Use of Oxidation Number

Use of Oxidation Number

Understanding Oxidation Numbers

  • The concept of an oxidation number or oxidation state helps us understand how electrons are transferred in redox reactions.
  • In simple terms, the oxidation number of an atom in a molecule or ion is the charge it would have if all the bonding electrons were assigned to the most electronegative atom.
  • Oxidation numbers can be positive, negative, or zero, and they are shown with a plus or minus sign.

Assigning Oxidation Numbers

  • In uncombined elements, regardless of whether they exist as monatomic or polyatomic species, the oxidation number is always zero.
  • For simple ions, the oxidation number is equal to the ion’s charge.
  • Hydrogen’s oxidation number is usually +1, except in metal hydrides such as NaH where it is -1.
  • Oxygen’s oxidation number is usually -2, but there are exceptions like in peroxide compounds (H2O2, Na2O2), where it is -1.
  • Halogens usually have an oxidation number of -1, except when they are more electronegative than other atoms in a molecule.

Balancing Redox Reactions by Oxidation Numbers

  • Oxidation and reduction reactions can be balanced using oxidation numbers, as both must occur simultaneously in a redox reaction.
  • Reduction is a decrease in oxidation number, while oxidation is an increase in oxidation number.
  • In a balanced chemical equation, the total increase in oxidation numbers of the oxidised species must equal the total decrease in oxidation numbers of the reduced species.

Applying Oxidation Numbers

  • Oxidation numbers are instrumental in determining the direction of redox reactions and in the electrochemical series.
  • They help identify oxidising and reducing agents, the substances that lose or gain electrons in reactions, respectively.
  • An oxidising agent causes other substances to be oxidised by being reduced itself, whereas a reducing agent causes other substances to be reduced by being oxidised itself.