3.1.11 Electrode potentials and electrochemical cells

• Half-cells

  • A typical half-cell comprises of a piece of metal dipped in an aqueous solution of its ions
  • All half-equations must be written as reduction reactions (IUPAC rules)

  The two strips of metals act as electrodes

  The two electrodes are connected by wires to a high resistance voltmeter

  The voltmeter measures the potential difference (p.d) between the two electrodes and shows the direction in which the current flows
• Example: Zinc and Copper

  There would be two half-cells

  • One half-cell would be made by placing a strip of zinc (Zn) into an aqueous solution of zinc sulfate (provides Zn2+)An equilibrium forms: Zn2+ (aq) + 2e ⇆ Zn (s)
  • The other half-cell would be made by placing a strip of copper (Cu) into an aqueous solution of copper sulfate (provides Cu2+) An equilibrium forms: Cu2+ (aq) + 2e ⇆ Cu (s)
• There is a standard way of writing electrochemical cells which is called a cell diagram or the convectional cell representation

  • All state symbols have to be included
  • A single vertical line (|) separates different states of matter
  • A double line ( || ) represents a salt bridge
  • The electrode where reduction occurs is always on the right (most positive)
• Standard Hydrogen Electrode

  → Each half-cell has a standard electrode Potential, E

  If you use a standard half cell with a standard electrode potential value zero then we can measure the standad electrode potential of the other half-cell relative to this standard half cell

  • Standard half-cell is called the standard hydrogen electrode (SHE)
• A Standard electrode Potential messured under

  Standard conditions:

  • Temp of 298 K
  • Pressure of 100 kPa
  • All solution @ concentration of 1 mol dm-3

  E° measured for half- cells which are conected to the SHE)
• SHE comprises of

  > Platinum electrode into HCI

  > Con of H+ ion ( 1 mol/dm3 ) or (0. 5 mol/dm3

  of H2SO4)

  > Hydrogen gas is bubbled through acid @ 100kPa

  > Temp of half cell maintained @ 298K
• The standard electromotive force is the potential difference between the 2 electrodes in a electrochemical cell
• E.M.F is calculated

  E cell = E right - E left

  positive value for E cell tells you the forward reaction occurs
• A positive E (V) means the forward reaction occurs (reduction)

  A negative E (V) means the backwards reaction occurs (oxidation)
• The factors which affect E® values are:

  • Cell concentration
  • Cell temperature
  • Cell pressure
• An increase in temperature will favour the backward reaction and the e.m.f. value decreases

  The effect of temperature on the e.m.f. value is small
• A cell is an electrochemical cell. A battery is two or more cells connected together
• There are three types of commercial electrochemical cells:

  • Primary cells - are not rechargeable and are thrown away after use - single use not good for the environment → equations not reversible

  • Secondary cells - can be recharged after use → reversible equation

  • Fuel cells - produce electricity from gaseous or liquid fuels
• In a cell what does the porous separator ?

  allows ions to pass like a salt bridge
• why does cell leak after time

  Zinc can get oxidised and wears away and paste comes out
• MnO2 can be mixed with graphite to increase its electrical conductivity
• Why use a high resistance voltmeter?

  The voltmeter needs to be of very high resistance to stop the current from flowing in the circuit. In this state it is possible to measure the maximum possible potential difference (E).

  The reactions will not be occurring because the very high resistance voltmeter stops the current from flowing.
• Why does a fuel cell not need to be electrically recharged 


  The reactants are supplied constantly