Electrode potentials

• What is the standard electrode potential?

  e.m.f of a half cell compared/connected to a standard hydrogen half cell
• what are the standard conditions of an electrode potential?

  concentration of solutions: 1 moldm⁻³

  temperature is 298K

  pressure is 100KPa
• electrode with the more reactive metal loses electrons

  • positive electrode
• electrode with the less reactive metal gains electrons

  • negative electrode
• The sign of the standard electrode potential shows the sign of the half cell connected to the hydrogen Electrode and the relative tendency to gain electrons in comparison to the hydrogen half cell
• standard hydrogen electrode

  
• standard electrode potential

  half cell is connected to a standard hydrogen electrode

  • wire allows the controlled flow of electrons
  • salt bride allows ions to flow

  
• standard hydrogen electrode by definition should be 0V
• more negative E° value shows a greater tendency to lose electrons and undergo oxidation
• less negative E° value shows a greater tendency to gain electrons and undergo reduction
• voltaic cells converts chemical energy into electrical
• what is an example of a salt bridge?

  a filter paper soaked in KNO3 (aq)
• how are voltaic cells made?

  by connecting together two half cells to allow electrons to flow

  • must be kept apart or electrons would flow in an uncontrolled way and heat energy would be released instead of electrical
• what do half cells contain?

  chemical species present in a redox half equation
• metals / non-metals in contact with their ions in aqueous solution

  -represented by a straight line for the phase boundary between aqueous solid and metal

  • at the phase boundary the metal will be in contact and in equilibrium with its ions

  
• when two half cells are connected the direction of electron flow depends upon the relative tendency of each electrode to release electrons
• in an isolated half cell there is no net transfer of electrons in or out of the metal
• ions of the same elements in different oxidation states are in contact with a platinum electrode.

  • both ions present must have the same concentration
  • no metal to transfer electrons so an inert metal is used
  • can also be lead as it is cheaper than platinum

  
• what is the calculation of the standard cell potential?

  E°cell = E°(positive electrode) - E°(negative electrode)
• what are the limitations of predictions using E° values?

  reaction rate: reactions may have a very large activation energy so reaction may occur but requires time to begin
• what are the limitations of predictions using E° values?

  only deduces the thermodynamic feasibility not the rate of reaction
• what are the limitations of predictions using E° values?

  concentration: standard electrode potentials are used but many reactions take place in concentrated or dilute solutions

  • if concentration of solution is not the standard the E° value will be different to the standard
• what are the limitations of predictions using E° values?

  actual conditions may be different to the standard conditions

  • standard also apply to aqueous equilibria, many reactions are not aqueous
• Feasibility

  when the standard cell potential is positive
• most negative to the right is the best reducing agent
• most positive on the left is the best oxidising agent
• when combining E° half cells you must flip the more negative one
• One advantage of using ethanol rather than fuel cell for vehicles

  -ethanol is a liquid/ less volatile

  -hydrogen is more flammable
• what are advantages of using hydrogen as a fuel compared with the conventional fuels?

  no carbon dioxide is formed

  only by product is water
• what is an important difference between a fuel cell and a modern storage cell

  fuel reacts with oxygen to give electrical energy