Electrode potentials

• Electrode or reduction potentials

  Half equations are always written as reduction processes, with the particle on the left gaining electrons to form the particle on the right
• Oxidation states


  Half equations always have two different oxidation states
• Standard electrode potential (E standard)

  A measure of the ability of a species to accept electrons, also known as reduction potential
• More positive standard electrode potential


  Greater ability to gain electrons
• Relative ability to gain electrons

  • Chlorine (Cl2) most able
  • Copper (Cu2+) middle
  • Vanadium (V2+) least able
• How the half equation is balanced does not affect the standard electrode potential
• Balancing half equations


  The important thing is to get the electrons to cancel, not to change the standard electrode potential
• Standard electrode potentials give a measure of the ability of a species to gain electrons
• Combining two half equations/cells

  The more positive standard electrode potential will proceed in the forward direction, forcing the less positive one to go in reverse
• Cell voltage (E cell)

  Calculated by taking the more positive standard electrode potential and subtracting the less positive one
• Determining if chlorine can oxidise Fe2+ to Fe3+

  1. Compare standard electrode potentials

  2. Cl2 is more positive, so it can oxidise Fe2+ to Fe3+

  3. Calculate cell voltage as 0.59V
• Determining if iodine can oxidise Fe2+ to Fe3+

  1. Compare standard electrode potentials

  2. Fe3+/Fe2+ is more positive, so it can oxidise I- to I2

  3. Iodine is not a powerful enough oxidising agent
• Copper 2+ copper half cell

  Copper metal electrode dipping into 1 M aqueous Cu2+ solution
• V2+ V half cell


  Vanadium metal electrode dipping into 1 M aqueous V2+ solution
• Electrochemical cell formation


  1. Connect the two half cells with a platinum wire and a salt bridge

  2. The more positive standard electrode potential half-reaction will occur in the forward direction, forcing the less positive half-reaction to occur in the reverse direction
• More positive standard electrode potential


  Substance can gain electrons more readily
• Reaction in the electrochemical cell

  1. Cu2+ ions are reduced to Cu metal at the cathode

  2. V metal is oxidized to V2+ ions at the anode
• Anode


  • The electrode with the less positive (more negative) standard electrode potential
  • Oxidation reaction occurs here
• Cathode


  • The electrode with the more positive standard electrode potential
  • Reduction reaction occurs here
• Electrons flow from the anode to the cathode through the external circuit
• Salt bridge


  Allows the flow of ions to maintain charge balance between the two half-cells
• Standard hydrogen electrode


  Half-cell with a standard electrode potential of 0 V, used as a reference to measure the standard electrode potentials of other half-cells
• Measuring standard electrode potentials

  1. Connect the unknown half-cell to the standard hydrogen electrode

  2. The voltage measured is the standard electrode potential of the unknown half-cell
• Electrochemical cell


  Formed when two half cells are combined
• Example electrochemical cell


  • Copper 2+ copper half cell + Vanadium 2+ Vanadium half cell
• Electrochemical cell setup


  1. Copper 2+ copper half cell on left

  2. Vanadium 2+ Vanadium half cell on right

  3. External circuit with voltmeter

  4. Salt bridge connecting solutions
• Salt bridge


  Allows flow of ions to balance charge imbalance between half cells
• Standard electrode potential


  Measure of how readily a substance can gain electrons
• Copper 2+ standard electrode potential

  More positive than Vanadium 2+ standard electrode potential
• Electrochemical cell reaction


  1. Copper 2+ ions accept electrons from Vanadium

  2. Vanadium 2+ ions formed

  3. Copper deposited on electrode

  4. Vanadium electrode disintegrates
• Cell voltage (E_cell)


  Calculated as most positive standard electrode potential minus least positive standard electrode potential
• Summary


  • Electrochemical cells made from two half cells
  • Standard electrode potential indicates which species accepts/donates electrons
  • Positive electrode has most positive standard electrode potential
  • Electrons flow from negative to positive electrode
  • E_cell = most positive - least positive standard electrode potential
• Standard electrode potential


  The voltage of a half cell when connected to a standard hydrogen electrode
• Standard conditions for standard electrode potential

  • 298 Kelvin temperature
  • 100 kilopascals pressure
  • Solutions at 1 mole per decimeter cubed
• Standard hydrogen electrode


  An electrode used as a reference to measure the standard electrode potential of other half-cells
• Standard hydrogen electrode


  1. Hydrogen gas at 100 kilopascals pressure

  2. Platinum electrode

  3. H+ ions at 1 mole per decimeter cubed

  4. Assigned 0 volts potential
• Positive standard electrode potential


  Indicates the half-cell has a greater tendency to accept electrons than the standard hydrogen electrode
• Negative standard electrode potential

  Indicates the half-cell has a lesser tendency to accept electrons than the standard hydrogen electrode
• Salt bridge


  Allows flow of ions between half-cells to address charge imbalance
• Types of half-cells


  • Metal-metal ion
  • Ion-ion
  • Gas-ion