Electrolysis

• How are electrochemical cells created?

  By joining two half cells together
• What is a half cell?


  One half of an electrochemical cell where either:

  • A metal is dipped in its ions
  • A platinum electrode with 2 aqueous ions
• Why is KNO3 used as a salt bridge:

  • Completes the circuit
  • Allows ions to flow through to balance charge
• Why are electrode potentials used?

  To measure the tendency of an electrode to gain or lose electrons in a redox reaction.
• What does the more negative half of the cell do?

  Undergoes oxidiation therefore loses electrons
• The more positive half of the cell undergoes reduction therefore gains electrons
• What are the standard hydrogen electrode conditions?

  100Kpa, 298K, 1mol of H+ ions
• How do you calculate the standard electrode potential of a cell?

  E = reduced - oxidised
• How do you identify the most oxidized cell?

  Most negative
• If an agent can be easily reduced it can gain an electron therefore poweful oxidising agent
• If an agent is easily oxidized it is powerful reducing agents
• How to predict reaction feasibility:

  • Identify oxidised equation and reverse it
  • Write it on top of reduction equation
  • Combine equations
• Rechargeable batteries:

  • Have a supply current
  • Reversible reaction
  • Forces electrons to flow in opposite way
• Equation at positive electrode of lithium cell:

  Li+ + CoO2 <-> Li+[CoO2]-
• Equation at negative electrode of lithium cell:

  Li <-> Li+ + e-
• Lithium cells use an non aqueous electrolyte, why?

  So that the lithium doesn't react with water
• Hydrogen fuel cell positive electrode equation:

  O2 + 2H2O + 4e- -> 4OH-
• Hydrogen fuel cell negative electrode equation:

  H2 + 2OH- -> 2H2O + 2e-
• Overall equation of hydrogen fuel cell:

  2H2 + O2 -> 2H20
• How do hydrogen fuel cells work:

  • Hydrogen is fed to the negative electrode which reacts with OH- ions to produce water and electrons
  • Electrons travel through the pt electrode
  • Oxygen is fed into a positive electrode which reacts with water and electrons to produce OH ions
  • The KOH electrolyte carries OH ions across ion membrane back to the negative electrode
  • Electrons travel around external circuit to generate current
• Advantages of fuel cells:

  • More efficient
  • Energy converted is more kinetic than thermal
  • Only waste product is water = not toxic
• Disadvantages of fuel cell:

  • H2 is flammable
  • Expensive to transport
  • Fossil fuels used to pass water through = contribute to CO2 emissions