23.4 - electrode potentials

• Anode


  Meaning up. The species at the anode will lose electrons (be oxidised) and they will flow up the electrode.
• Cathode


  Meaning down. The species at the cathode will gain electrons (be reduced) and they will flow down the electrode.
• Voltaic cell


  A type of electrochemical cell that converts chemical energy into electrical energy.
• What do you need to make a voltaic cell?


  As electrical energy results from the movement of electrons, you need chemical reactions that transfer electrons from one species to another. These are redox reactions.
• What does a half-cell contain?


  The chemical species present in a redox half equation.
• How can a voltaic cell be made up?


  By connecting together two different half-cells, which then allows electrons to flow.
• Why must the chemicals in the two half-cells be kept apart in a voltaic cell?


  If the chemicals in the two half-cells are allowed to mix, electrons would flow in an uncontrolled way and heat energy would be released rather than electrical energy.
• Metal/metal ion half-cells


  A metal rod dipped into a solution of its aqueous metal ion.

  
• How are metal/metal ion half-cells represented?


  Using a vertical line for the phase boundary between the aqueous solution and the metal. E.g. Zn²⁺(aq)|Zn(s) and Cu²⁺(aq)|Cu(s).

  
• Phase boundary


  Where the metal (rod) is in contact with it's (aq) ions.
• What happens at the phase boundary in metal/metal ion half-cells?


  An equilibrium is set up.
• How is the equilibrium in a metal/metal ion half-cell written?


  So that the forward reaction shows reduction and the reverse reaction shows oxidation.

  
• What happens in an isolated half-cell?


  There is no net transfer of electrons either into or out of the metal.
• What does the direction of electron flow depend on when two half-cells are connected?


  The relative tendency of each electrode to release electrons.
• Ion/ion half-cells


  A half-cell that contains ions of the same element in different oxidation states. E.g. a half cell can be made containing a mixture of aqueous iron (II) and iron (III) ions.

  
• How are electrons transferred in or out of an ion/ion half-cell


  There's no metal to transport electrons, so an inert metal electrode made out of platinum is used.

  
• How do you know which electrode has a greater tendency to gain or lose electrons, in a cell with two metal/metal ion half-cells connected?


  The more reactive metal releases electrons more readily and is oxidised.
• What happens in an operating cell?


  - The electrode with the more reactive metal loses electrons and is oxidised - this is the negative electrode.

  - The electrode with the less reactive metal gains electrons and is reduced - this is the positive electrode.
• Standard electrode potential


  The potential difference produced when a standard half-cell is connected to a standard hydrogen cell under standard conditions.

  
• What does electrode potential depend on?


  The position of equilibrium and therefore the electrode potential depends on factors such as:

  - temperature.

  - pressure of gases.

  - concentration of reagents.
• How can we compare the electrode potentials of different species?


  By measuring them against a common reference or standard. As well as using standard conditions: 1 mol dm⁻³, 298K, 100kPa.
• What type of voltmeter is used to measure standard electrode potential?


  A high resistance voltmeter so that no current flows and maximum potential difference is acheived.
• What are electrode potentials measured relative to, when trying to find standard electrode potentials?


  Standard hydrogen electrode.
• Standard hydrogen electrode


  A half-cell containing hydrogen gas, H₂(g), and a solution containing H⁺(aq) ions. An inert platinum electrode is used to allow electrons into and out of the half cell.

  
• Standard electrode potential of a standard hydrogen electrode


  0V
• Measuring standard electrode potential


  The half-cell is connected to a standard hydrogen electrode:

  - The two electrode are connected by a wire to allow a controlled flow of electrons.

  - The two solutions are connected with a salt bridge which allows ions to flow.

  
• Salt bridge


  Contains a concentrated solution of an electrolyte that does not react with either solution. E.g. a strip of filter paper soaked in aqueous potassium nitrate.
• The more negative the standard electrode potential value...


  - The greater the tendency to lose electrons and undergo oxidation.

  - The less the tendency to gain electrons and undergo reduction.
• The more positive the standard electrode potential value...


  - The greater the tendency to gain electrons and undergo reduction.

  - The less the tendency to lose electrons and undergo oxidation.
• What type of standard electrode potential values do metals tend to have?


  Negative values and lose electrons.
• What type of standard electrode potential values do non-metals tend to have?


  Positive values and gain electrons.
• In general, the more negative the electrode potential value...


  The greater the reactivity of a metal in losing electrons.
• In general, the more positive the electrode potential value...


  The greater the reactivity of a non-metal in gaining electrons.
• Steps of measuring standard cell potentials


  1. Prepare two standard half-cells.

  - For a metal/metal ion half-cell, the metal ion must have a concentration of 1 mol dm⁻³.

  - For an ion/ion half-cell, both metal ions present in the solutions must have the same concentration. There must be an inert electrode, usually platinum.

  - For half cells containing gases (e.g. hydrogen half-cell), the gase must be 100kPa pressyre, in contact with a solution with an ionic concentration of 1 mol dm⁻³ and an inert platinum electrode.

  - Temperature must be 298K.

  
  

  2. Connect the metal electrode of the half-cells to a voltmeter using wires.

  
  

  3. Prepare a salt bridge by soaking a strip of filter paper in a saturated aqueous solution of potassium nitrate.

  
  

  4. Connect the two solutions of the half-cells with a salt bridge.

  
  

  5. Record standard cell potential from the voltmeter.
• Electromotive force


  The same value as the cell potential.
• What do standard cell potentials quantify?


  The tendency of redox systems to gain or lose electrons.
• How can standard cell potentials be calculated?


  Directly from standard electrode potentials:

  Standard cell potential = standard electrode potential (positive electrode) - standard electrode potential (negative electrode).