Electrode Potentials

• Electrochemical cells use redox reactions as the electron transfer between products creates a flow of electrons
• This flow of charged particles is an electrical current which flows between electrodes in the cell
• A potential difference is produced between the two electrodes which can be measured
• Most electrochemical cells consist of two solutions with metal electrodes and a salt bridge
• A salt bridge is a tube of unreactive ions that can move between the solutions to carry the flow of charge but will not interfere with the reaction
• Each solution is a half-cell which makes up the full chemical cell
• Half-cells have a cell potential which indicates how it will react, either as an oxidation or reduction reaction
• Cells are represented in a simplified way with specific rules:

  • The half-cell with the most negative potential goes on the left
  • The most oxidised species from each half-cell goes next to the salt bridge
  • A salt bridge is shown using a double line
  • Always include state symbols
• The standard hydrogen electrode is the measuring standard for half-cell potentials with a cell potential of 0.00V under standard conditions
• Standard conditions include solutions of 1.0 moldm^-3 concentration, a temperature of 298K, and 100 kPa pressure
• The standard hydrogen electrode consists of Hydrochloric acid, Hydrogen gas, and uses Platinum electrodes
• Cell potentials are measured compared to the SHE to give a numerical value for the half-cell potential
• Negative potentials mean the substances are more easily oxidised and will lose electrons
• Positive potentials mean the substances are more easily reduced and will gain electrons to become more stable
• Standard cell potential values are used to calculate the overall cell EMF
• Overall cell EMF is calculated as the potential of the right of the cell minus the potential of the left of the cell
• If the overall cell potential is a positive value, the reaction taking place is spontaneous and favourable
• In a similar way to redox reactions, half-cell reactions can be combined to give the overall cell reaction
• The ‘anti-clockwise rule’ is a method for ensuring the reaction is formed correctly:

  • Write the most negative EMF out of the pair on top
  • Draw anticlockwise arrows around the reactions
  • Balance the electrons on both sides of the reaction
  • Write out the cell reaction
• Electrode potentials that are very positive are better oxidising agents and will oxidise those species more negative than it
• Species that are very negative are better reducing agents and will reduce those less negative than it
• Increasing the concentration of the solutions used in the electrochemical cell makes the cell EMF more positive as fewer electrons are produced in the reaction
• Increasing the pressure of the cell will make the cell EMF more negative as more electrons are produced
• Electrochemical cells can be non-rechargeable, rechargeable, or fuel cells
• Rechargeable cells involve a reversible reaction where reactants can reform, such as Lithium ion cells
• Lithium ion cells consist of a Lithium Cobalt Oxide electrode and a Graphite (Carbon) electrode with an electrolyte of a Lithium salt in an organic solvent
• To be recharged, a current has to be applied over the cell which forces electrons to move in the opposite direction, causing the reaction to reverse
• Non-rechargeable cells are not able to reverse reactions as they are impossible to reverse
• Fuel cells, like the Hydrogen fuel cell, generate an electrical current without needing to be recharged
• Hydrogen fuel cells use a continuous supply of Hydrogen and Oxygen from air to generate a continuous current
• The reaction in Hydrogen fuel cells produces water as the only waste product, making them environmentally friendly
• Downsides to Hydrogen fuel cells include the high flammability of Hydrogen and their expensive production costs