electrode potentials + EC cells

why does does Ba react with H2SO4 quickly initially, but then stop despite unreacted Ba being present?

• reacts to form BaSO4 + H2

• BaSO4 coats Ba preventing it from reacting further

how can electrochemical cells be recharged?

electrode reactions can be reversed by applying a reverse potential

• if the forward reaction occurs on discharge, giving out charge reversing the reaction

only work if the product stays attached to the electrode and does not disperse

purpose of salt bridge?

filter paper soaked in solution to complete the circuit so ions can move between half cells

what are suitable ionic compounds that can be used in salt bridges?

any soluble ionic compound that cannot react with the components in the cell/the ions in solution

usually metal nitrate [KNO3]

why is a high resistance voltage meter used in electrochemical cells?

• to stop the current flowing in the circuit

• to measure the max. possible E difference

what happens if current is allowed to flow in an EC cell?

• reactions occur separately

• V falls to zero as products get used up

how can electrical contact be improved between the electrodes and the solution?

• rub electrodes with sandpaper to remove any oxide layer on its surface

• wipe electrodes with propanone to remove any grease on surface

what electrode is used for reactions without a solid or metal conducting surface?

• Pt wire - un-reactive metal + can conduct

• coated in finely divided Pt black in standard hydrogen electrode as it is porous and can absorb H2 gas

why are standard conditions needed?

position of the redox equilibrium will change with different conditions

eg. Mg2+ + 2e- → Mg

• if conc. is increased, equ. shifts right, making potential more positive

what is happening if forward reaction is discharge?

battery producing energy, chem energy into electrical, spontaneously

what does it mean if reaction is recharging?

forward discharge reaction reversed, external potential forcing reaction backwards, reforming the products for the cell to be used again

pros of fuel cells over conventional petrol/diesel powered vehicles?

• less pollution + CO2, pure H only produces water

• more efficient

limitations of H fuel cells?

• expensive

• storing and transporting hydrogen, in terms of safety, feasibility of a pressurised liquid

• limited life cycle of the solid its stored in

• limited lifetime (requiring regular replacement and disposal)

• high production costs

• toxic chems. used in their production

• H is readily available by the electrolysis of water, this is expensive - to be a green fuel the electricity needed would need to be produced from renewable resources
  

how can H be stored in fuel cells?

• as a liquid under pressure

• adsorbed on the surface of a solid material

• absorbed within a solid material

ethanol fuel cell advantages?

• ethanol can be made from renewable sources in a C neutral way

• raw materials to produce it by fermentation are abundant

• less explosive + easier to store than H

• new petrol stations not req. as ethanol is a liquid fuel

adv of methanol fuel cell > H fuel cell?

easier to store + transport as it is liquid

O2 half equation in an alkaline H-O2 fuel cell?

O2 + 2 H2O + 4e- → 4 OH

suggest why the EMF values of the acidic and alkaline hydrogen–oxygen fuel cells are the same

same overall reaction

2 H2 + O2 → 2 H2O