electrochemical cells Flashcards
why do you use a high resistance voltmeter
to stop the current from flowing in the circuit, so that it is possible to measure the maximum Potential difference. It prevents the reaction from occurring.
why use a platinum electrode
it is unreactive and can conduct electricity
conditions of the standard hydrogen electrode (SHE)
Hydrogen gas at a pressure of 100kPa
solution containing hydrogen ions at 1 mol dm-3
temperature of 298K
platinum electrode
why is platinum black used
it is porous and so can absorb the hydrogen gas
what are standard conditions
All solutions at 1 mol dm-3
temperatures at 298K
gasses at 100kPa
no current flowing
E cell =
RHS - LHS
or eRed - eOx
E cell is a measure of how far from equilibrium the cell reaction lies.
Give the half equations and the full equation for the hydrogen fuel cell in alkaline conditions
4e- + 4H2O –> 2H2 +4OH- E=-0.83V
4e- + 2H2O +O2 –> 4OH- E=+0.4V
Overall reaction 2H2 + O2 –> 2H2O
what is the electrolyte in a hydrogen fuel cell
potassium hydroxide
how does a fuel cell work
a fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage. Fuel cells will maintain a constant voltage over time because they are continuously fed with fresh O2 and H2, so maintaining a constant concentration of reactants. This differs from ordinary cells where the voltage drops over time as the reactant concentrations drop.
advantages of fuel cells over conventional petrol etc
less pollution and less CO2 as pure hydrogen only emits water.
It has a greater efficiency.
Limitations of the hydrogen fuel cells
expensive
storing and transporting hydrogen is hard to do safely .
Limited lifetime as requires regular replacement and disposal = high costs
use of taxi chemicals in their production
Hydrogen is readily available from the electrolysis of water, however this is expensive. to be a green fuel the electricity needed would need to be produced from renewable resources.
ways in which hydrogen can be stored in fuel cells
as a liquid under pressure
adsorbed on the surface of a solid material
absorbed within a solid material
what are lithium ion cells used for
powering cameras and mobile phones
half equations for lithium ion cell and overall equation
Li+ + CoO2 + e- –> Li+[CoO2] -
Li+ + e- –> Li
Li + CoO2 –> LiCoO2
Lithium cell conventional cell representation
Li | Li+ || Li+ , CoO2| LiCoO2| Pt
the reaction for the lithium cell recharging
Its just the overall reaction reversed.
comment on the support medium used in a lithium cell
the reagents are absorbed onto powdered graphite that acts as support medium. The support medium allows the ions to react in the absence of a solvent such as water. water would not be good as a solvent because it would react with the lithium metal.
