1.11 Electrode Potentials

• What is redox?


  When an element is oxidised then another must be reduced
• What must you remember when writing half equations?


  - only one element loses or gains electrons

  - the equation must balance for atoms

  - the equation must balance for charge

  - for reactions in soultion, surplus oxygen is converted to water with hydrogen ions from the acid
• What are standard conditions?


  - 298K

  - 100kPa

  - 1 mol dm-3 solution of ions
• When and why is platinum used as an electrode?


  - when there is no solid metal electrode

  - Platinum is unreactive
• Why is the E* of the standard hydrogen electrode 0V?

  By definition
• What are the conditions of the standard hydrogen electrode?


  - 298K

  - 100kPa of H2 gas

  - 1 mol dm-3 of H+ ions

  - Pt electrode
• What is a salt bridge?


  Piece of salt paper soaked in salt solution e.g. KNO31
• Why is a salt bridge used?


  Allow the flow of ions between solutions
• What is the electrochemical series?

  A list of electrode potentials in order of decreasing or increasing potential.
• How do electrons flow in a circuit?

  From the more reactive metal to the less reactive metal
• Which is the more reactive metal?

  The metal that is more likely to give up electrons (oxidation)
• Which are the best oxidising agents?


  - most positive value

  - species on the left of the reduction equation
• Which are the best reducing agents?


  - most negative value

  - species on the right
• Which cell is the oxidation cell?


  - cell with the most negative/least positive value

  - on the left
• Which cell is the reduction cell?


  - cell with the least negative/most positive value
• How do you measure the emf of a cell?

  Reduction - oxidation
• When will a reaction not take place?

  When a stated reaction has a negative value
• How do you write cell notation?


  OXIDATION || REDUCTION
• What are some key features to remember when writing cell notation?


  - Metal on the outside, ions on the inside

  - | represents phase boundary

  - || represent salt bridge

  - species of the same state are separated by a coma

  - no solid in the half equation, then a Pt electrode is used
• What are non-rechargeable cells?


  - reactions in the half-cells are irreversible therefore the battery is non-rechargeable (primary cells)

  - chemicals are used up over time and the EMF drops

  - once one or more of the chemicals are completely used up the cell is flat and the emf is 0V
• What are rechargeable cells?


  - reactions are reversible, the battery is rechargeable (secondary cell)

  - by connecting the battery to another power supply with a larger em, electrons and ions are forced around the circuit in the opposite direction

  - reverses chemical reaction which regenerates the chemicals
• Give equations for lithium cells


  Li+ + CoO2 + e- ->[Li]+[CoO2]-

  Li(s) -> Li+ + e-
• Which is the positive electrode in a lithium cell?


  Li+ + CoO2 + e- ->[Li]+[CoO2]-
• Which way do electrons flow on discharge?


  Negative to positive electrode
• Which way do electrons flow on recharge?

  Positive to negative electrode
• What is a fuel cell?


  - when a chemical reaction between a fuel and oxygen is used to create a voltage

  - they have a continuous supply to chemicals into the cell no so neither run out or need re-charging
• How are the two half cells connected in a fuel cell?


  - semi-permeable membrane
• What is the most common fuel cell?

  Hydrogen-oxygen fuel cell
• What is the overall equation for a hydrogen fuel cell in alkaline conditions?

  2H2 + O2 -> 2H2O
• What is the overall equation for a hydrogen fuel cell in acidic conditions?

  2H2 + O2 -> 2H2O
• Which cell undergoes reduction?


  Oxygen half cell as it is more positive
• Which cell undergoes oxidation?

  Hydrogen half cell as it is more negative
• What are some of the advantages of a fuel cell?


  - waste product is water

  - doesn't need recharging therefore more efficient

  - less CO2 produced compared to combustion engine
• What are some of the disadvantage of fuel cells?


  - hydrogen is difficult to store and transport

  - hydrogen produced from methane (fossils fuels) therefore process is not carbon neutral