Electrode Potentials

• Most positive E value shows the equation that wants to be reduced the most
• Most negative E value shows the equation that wants to be oxidised the most
• Joining two different metals in a salt solution makes electrons travel from the most reactive metal to the least reactive metal - current produced that can power devices
• Electrode or half cell is when a strip of metal is dipped in a solution of its own ions
• Placing a metal in a solution of its own ions will create an equilibrium and creates a potential difference between the metal strip and the solution
• A LARGE voltage means the equilibrium is to the right
• A SMALL voltage means the equilibrium is to the left
• The voltage of a half cell cannot be measured on its own
• The positive electrode is usually to the right and the negative electrode is usually to the left
• A salt bridge is needed to complete the circuit and allow the current to flow through the circuit using ions that are UNREACTIVE with the electrodes and ions are FREE to move
• The voltmeter prevents electrons flowing to the voltage can be measured by the voltmeter
• If the voltmeter is replaced with an ammeter or bulb then the electrons can flow from left to right and a current is produced
• Reaction goes to completion if the voltmeter is not present
• Why might a current produced by a cell fall to zero?

  All the reactants are used up
• What happens to a cell once the reactants are used up?

  Stops working OR starts to leak
• Oxidation always occurs at the negative electrode
• Electrons flow from the negative electrode to the positive electrode
• Reduction always happens at the positive electrode
• Platinum electrodes are used when there is no solid metal and there are only metal ions with different charges
• Why is platinum a suitable electrode?

  Platinum is unreactive AND conducts electricity
• To compare single electrodes to one another, a half cell called the standard electrode is used
• Standard hydrogen electrode conditions:

  • 100kPA (with hydrogen)
  • Platinum electrode
  • 298K
  • ALL solutions in 1 mol dm-3
  • Salt bridge
• The voltage of the standard hydrogen electrode is defined as ZERO
• Standard hydrogen electrode is always on the LHS
• Why the actual electrode potential may not be the standard electrode potential?

  The concentration of the solution might not be 1 mol dm-3
• Species with the highest oxidation is written closest to the salt bridge
• No phase boundary between species with the same state
• Ecell = Reduction - oxidation
• Cell discharge - If the Ecell value is positive then the reaction is feasible and the cell discharges (produces a current)
• Cell recharge - If the reaction is reversible then the cell can be recharged by plugging it into the mains (reverse reaction occurs when cell recharged)
• An environmental advantage of using rechargeable cells:

  • Metals are reused
• An environmental disadvantage of using rechargeable cells:

  • Mains electricity used to recharge may be powered by fossil fuel combustion which releases CO2
• How EMF changes if concentration changes:

  • EMF increases/decreases
  • Equilibrium shifts left/right
  • More/less electrons donated
• Electrons will flow towards the electrode where the reduction reaction occurs
• Single-use batteries:

  • Electrochemical reaction is irreversible
  • Once the reactants are used up it cannot be used again
• Rechargeable batteries:

  • Electrochemical reaction is reversible
  • Reactants can be remade
  • Discharge occurs when the device is being used
  • Recharge is the opposite of the discharge reaction
• When lithium battery used:

  Li + CoO2 -> Li(CoO2)
• When lithium battery recharged:

  Li(CoO2) -> CoO2 + Li
• Fuel cell - Uses energy from the reaction of a fuel with oxygen to create a voltage
• In fuel cells reactants need to be constantly supplied to provide a constant voltage