Standard electrode potential

• The position of equilibrium and therefore the electrode potential depends on factors such as:
  • Temperature
  • Pressure of gases
  • Concentration of reagents

   

• So, to be able to compare the electrode potentials of different species, they all have to be measured against a common reference or standard
• Standard?conditions?also have to be used when comparing electrode potentials
• These standard conditions are:
  • Ion concentration of 1.00 mol dm-3
  • A temperature of 298 K
  • A pressure of 100 kPa

   

• Standard measurements are made using a?high resistance voltmeter?so that no current flows and the maximum potential difference is achieved

• The electrode potentials are measured relative to a?standard hydrogen electrode
• The standard hydrogen electrode is given a value of 0.00 V, and all other electrode potentials are compared to this standard
• This means that the electrode potentials are always referred to as a?standard electrode potential?(E?)
• The?standard electrode potential?(E?)?is the potential difference ( sometimes called voltage) produced when a?standard half-cell?is connected to a?standard hydrogen cell?under standard conditions
• For example, the standard electrode potential of bromine suggests that relative to the hydrogen half-cell it is more likely to get reduced, as it has a?more positive?E??value

Br2(l) + 2e–?? 2Br–(aq)? ? ? ??E??= +1.09 V

2H+(aq) + 2e–?? H2(g)? ? ? ??E??= 0.00 V

• The standard electrode potential of sodium, on the other hand, suggests that relative to the hydrogen half-cell it is less likely to get reduced as it has a?more negative?E??value

Na+?(aq) + e–?? Na(s)? ? ? ??E??= -2.71 V

2H+?(aq) + 2e–?? H2(g)? ? ? ??E??= 0.00 V

• The?standard hydrogen electrode?is a half-cell used as a?reference electrode?and consists of:
  • Hydrogen gas in equilibrium with H+?ions of concentration 1.00 mol dm-3?(at 100 kPa)

   

2H+?(aq) + 2e-?? H2?(g)

• • An inert platinum electrode that is in contact with the hydrogen gas and H+ ions
• When the standard hydrogen electrode is connected to another half-cell, the?standard electrode potential?of that half-cell can be read off a high resistance voltmeter

The standard electrode potential of a half-cell can be determined by connecting it to a standard hydrogen electrode