Electrochemistry

• Half cell

  one half of electrochemical cell and consists of metal dipped in its ions or platinum electrode
• what makes an electrochemical cell?

  2 half cells joined by salt bridge and a voltmeter
• Where do electrons flow between an electrochemical cell?

  electrons flow from more reactive metal and less reactive metal, flow to react at electrode
• Why is a high resistance voltmeter used?

  to stop current from flowing into the circuit to measure maximum possible potential difference
• Salt bridge

  -free moving ions conduct charge, maintain charge, complete circuit

  -soaked in potassium nitrate

  -inert, unreactive with electrode and electrode solution
• Why is a salt bridge used instead of a wire?

  wire is not used because metal wire would set up its own electrode system with solutions
• What happens if current is allowed to flow?

  Reaction would occur at separate electrode, voltage will go to 0 as reactants are used up, concentration become the same
• How can current flow in the circuit?

  -remove voltmeter and replace with bulb or if the circuit is short circuited
• Standard conditions for SHE

  -100kPa

  -298K

  -H+ ions at 1 moldm3

  -platinum electrode
• What is SHE for?

  to measure electrode potential of other electrodes
• Why is SHE used?

  it has 0.00V
• Why is Platinum used?

  -conduct electricity for electron transfer

  -inert
• Why is platinum black used?

  because porous and can absorb H2 gas
• Why are standard conditions used?

  position of the equilibrium used change with condition
• Secondary standard

  -sometimes SHE is difficult to use, but Secondary standard is calibrated against SHE
• 2 Secondary electrodes

  -silver/silver chloride

  -calomel electrode
• What shows that a reaction is feasible?

  Ecell value is posititve
• Why might reaction not occur, even when Ecell value is positive?

  Ea too high
• When current flow, cell reaction will occur, Ecell will fall to 0 as reaction proceed and react concentration drops
• Most reactions are exothermic, so when temperature rise. E cell decrease because equilibrium moves backwards
• rechargeable batteries

  -reversible, cheaper long term

  -can store electrical energy
• non-rechargeable batteries

  cheaper
• forward reaction is discharge, reversed reaction is charging (rechargeable)
• Reversible cells only work if the product stays attached to the electrode and does not disperse

  -plug into supply of current
• The reagent in the cell are absorbed onto powdered graphite that act as a support medium.
• Support medium allows the ions to react in the absence of a solvent such as water
• Why would water be a good solvent for LiCoO2?

  it would react with lithium
• Fuel cells

  uses energy from reaction of a fuel with oxygen to create voltage
• What electrolytes does hydrogen fuel cell use?

  potassium hydroxide
• Hydrogen feed

  2H2 + 4OH- -> 4H2O + 4e-
• Oxygen feed

  O2 + 2H2O + 4e- -> 4OH-
• Overall equation for hydrogen fuel cell

  2H2 + O2 -> 2H2O
• Why are platinum electrode used in hydrogen fuel cells?

  conducting surface for electrons to flow to power something (a car)
• Why are electrolytes used in hydrogen fuel cells?

  to carry OH-
• How do fuel cell differ from ordinary cell?

  Fuel cells will maintain constant voltage over time because they are constantly fed with O2 and H2 so maintaining constant concentration of reactants. Ordinary cells, voltage drops to 0 as reaction proceeds and concentration of reagents drop to 0
• Why aren't standard conditions used for Fuel cells?

  rate too slow to produce appreciable current
• How to increase rate in fuel cells?

  higher temperature, but reaction is exothermic, so E cell would decrease because of Le Chatelier. So higher pressure is used
• Advantage of fuel cells

  -less pollution and less CO2, water is the only waste

  -more efficient, more energy to kinetic energy, combustion engines waste more energy
• Disadvantage of fuel cell

  -expensive transport and storage

  -highly flammable, dangerous

  -limited lifetime and high production costs
• Hydrogen is readily available by electrolysis of water but is expensive. To be green fuel, electricity would be needed to be produced from renewable source. Some sources of electricity is produced from fossil fuels