3.1.11 - Electrode potentials

• What is the purpose of the salt bridge in an electrochemical cell? allows ions to flow to complete the circuit
• What is the salt bridge made up of in an electrochemical cell? filter paper soaked in KNO3 cell
• EӨ values are always quoted for reductions
• Soluble ionic compound used in a salt bridge must not react with electrolyte in beaker
• Reactions are written in the electrochemical series as reductions
• An oxidising agent will oxidise anything that is in the electrochemical series above
• Reasons an expected reaction would not happen in a cell:

  1. Kinetically stable
  2. Solution too dilute
• What are the three types of cells?

  • Rechargeable
  • Non-rechargeable
  • Fuel
• Advantages of non-chargeable batteries:

  • Cheaper to buy
  • Work for longer
  • Can be recycled
  • Don't contain toxic Pb / Cd so less dangerous in landfill
• What are the advantages of rechargeable batteries?

  • Can be reused so cheaper long term
  • Can be recycled
• What are the advantages of fuel cells?

  • No recharging needed
  • H2O is only product
• What are the disadvantages of fuel cells?

  • Expensive as H2 produced by electrolysis of water
  • H2 highly flammable
• Features of the electrochemical series:

  • Oxidising agents on the left 
  • Reducing agents on the right
  • Most powerful oxidising agent on the bottom left
  • Most powerful reducing agent on top right
• Salt bridges are necessary to:

  • Complete the circuit
  • Allows ion movement to balance the charge. Do not react with electrodes
• What type of species goes is furthest from the salt bridge in standard cell notation?

  
  

  The most reduced species
• What does | indicate?

  Phase boundary (solid/liquid/gas)
• What happens at the left-hand electrode?

  • Left hand electrode is where oxidation occurs.
  • Left hand electrode is the half cell with the most negative Eo value
• Right hand electrode:

  • right hand electrode is where reduction occurs.
  • Right hand electrode is the half cell with the most positive Eo value
• Which side of the cell has the most negative Eo value?

  
  

  Left hand electrode
• Label this standard hydrogen electrode

  A) High resistance voltmeter

  B) Platinum wire

  C) Hydrogen Gas

  D) Salt bridge

  E) Porous platinum metal

  F) Dilute Acid

  
• What conditions is the standard hydrogen electrode used in?

  • Temperature = 298 K
  • Pressure = 100 kPa
  • [H+ ] = 1.00 mol dm-3
• Why might you use other standard electrodes occasionally?

  • They are cheaper/easier/quicker to use and can provide just as good a reference.
  • Platinum is expensive
• If an Eo value is more negative, what does it mean in terms of oxidising/reducing power? Better reducing agent (easier to oxidise)
• What factors will change Eo values?

  • Concentration of ions
  • Temperature
• When would you use a Platinum electrode? When both the oxidised and reduced forms of the metal are in aqueous solution
• Why is Platinum chosen?

  • Inert so does not take part in the electrochemistry
  • Good conductor to complete circuit
• Where are lithium-ion cells used? Mobile phones and Laptops
• A fuel cell is a cell that is used to generate electric current; does not require electrical recharging
• Label this diagram of a hydrogen fuel cell

  A) Anode

  B) Electrolyte

  C) Cathode

  D) h2

  E) o2

  F) h20