Electrode Potentials Flashcards
what happens when a rod of metal is dipped into a solution of its own ions
an equilibrium is set up between the solid metal and the aqueous metal ions
what is the simplest salt bridge made of
filter paper soaked in saturated solution of KNO3 (potassium nitrate)
why are salt bridges necessary
complete the circuit but avoid further metal/ion potentials as does not perform electrochemistry
allows ion movement to balance the charge
do not react with electrodes
symbol for salt bridge in standard notation
II
what type of species goes on the outside in standard cell notation
most reduced
what does I indicate in standard cell notation
phase boundary
what happens at left hand electrode
oxidation
half cell with most negative E value
what happens at right hand electrode
reduction
half cell with less negative E value
conditions for standard hydrogen electrode
298 K
100 kPa
[H+] = 1.00 moldm-3
what is the standard hydrogen electrode used for
comparing other cells against
E of SHE is defined as 0, so all other E values are compared against it
why might you use other standard electrodes occasionally
cheaper/easier/quicker to use and can provide just as good as reference
platinum is expensive
if E value is more negative what does it mean in terms of oxidising/reducing power
better reducing agent
if E value is more positive what does it mean in terms of oxidising/reducing power
better oxidising agent
what factors change E value
concentration of ions
temperature
what happens if you reduce the concentration of the ions in the left hand half cell
equilibrium moves to the left to oppose the change of removing ions; releases more electrons, the E of the left hand cell becomes more negative, so e.m.f. of the cell increases
how do you calculate the emf of a cell from E values
Ecell = Eright - Eleft
aka less negative - more negative
when would you use platinum electrode
when both the oxidised and reduced forms of the metal are in aqueous solution
why is platinum chosen
inert
good conductor to complete circuit
how would you predict if a reaction would occur
take 2 half equations
find species that is being reduced
calculate its E value minus E value of oxidised
if E value > 0 reaction will occur
what was the first commercial cell made from (Daniell cell)
zinc / copper (II)
what are zinc/carbon cells more commonly known as
disposable batteries
what are two reactions that take place in zinc/carbon cells
Zn oxidised to Zn 2+
NH4 + reduced to NH3 at carbon electrode
reactions that occur in a lead/acid battery
Pb + SO4 2- –> PbSO4 + 2e-
PbO2 + 4H+ + SO4 2- –> PbSO4 + 2H2O
how are cells rechargeable
reactions are reversible and are reversed by running a higher voltage through the cell than the cells E value
nickel/cadmium cells are rechargeable AA batteries etc.
what reactions occur at electrodes
Cd(OH)2 (s) + 2e- –> Cd (s) + 2OH-
NiO(OH) (s) + H2O + e- –> Ni(OH)2 (s) + OH-
where are lithium-ion cells used
mobile phones
laptops
reactions occur in lithium-ion cells
Li+ + CoO2 + e- –> Li+[CoO2]-
Li –> Li+ + e-
what is a fuel cell
cell used to generate electric current
does not require electrical recharging
what are the reactions that take place at the two electrodes in an alkaline hydrogen fuel cell
2H2 + 4OH- –> 4H2O + 4e-
O2 + 2H2 + 4e- –> 4OH-
why is it better to use a fuel cell than to burn H2 in air
in combustion, sulfur containing compounds and nitrogen containing compounds are produced due to high temperatures and the S and N in air. These are bad for the environment.
this does not occur in a fuel cell; only product is water.
more efficient
disadvantages of fuel cells
hydrogen is a flammable gas with a low boiling point –> hard and dangerous to store and transport –> expensive to buy
fuel cells have a limited lifetime and use toxic chemicals in their manufacture
how do you find the weakest reducing agent from a table of electrode potential data
most positive E value
then it is the PRODUCT of the reduction equation i.e. imagine equation going from right to left
what is the reason that some cells cannot be recharged
reaction of the cell is not reversible - a product is produced that either dissipates or cannot be converted back into their reactants
why might the e.m.f. of a cell change after a period of time
concentrations of the ions change - the reagents are used up
how can the e.m.f. of a cell be kept constant
reagents are supplied constantly, so the concentration of ions are constant, E remains constant
