electrode potentials and cells

• 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 2 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. these half cells have a cell potential which indicates how it will react, either as an oxidation or reduction reaction
• conventional cell representatio:

  cells are represented in a simplified way so they dont have to be drawn out each time. this has 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 always shown using a double line

  • always include state symbols
• the standard hydrogen electrode is the measuring standard for half-cell potentials. it has the cell potential of 0.00V, measured under standard conditions. these conditions are:

  •solutions of 1.0moldm-3

  •298K

  •100kPa

  the cell consists of hydrochloric acid, hydrogen gas and uses platinum electrodes. these are very useful as they are metallic so theyll conduct electricity but are also inert so wont interfere with the reaction
• if measured under standard conditions, cell potentials are measured compared to the standard hydrogen electrode to give a numerical value for the half cell potential.

  negative potentials mean that 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
• calculating cell EMF (electromotive force)

  standard cell potential values are used to calculate the overall cell EMF. this is always done as the potential of the right of the cell minus the potential of the left.

  if the overall cell potential is a positive value, the reaction taking place is spontaneous and favourable.
• anticlockwise rule for cell reactions

  half cell reactions can be combined to give the overall cell reaction.

  1. write the most negative EMF out of the pair on top
  2. Draw anticlockwise arrows around the reactions
  3. balance the electrons on both sides of the reaction
  4. 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
• electrochemical cells can be a useful source of energy for commercial use. they can be produced to be non-rechargeable, rechargeable or fuel cells
• a reaction that takes place within a rechargeable cell is a reversible reaction meaning the reactants can reform. therefore the cell can be reformed meaning it is a rechargeable cell
• lithium ion cells are commonly used as rechargeable batteries in phones, laptops and cars. they consist of a lithium cobalt oxide electrode and a graphite electrode. an electrolyte of a lithium salt in an organic solvent is used to carry the flow of charge
• half-cell equations for lithium ion cells

  negative electrode: Li <-> Li+ + e-

  positive electrode: e- + Li+ + CoO2 <-> Li+[CoO2]-

  Li + Li+ + CoO2 <-> Li+ + Li+[CoO2]-
• in order for a rechargeable cell to be recharged, a current has to applied over the cell which forces electrons to move in the opposite direction. this causes the reaction to reverse, recharging the cell.

  non-rechargeable cells are not able to do this because the reactions are impossible to reverse
• fuel cells can be used to generate an electrical current without needing to be recharged. the most common type of fuel cell is the hydrogen fuel cell which uses a continuous supply of hydrogen and oxygen from air to generate a continuous current.

  the reaction that takes place produces water as the only waste product meaning the hydrogen fuel cell is seen as being much more environmentally friendly.
• downsides to hydrogen fuel cells include the high flammability of hydrogen and that they are expensive to produce meaning they are not used commonly
• hydrogen fuel cell:

  anode: H2(g) + O2- = H2O(g) + 2e-

  cathode: 2e- + 1/2 O2(g) = O2-

  overall: H2(g) + 1/2 O2(g) = H2O(g)
• an anode is a positive electrode where oxidation takes place
• a cathode is a negative electrode where reduction takes place
• electrochemical cells are a combination of 2 half-cells, consisting of 2 electron conductors (electrodes) separated by an ionic conductor (electrolyte). cells are used to measure electrode potentials by reference to the standard hydrogen electrode
• electrochemical series is a series of elements arranged in order of their standard electrode potentials
• an electrode is a conductor where electricity enters or leaves a substance/region
• EMF is the difference between the potential difference of the cathode and anode in an electrochemical cell
• a fuel cell is an electrochemical cell that converts the chemical energy of fuel (usually hydrogen) and an oxidising agent (usually oxygen) into electricity through redox reaction. they do not need to be recharged
• non-rechargeable cells are cells designed to be used only once since the reactions involved are irreversible
• rechargeable cells are where reversible reactions occur, allowing them to be recharged to regain their cell potential
• redox reactions are where oxidation and reduction occur simultaneously
• a salt bridge is a porous substance soaked with a solution of an inert, strong electrolyte (e.g filter paper soaked in KNO3(aq)). the salt ions flow through the bridge to complete the cell and balance charges in solutions
• when a rod is dipped into a solution of its own, an equilibrium is set up between the solid metal and the aqueous metal ions
• Zn(s) <-> Zn2+(aq) + 2e-
• Cu2+(aq) <-> Cu3+(aq) + e-
• the simplest salt bridge is made of filter paper soaked in a saturated solution of potassium nitrate (KNO3)
• Salt bridges are necessary because they complete the circuit but avoid further metal/ion potentials as they dont perform electrochemistry. They allow ion movement to balance the charge and do not react with electrodes
• Al(s) | Al3+(aq) || Cu2+(aq) | Cu(s)
• the left hand electrode is where oxidation occurs.
• A platinum electrode is used when both the oxidised and reduced forms of the metal are in aqueous solutions
• platinum is chosen because it is inert and doesnt take part in electrochemistry. It is a good conductor
• How to predict if a reaction would occur

  1. Take the 2 half equations

  2. Find the species that is being reduced (this is effectively the right hand electrode)

  3. Calculate its E° value minus the E° value of the species that is being oxidised (effectively the left hand cell)

  4. If E° overall > 0, reaction will occur