m5 - electrode potentials Flashcards
what is a half cell
a metal dipping into a solution of its ions eg Cu in Cu2+ ions
or could do 2 ions eg Fe3+ +2e- -><- Fe2+ (now we have to dip our own electrode into solution, use Platinum)
or gases eg 2H+ +2e- -><- H2 (trap the gas and bubble into solution, Pt electrode also present)
measuring standard electrode potentials
using hydrogen electrode as a standard (0)
voltmeters measures electromotive force of that cell
standard electrode potential def (E ^θ)
the emf of a half cell compared with a standard hydrogen half cell, measured at 298K , 1 moldm-3 conc solutions, and 100kPa
salt bridge in a circuit
strip of filter paper soaked in KNO3 solution (used bc it doesn’t form precipitates)
they replace ions being removed to complete the circuit and keep electrical balance
electrochemical series order
written as half equations X+ +e- -><- X
most -ve at top (experiences oxidation, backwards reaction)
hydrogen = 0
most +ve at bottom (experiences reduction, forward reaction)
how to work out cell potential
difference between 2 electrode potentials eg -5 and +75 would be 1.25
limitations of predictions using electrode potentials
based on standard conditions, actual conditions may be diff eg conc not 1moldm-3
they predict eq position not rate, may be too slow due to a high Ea
they are for aqueous equilibria, different states may give diff results
larger the difference the more likely a reaction will take place
how to tell if a reaction is feasible using electrode potentials
more -ve potential on the right will react with more +ve potential below it on the left
3 types of electrochemical cells
non rechargeable: when the chemicals have run out the cell is discarded
rechargeable: cell reaction can be reversed eg Nickel-Cadmium batteries
fuel cell: will carry on as long as there’s a constant supply of fuel and oxidant
what does a fuel cell do
uses energy from reaction of a fuel eg hydrogen or methane, with an oxidant eg O2 to create a voltage
reactants flow in and products flow out while the electrolyte remains in the cell so they can operate continuously as long as there’s fuel and oxidant
benefits and risks of fuel cells
+ work continuously as long as constant fuel oxidant supply
+ don’t have to be recharged
- toxic if ingested eg lithium cells
- rapid discharge can cause fires/explosions
disadvantages of fuel cells
a lot of energy needed to make them
difficult to dispose of
making hydrogen uses electricity and storing hydrogen is hard (under pressure as a liquid) is flammable/explosive
