Electrode potentials

Revision of oxidation states

  • The oxidation state of uncombined elements is always zero

  • For a monatomic ion, the oxidation state is its charge

  • The sum of all the oxidation states in a neutral compound is 0

  • The sum of all the oxidation states in a polyatomic ion is the charge on the ion

  • The oxidation state of hydrogen is +1 except when bonded to a metal (metal hydride) when it is -1

  • Group 1 elements are always +1

  • Group 2 elements are always +2

  • Aluminium in a compound is always +3

  • Fluorine is always -1 in a compound

  • Oxygen is a compound is -2, except when it is in a peroxide or bonded to fluorine

  • Chlorine is -1 unless it is in a compound with a more electronegative element such as F or O

Redox reactions

  • Any redox reaction is a combination of two half equations

  • Consider the two ionic equations

    • Cu2+(aq) + H2(g) → Cu(s) + 2H+

      • Cu2+(aq) + 2e- → Cu(s)

      • H2(g) → 2H+ + 2e-

    • 2H+(aq) + Fe(s) → Fe2+(aq) + H2(g)

      • 2H+(aq) + 2e- → H2(g)

      • Fe(s) → Fe2+(aq) + 2e-

  • The half equation for the hydrogen species is the same but in reverse

  • The half equation can be written as a reverse reaction

    • 2H+(aq) + 2e- ⇌ H2(g)

  • The reversible reaction arrow indicates that either reduction or oxidation can take place

  • Whether oxidation or reduction takes place depends on the other half equation being used

  • Half equations are listed with reductions in the forward direction

Standard electrode potential E0

  • The standard electrode potential is the EMF of a half-cell connected to a standard hydrogen half-cell, under standard conditions of 298K, solution concentration of 1.00 moldm-1 , and a pressure of 100kPa

  • EMF = electromotive force i.e. the potential difference or voltage between 2 half cells

  • A half-cell contains the chemical species present in a redox half-equation

  • Electricity flows when two half cells are connected together

Electrode potentials

  • It is not possible to measure the voltage for an individual half-cell

  • It is only possible to measure the difference in voltage between two half-cells

  • To do this one half-cell is used as a standard. This, by definition, has an electrode potential of 0.00V

  • This is the standard hydrogen electrode

  • All other half-cells electrode potentials are measured compared to this half-cell