Redox reactions

Oxidation numbers

A system used to assign numbers to keep track of electron transfers in molecules, ions and atoms.

  • The oxidation number of a free element is always 0.
  • The oxidation number of an ion is always equal to the charge of the ion
  • The ON of hydrogen is always +1 except when acting as a hydride ion (-1) in compounds containing elements that are less electronegative than hydrogen
  • The ON of oxygen is always 2 except when combined with elements more E/N (e.g. OF2 = +2) or in peroxides (-1)
  • The ON of Group 1 and 2 elements in a compound are always 1 and 2
  • The ON of Group 17 are nearly always -1
  • The sum of ONs in a neutral compound is always 0
  • The sum of ONs in a polyatomic ion is always equal to the charge of the ion

In order of precedence,

  1. A substance in elemental form has an ON of 0
  2. The ON of an monoatomic ion is the charge on that ion
  3. Hydrogen has an ON of +1 unless Rule 2 requires it to be -1
  4. Oxygen has an ON of -2 unless Rules 2 and 3 dictate otherwise
  5. The sum of ONs of atoms in a neutral compound 0, in a polyatomic ion = charge

Balancing redox equations

  1. Find the unbalanced half reaction
  2. Add the two half equations, using suitable coefficients to cancel out the electrons
  3. Check that RHS = LHS in terms of atoms, charge, mass
For each half reaction,
  1. Assign oxidation numbers
  2. Balance the atoms/ions involved in the oxidation/reduction
  3. Balance excess O by adding H2O
  4. Balance excess H by adding H+
  5. For reactions in an alkaline solution, neutralise H+ by adding OH- to both sides
  6. Balance the charges by adding electrons


Oxidation and reduction involves the transfer of electrons between species

An Oil Rig Cat

  • Oxidation is loss of electrons
  • Reduction is gain of electrons
  • An oxidant (oxidising agent) oxidises the other species, and it itself is reduced
  • A reductant (reducing agent) reduces the other species, and it itself is oxidised

From the data sheet, on the back page:

  • Fluorine gas at the top is the strongest oxidant, as it gains electrons to form fluorine ions
  • Conversely, K+ at the bottom is the weakest oxidant
  • Solid potassium is the strongest reductant, whilst fluorine ions are the weakest.









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