Electrode Potentials and Electrochemical Cells

• What is the acronym for remembering at what electrode oxidation occurs?

  NO PR

  Negative= oxidation

  Positive= reduction
• What is the acronym for remembering what side that oxidation occurs on?

  OIL

  Oxidation is left
• If the emv value is more negative, is the species being oxidised or reduced?

  Oxidised
• What is the acronym for remembering whether a species is being oxidised or reduced in an emv cell?

  NO PR

  More negative= oxidised

  More positive= reduced
• Why does the more reactive element form the negative electrode?

  • More easily oxidised
  • Loses electrons
  • Accumulation of electrons causes electrode to become more negative
• What is a salt bridge?

  A piece of filter paper soaked in KCl or KNO3 or a tube of these ions in agar gel
• What is the purpose of a salt bridge?

  Allows for the movement of ions and completes the circuit
• What is a compound we can use in the salt bridge?

  KNO3
• Why do we use KCl or KNO3 in the salt bridge?

  They are soluble so no precipitation
• Why do we use a high resistance voltmeter?

  • Measure potential difference
  • Maintains concentration of ions in the solution
• What is the standard hydrogen electrode?

  Measures half cell and all standard electrode potentials are written as reductions
• What are the conditions in a standard hydrogen electrode?

  • 1.00moldm3 ion (H+) solution
  • 100kPa pressure of gases
  • Temperature at 298K
  • Platinum electrode
  • Glass bell inverted in the solution in a beaker with a wire and an opening to allow gas to bubble out of the solution
• Why do we use platinum in electrochemical cells?

  • It is a good electric conductor
  • Inert
  • Porous for a larger surface area
• What is the acronym for remembering what happens in the shorthand equation for a standard chemical cell?

  R O O R

  reduction oxidation oxidation reduction
• What does | represent in the shorthand equation for cells?

  A change in state
• What does || represent in the shorthand way of writing what happens in a chemical cell?

  Salt bridge
• What is represented in the shorthand equation in cells?

  The flow of electrons in a cell
• Here are two half equations

  Zn2+(aq) + 2e- --> Zn(s) E= -0.76V

  Pb2+(aq) + 2e- --> Pb (s) E= -0.13V

  Write the shorthand equation.

  Zn(s)|Zn2+(aq)||Pb2+(aq)|Pb(s)
• Does changing the surface area of the platinum electrode affect the EMF?

  No, it does not affect the equilibrium
• What is used to separate components in the same phase in the shorthand equation?

  Commas
• Where do we put Pt in the shorthand equation?

  Far left or right
• What is the equation for emf?

  Emf= reduced - oxdised
• If equilibrium lies to the left...

  There is a negative charge
• If equilibrium lies to the right...

  There is a positive charge
• What is a metal dipping into a solution of its ions called?

  A half cell
• How is the standard hydrogen electrode known?

  primary standard
• Features of an electrochemical cell.

  • two metals joined by a wire
  • joined by a salt bridge
  • a high resistance voltmeter
• When do we use platinum in the shorthand equation?

  If there is no solid metal in the half equations
• Why is the SHE difficult to use?

  Uses a flammable gas
• What is a secondary standard?

  A standard electrode that has been calibrated against the primary standard
• If a species has a very positive emf value, is it a good oxidising or reducing agent?

  Oxidising agent as it is good at gaining electrons thus oxidising the other component
• Define the term electrochemical series.

  List of electrode potentials in numerical order
• What is a cell made up of?

  Two half cells
• Write the shorthand equation for a Sn2+ standard hydrogen electrode.


  Pt | H2 | 2H+|| Sn2+| Sn
• Why do we use a high resistance voltmeter?

  Prevents the current from flowing and the reaction from stopping so that we can measure the potential difference
• What systems is a KCl salt bridge not compatible with?

  Copper systems because copper can form complexes with Cl- ions
• What do we replace a voltmeter with in a circuit?

  A bulb or an ammeter
• What happens if we remove the voltmeter?

  Potential difference falls to zero as the concentrations of each solution in each electrode is the same and reactants are used up
• When can we not include H+ or H2O or numbers in the shorthand equation?

  If the system contains several species
• What is the potential of a hydrogen electrode under standard conditions?

  0V