?
the voltage meausred under standard conditions when a half-cell is connected to SHE (standard hydrogen electrode)
how do you calculate standard cell potential? (Eocell)
Eocell = Eo(reduction)-Eo(oxidation)
it is ALWAYS a positive value
how do you draw a standard electrode potential diagram (both solids)?
- draw 2 poles showing each element; each pole should be dipped in an aqueous solution with the same ion
- external wire connects the 2 poles with a high resistance voltmeter in between
- salt bridge dipped in both solutions
how do you draw a standard electrode potential diagram (when it is both aqueous)?
- external wire is connected to a THIN pole of platinum (Pt)
- dipped in an equimolar solution of both ions e.g. Fe(2+) and Fe(3+)
how do you display cell conventions?
e.g. Sn(s) -> Sn2+(aq) + 2e-
Ag+(aq) + e- -> Ag(s)
Sn(s)|Sn2+(aq)||Ag+(aq)|Ag(s)
- the left shows oxidation; the right shows reduction
which side does the oxidised and reduced species go on the diagram?
left - oxidised
right - reduced
what are the standard conditions for SHE (standard hydrogen electrode)?
- temperature - 298K
- concentration - 1moldm(-3)
- pressure - 1atm/100kPa
how do you predict the feasibility of a reaction?
if the standard cell potential is negative, the reaction is NOT feasible
what is meant by ‘oxidising agent’ in terms of electrons and what is meant by ‘reducing agent’ in terms of electrons?
electron acceptor; electron donor
what are the characteristics of a non-rechargeable cell?
- irreversible reaction
- battery is flat when all the reactants have been used up
give an example of a non-rechargeable battery
electrochemical cell
what are the characteristics of a rechargeable battery?
- reversible reaction
- when the battery is flat, electricity is supplied to force the electrons to move in the opposite direction so the reaction is reversed
what is the general equation of a lithium battery (rechargable)?
Li+ + CoO2 + e- -> Li+[CoO2]-
what is meant by ‘electrolyte’ in a rechargeable lithium battery?
a lithium salt in an organic solvent
why aren’t aqueous electrolytes used in a rechargeable lithium battery?
Li is a powerful reducing agent and reduces water to OH- ions, resulting in an alkaline solution
overall reaction between lithium and water?
H2O + Li -> Li + H2 + 2OH-
how does an ‘alkaline-hydrogen-oxygen’ cell work?
- H2 is fed into the anode (oxidation occurs) - H2 -> 2H+ + 2e-
- O2 is fed into the cathode (reduction occurs) - O2 + 4H+ + 4e- -> 2H2O
- electrons pass through the external circuit from the anode to the cathode
- an ion exchange membrane allows smaller size ions e.g. H+, OH- but not H2O and H2
what are the uses of non-rechargeable cells?
doorbells, torches
what are the advantages of non-rechargeable cells?
- cheap
- easy to use + portable
what are the disadvantages of non-rechargeable cells?
- casing eventually dissolves and leaks acidic ammonium chloride paste
- made from finite materials; unsustainable
- toxic chemicals can leach from landfill sites to water sources after disposal
what are the advantages of rechargeable cells?
- light metal, won’t leak as electrolyte is solid polymer
- can be recharged without having to run down completely first
what are the disadvantages of rechargeable cells?
- costly, sensitive to high temperatures, may explode on heating
what are the advantages of fuel cells?
clean emissions
what are the disadvantages of fuel cells?
- sourcing raw materials, storage space of gases, explosion hazard
