Standard Electrode Potential (Edexcel International A Level Chemistry): Revision Note

Standard Electrode Potential Definition

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^ꝋ)

• The standard electrode potential (E^ꝋ) 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^ꝋ value

Br₂(l) + 2e^– ⇌ 2Br^–(aq)        E^ꝋ = +1.09 V          

2H⁺(aq) + 2e^– ⇌ H₂(g)        E^ꝋ = 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^ꝋ value

Na⁺ (aq) + e^– ⇌ Na(s)        E^ꝋ = -2.71 V

2H⁺ (aq) + 2e^– ⇌ H₂(g)        E^ꝋ = 0.00 V

The Standard Hydrogen Electrode, SHE

• 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

  • The inert platinum electrode often has a coating of finely divided platinum black

  • The increased surface area of the platinum black increases the rate of equilibrium between the hydrogen gas and hydrogen 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