topic 02 - electrode potentials and electrochemical cells.

• what happens to metal atoms when they lose electrons?


  they become positively charged metal ions
• what happens when a strip of a metal element is placed into a solution of its own ions?

  an equilibrium is set up where the conc of metal atoms and ions are equal
• what is the equation to show a metal being ionised?


  M (S) = M z+ (aq) + ze-

  M = any metal

  m+ = metal ion

  z = no. of the charge on metal ion and no of electrons gained by metal ion
• what is the half cell/electrode?


  the equilibrium containing the solid metal and metal ions in solution
• what happens when a half cell doesn't contain a solid metal electrode in the half equation?


  they ca use a platinum electrode.
• why do we use a platinum electrode when there is no solid metal electrode?


  • its a metal so conducts electricity
  • its inert so won't interfere/take part with the reaction in the half cell
• how does an electrochemical cell form?


  if 2 half cells are connected together an electrochemical cell forms
• how are the 2 half cell attached in an electrochemical cell


  • through a wire attached to the top of each metal electrode
  • a salt bridge which is immersed into the solution of metal ions one each half cell
• what is the function of the wire in the electrochemical cell?


  • allows the flow of electrons from one half cell to the other half cell
• what is the function of the salt bridge in the electrochemical cell?


  • allows the flow of ions between the half cells
• what is the salt bridge usually made from?

  • usually made of a piece of filter paper that has been soaked in a salt solution, eg: KNO3 or KCL
• how do we choose the salt we use in the salt bridge of an electrochemical cell?


  • the choice of salt varies depending on the solution's identity in the half cells
  • the salt used can't react with either of the solutions in the half cells
  • KNO3 is used often as the K- and the NO3 - ions tend to react with most solutions used
  • KCL wouldn't be suitable when the half cells contain silver or lead ions bc the chloride ions would react with the metal ions to form a precipitate
• in the electrochemical cell, what must the concentrations be of each solution?


  1.0 moldm^-3
• what is an electrochemical cell an example of?

  a redox system
• how is an electrochemical cell a redox system?

  • oxidation occurs in one half cell where the metal atoms are oxidised to metal ions which releases electrons
  • as long as the 2 half cells are connected via wire in a complete circuit, the electrons can move through the wire to the second half cell
  • in the second half cell, reduction occurs as the metal ions in the solution gain electrons and are reduced to metal atoms
• how long do the reactions occur in the electrochemical cell?


  • reactions occur until all the reactants are used up
  • once all the metal atoms in the oxidation half cell have been used up, no more electrons are released
  • so no electrons can be moved through the wire to the reduction half cell
  • the redox reaction has stopped as no further oxidation or reduction can occur
• why does each half cell have the potential to release or accept electrons?


  • because each half cell is a system in equilibrium, they have the potential to release or accept electrons depending on the direction the equilibrium shifts.
• what can cause the half cell's equilibrium to shit towards or backwards?


  • any change in conditions, eg: temp or conc
• what is the forwards direction?


  reduction
• what is the backwards direction?

  oxidation
• what can the equilibrium shifting cause?

  • can cause visible changes which can be observed in the half cell
  • changes observed could be : a change in size of the electrode or a change in the colour of the metal ion solution
• what visible changes would be seen if equilibrium shifts in the forwards direction?

  • forward direction means the metal atoms are oxidised and lose electrons to become metal ions
  • so there would be a gradual decrease in size of the metal electrode
  • a gradual increase in intensity of the solution's colour as there would be an increase in conc of metal ions
• what visible changes would be seen if equilibrium shifts in the backwards direction?

  • backwards direction means the metal ions are reduced and gain electrons to become metal atoms
  • so there would be a gradual increase in size of the copper atoms on the electrode so increased size of electrode
  • the colour of the solution would gradually decrease as the conc of metal ions decreases
• what would happen if the wire the connects the 2 half cells in an electrochemical cell is removed and replaced with a voltmeter?


  • the circuit is no longer complete
  • electrons can't flow between the half cells
  • no oxidation or reduction can occur
• what is the electrode potential?


  • the potential of a half cell to release electrons
  • uses volts
• what can change the position of equilibrium in a half cell?


  • temp changes
  • conc changes
  • pressure changes
• what can the position of equilibrium changing affect?


  • can affect the accuracy of the potential measurement
• how do we get an accurate value for the electrode potential?


  • we use standard conditions
• what is the standard electrode potential of the half cell?


  • if the electrode potential is measured under standard conditions, the value obtained is the standard elected potential of the half cell.
• what are standard conditions?


  • temp of 298 K
  • pressure of 100kPa
  • conc of H+ ions is 1.0 moldm^-3
• what does the voltmeter do when placed between 2 half cells?


  • can be used to measure the potential for electron release in the electrochemical cell
  • but we can't measure electron releasing potential of a single half cell, we need to connect if to a standard half cell
• what is the electrode potential of a standard half cell?


  0V
• what is the standard half cell called that we use to measure the electron potential of a half cell?


  a standard hydrogen electrode or SHE cell
• what electrode is used in a SHE cell and why?


  • platinum electrode
  • because there is no solid in the H2/H+ half cell
• in a SHE cell, what must the conc of H+ be?


  • conc H+ ions = 1 moldm^-3
• what are some suitable solution to use in a SHE cell?


  • 1 moldm^-3 HCL
  • 1 moldm^-3 HNO3
  • 0.5 moldm^-3 H2SO4
• what is the elected potential of a SHE cell?


  • by definition it is 0V
• how does it work that when attached a SHE cell to a half cell we get the electrode potential of the half cell?


  • when a half cell is connected to a SHE cell with a voltmeter placed between them, any potential measured by the voltmeter must be the electrode potential of that half cell
• what reaction is always occurring in the SHE cell?

  • oxidation, the hydrogen atoms lose electrons to become hydrogen ions
  • H2 (g) = 2H+ (aq) + 2e-
• why are all the half equations in the electrochemical series written as reduction reactions?

  SHE cells only have reduction