Electrode potential

• The?electrode (reduction) potential?(E) is a value which shows how easily a substance is?reduced
• These are demonstrated using reversible half equations
  • This is because there is a redox equilibrium between two related species that are in different oxidation states
  • For example, if you dipped a zinc metal rod into a solution which contained zinc ions, there would be zinc atoms losing electrons to form zinc ions and at the same time, zinc ions gaining electrons to become zinc atoms
  • This would cause a redox equilibrium
• When writing half equations for this topic, the electrons will always be written on the left-hand side (demonstrating reduction)
• The position of equilibrium is different for different species, which is why different species will have electrode (reduction) potentials
• The more?positive?(or less negative) an electrode potential, the?more likely?it is for that species to undergo?reduction
  • The equilibrium position lies more to the?right
• For example, the positive electrode potential of bromine below, suggests that it is likely to get reduced and form bromide (Br^-) ions

Br₂(l) + 2e^-?? 2Br^-(aq)? ? ? ? voltage = +1.09 V

• The more?negative?(or less positive) the electrode potential, the less likely it is that reduction of that species will occur
  • The equilibrium position lies more to the?left
• For example, the negative electrode potential of sodium suggests that it is unlikely that the sodium (Na⁺) ions will be reduced to sodium (Na) atoms

Na⁺(aq) + e^-?? Na(s)? ? ? ? voltage = -2.71 V

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 1 atm

• The electrode potentials are measured relative to something called 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 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

Standard cell potential

• Once the?E^?of a half-cell is known, the?voltage?of an?electrochemical cell?made up of two half-cells can be calculated
  • These could be?any?half-cells and neither have to be a standard hydrogen electrode
• This is also known as the?standard cell potential (E_cell^?)
  • The standard cell potential is the difference in?E^??between two half-cells
  • For example, an electrochemical cell consisting of bromine and sodium half-cells has an?E_cell^??of:

E_cell^??= (+1.09) - (-2.71)

= +3.80 V

• When a metal rod is placed in an aqueous solution, a?redox equilibrium?is established between the metal ions and atoms
  • For example, the copper atoms get?oxidised?and enter the solution as copper ions

Cu(s) → Cu²⁺(aq) + 2e^-

Oxidation of copper ions

• • The copper ions gain electrons from the metal rod and deposit as metal atoms on the rod

Cu²⁺(aq) + 2e^-?→ Cu(s)

Reduction of copper ions

• • When equilibrium is established, the?rate?of oxidation and reduction of copper is?equal

• The position of the redox equilibrium is different for different metals
  • Copper is more easily?reduced, thus the equilibrium lies further over to the?right

Cu²⁺?(aq) + 2e^-?? Cu (s)

• • Vanadium is more easily?oxidised, thus the equilibrium lies further over to the?left

V²⁺?(aq) + 2e^-?? V(s)

• The metal atoms and ions in solution cause an?electric potential?(voltage)
• This potential cannot be measured?directly?however the potential between the metal/metal ion system and?another?system?can?be measured
• This value is called the?electrode potential?(E) and is measured in?volts
  • The electrode potential is the?voltage?measured for a half-cell compared to another half-cell
  • Often, the half-cell used for comparison is the?standard?hydrogen?electrode

Standard hydrogen electrode

• The?standard hydrogen electrode?is a half-cell used as?reference electrodes?and consists of:
  • Hydrogen gas in equilibrium with H⁺?ions of concentration 1.00 mol dm^-3?(at 1 atm)

2H⁺?(aq) + 2e^-?? H_2?(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 voltmeter

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