electrochemistry and cells Flashcards
standard conditions for measuring electrode potential (3)
- conc. of solutions = 1.0 mol/dm3
- 298K
- 100kpa / 1atm
standard cell for comparison (primary)
- electrode used
hydrogen cell
- platinum
metal dipping into solution =
half cell
if eq. lies to the left, charge will be … due to … making the potential …
negative
due to build up of electrons on metal
negative
if eq. lies to the right, charge will be … due to … making the potential … ??
positive
as electrons used up to form the metal
positive
more reactive metals tend to have ….. potentials
negative
positive potential = … hand electrode = ….
right
cathode
emf of cell =
E right - E left
secondary standard
one which has been calibrated against primary standard
anode…
oxidation
negative
which way do reduction reactions go
left to right
which what type of reaction is the more negative V
oxidation
which way do oxidation reactions go
right to left
what is always needed for fuel cells?
- what can be used?
- oxygen!!
- hydrogen/alternatives
set up used to investigate potential of an electrode
- use standard hydrogen electrode
- salt bridge - potassium nitrate
for a positive potential difference, electrons moe from…. to …
left
right
zinc-carbon cell - features
- cheap
- non-rechargable
- fairly short life
alkaline cell features
- higher cost then zinc-carbon
- longer life “
lithium ion cell features
- rechargable
- used in phones, computers etc.
lead acid battery features
- made of 6 cells
- used in cars
how are fuel cells different to other cells?
continuous supply of chemicals into the cell, so neither run out or need recharging
fuel =
something that reacts with oxygen to release energy
most common methods of producing H2
- electrolysis of sea water
- methane + steam
benefits of hydrogen-oxygen fuel cells (3)
- only waste product is H2O
- efficient
- dot need recharging
cons of hydrogen-oxygen fuel cells (4)
- needs constant fuel supply
- hydrogen = flammable and expensive
- H2 production usually involves fossil fuels
- high cost of cell
voltaic cell =
connecting 2 half cells together
why must 2 different half cell species be kept separate?
otherwise electrons would flow in uncontrolled way and heat energy produced instead of electrical
electrode with MORE reactive metal will … electrons and is ….. making it the ….
loose
negative
anode
standard electrode potential =
tendency to be reduced and gain electrons
limitations of predictions using standard electrode potentials (3)
- rate of reaction - large activation energy - doesn’t give indication of rate
- concentration - will affect, not all cells have 1.0 mol/dm3
- conditions may vary
what are electrodess in hydrogen fuel cel made from
platinum
overall reaction for the hydrogen-oxygen fuel cell
2H2(g) + O2(g) → 2H2O(g)
what happens to the metal in a cell with the MORE POSATIVE electrode potential
is REDUCED and GAINS electrons
positive electrode is one with
lowest standard electrode potential
standard cell potential (eq)
Eθcell = Eθright half-cell − Eθleft half-cell
why is a high resistance voltmeter used to measure electrochemical cells?
- prevents current flowing through circuit
- if current flowed, reactants would be used up at both electrodes
- E cell would then decrease
the more negative the E°, the further the eq lies to the …
LEFT
the more positive the E°, the further the eq lies to the …
RIGHT
why must chemicals in a cell be kept separate
otherwise electron flow would be uncontrolled and heat energy would be produced instead of electrical
standard electrode potential
emf of half cell when connected to a standard hydrogen half-cell under standard conditions
limitations of predictions using Eθ values
- no indication of rate
- measured using 1 mol/dm3 conc - if conc higher then standard the value will be different
- actual conditions will be different from standard conditions
- standard electrodes apply to AQUEOUS equilibria - many reactions aren’t aqueous
3 types of cell
- primary
- secondary
- fuel
