topic 9 - electrode potentials and cells

• if Eºcell is +ve:

  ~ the electron flow is left to right

  ~ the right side is more reactive
• if Eºcell is -ve:

  ~ the electron flow is right to left

  ~ the left side is more reactive
• the more +ve an Eº electrode is, the better it reduces

  the more -ve an Eº electrode is. the better it oxidises
• Eº cell = Eº right - Eº left
• in a conventional cell diagram, the highest oxidation state should go in the middle
• a solution that can be used in a salt bridge is silver nitrate as it allows ions to flow across but is generally unreactive
• general conventional cell diagram rules

  1. work out which equation is on the left and right sides of the cell
  2. cross out electrons and work out oxidation states of each atom
  3. find common element and sort by oxidation state (with highest towards the middle)
  4. if same oxidaion state, put more -ve one as a lower oxidation state
  5. sort into states
  6. group states together
  7. add any carbon / platinum electrodes at the end
• in a conventional cell diagram, water may not be included if there are aqueous compounds in the cell
• a standard hydrogen electrode has:

  ~ a platinum electrode

  ~ hydrogen gas bubbling into the solution

  ~ a solution of 1 mol hydrochloric acid

  ~ a salt bridge

  ~~ this is connected to a high resistance voltmeter if measuring cell potentials
• what is an electrochemical series

  a list of half equations in order of electrode potential
• standard conditions

  298 K

  100 kPa
• why would an emf value be different to a calculated value

  non - standard conditions
• what is the purpose of a salt bridge

  to complete the circuit by allowing ions to flow