Redox Reactions

Defining Redox Reactions

Redox Reactions Through Oxygen

• Redox reactions involve two parts.
  • An oxidation reaction.
    • This involves the gain of oxygen by an element or compound.
    • When something has oxidised, it has gained oxygen.
  • A reduction reaction.
    • This involves the loss of oxygen from an element or compound.
    • When something has been reduced, it has lost oxygen.
• As an example of this, for magnesium to combust, it must be burnt in oxygen.

Magnesium Combustion Reaction

$2Mg(s)+O_2(g)\to 2MgO(s)$

• In this case, magnesium has oxidised.
• The oxygen molecule has also lost an oxygen atom which means it has been reduced.

Redox Reactions and Electrons

• Redox reactions can also involve the movement of electrons between different chemical species.
• This goes through the same concepts of oxidation and reduction reactions.
  • For instance, oxidation reactions can be defined as the chemical species losing electrons.
  • Reduction reactions can be defined as the chemical species gaining electrons.
  • To remember these reactions, you can use an acronym like OILRIG.
    • “OILRIG” stands for “Oxidation is Loss, Reduction is Gain”
• Remember, electrons cannot be created nor destroyed. When a chemical species loses electrons (oxidising), one must take the lost electrons (reduction). Thus, there is no net gain or loss in a redox reaction.

Magnesium Combustion Reaction

$2Mg(s)+O_2\to 2M{g}^{2+}+2O^{2-}$

• $M{g}^{2+}+M{g}^{2+}\to O^{2-}+O^{2-}$ (ionic explanation)
  • In this scenario, magnesium loses electrons and therefore, oxidises.
  • As magnesium loses electrons, oxygen gains these electrons and reduces.

Redox Reactions Through Hydrogen

• Redox reactions can also involve the transfer of hydrogen atoms.
• This goes through the same concepts of oxidation and reduction reactions.
  • Oxidation is the loss of hydrogen.
  • Reduction is the gain of hydrogen.

Recap

• Oxidation involves a species gaining oxygen, losing hydrogen OR losing electrons.
• Reduction involves a species losing oxygen, gaining hydrogen OR gaining electrons.

Terminology

• See Recap for the definitions of oxidation and reduction.
• A reducing agent is a species that is being oxidised or making something else be reduced.
• An oxidising agent is a species that is being reduced or making something else to be oxidised.
• Using our magnesium combustion reaction, we can say that…
  • Magnesium is our reduction agent, as it is the chemical species being oxidised or making something else be reduced (lose electrons).
  • Oxygen is our oxidising agent, as it is the chemical species being reduced or making something else be oxidised (gain electrons).

Summary

• Redox reactions involve a reduction reaction and an oxidation reaction.
  • See Terminology for more.
  • See Defining Redox Reactions for more.