electrochem

• electrolyte


  -compound which will conduct an electric current when it is in aq solution or in molten state, due to its flow of charge carried by its ions
• strong electrolyte: fully ionised

  weak electrolyte: partially ionised
• electrode + solution = half-cell
• electrode potential of metal: potential difference btwn metal and solution (over time, metal becomes increasingly neg charged, solution becomes increasingly positively charged)
• standard hydrogen electrode (SHE)

  • pt electrode coated with finely divided pt
  • immersed in solution where [H+] = 1 mol/dm3
  • H2 gas at 1 bar
  • 298K
• why pt used for SHE?

  • inert
  • coated with finely divided pt to increase SA so eqm btwn H2 and H+ can be established quickly
• SHE

  0.00 V
• half cell

  • contains same element in diff oxidation states
  • pt electrode used in abs of metal in half-cell
• definition of standard electrode potential

  the SEP of a half-cell is the electromotive force (potential difference) measured at 298k, between the half-cell and the SHE, in which conc of any reacting species in solution is 1 moldm-3 and any gaseous species is at a pressure of 1 bar
• • when direction of half-rxn is reversed, sign of E also reversed
  • doubling coeff does not double E
• steps of setting up:

  • connect half-cell to SHE, ensuring conc of all solutions are 1 moldm-3, temp is 298k and pressure of gases (if any) is 1 bar
  • connect the 2 solutions in the 2 half cells with a salt bridge, saturated potassium nitrate solution
  • using a high-resistance voltmeter, measure the potential difference btwn half-cell and SHE. value shown on voltmeter is the SEP of the half-cell connected
• standard cell potential (Ecell): not compared to SHE
• Daniell Cell: Zn-Cu cell
• salt bridge roles:

  1. completes circuit
  2. allows ions to flow through cell without the mixing of electrolyte
  3. maintain electrical neutrality of each half-cell

  -ions in half bridge migrate to neu increasing charge in the 2 half cells, cations migrate to cathode while anions migrate to anode
• SCP definition: potential diff btwn 2 half cells under standard conditions, giving a measure of emf of the cell
• E cell = E cathode - E anode
• • E cell > 0: reaction thermodynamically feasible and spontaneous
  • E cell + 0: reaction at eqm
  • E cell < 0: reaction not thermodynamically feasible and not spontaneous

  opp of G
• limitations of using Ecell to predict spon:

  1. kinetic factor not taken into consideration: Ecell ia a measure of POE but no indication of rate (eg due to high Ea may be thermodynamically feasible but not kinetically feasible)
  2. predictions using Ecell under non-standard conditions may not be valid
  3. side reactions may occur
• other cell types


  fuel cell:

  • combustion of H2 & O2 to produce electricity (forms H2O)
  • 2H2 + O2 -> 2H2O

  ethane cells:

  • anode: always start w 1 mol ethane
  • cathode: always start w 1 mol O2