4.1.4 Oxidation and Reduction in terms of Electrons (HT only)

In this lesson, we will explore the definitions of oxidation and reduction and learn how to write ionic equations for displacementThe forced removal of people from their land. reactions.

Oxidation and Reduction

Oxidation is defined as the loss of electrons from an atom, ion, or molecule. It involves an increase in the oxidation state of the species. Reduction, on the other hand, is the gain of electrons by an atom, ion, or molecule. It leads to a decrease in the oxidation state of the species.

Electron Transfer and Redox Reactions

Oxidation and reduction always occur together in a process known as a redox (reduction-oxidation) reaction. In a redox reaction, one species loses electrons (oxidation) while another species gains those electrons (reduction). This transfer of electrons allows for the balancing of charges and the formation of new chemical compounds.

Writing Ionic Equations for Displacement Reactions

Displacement reactions are common examples of redox reactions. In these reactions, one element displaces another from a compound. To write the ionic equation for a displacement reaction, follow these steps:

1. Write the balanced chemical equation for the reaction.
2. Identify the oxidation states of each species involved.
3. Write the half equations for oxidation and reduction by showing the transfer of electrons.
4. Balance the number of electrons transferred in each half equation.
5. Combine the half equations to form the overall ionic equation.

Here’s an example involving the reaction of zinc with copper sulfate to form zinc sulfate and copper:

• Write the balanced chemical equation for the reaction:
  • Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
• Identify the oxidation states of each species involved in the reaction:
  • Zinc (Zn) is a solid element, so its oxidation state is 0.
  • Copper (Cu) is also a solid element, so its oxidation state is 0.
  • In copper sulfate (CuSO₄), copper (Cu) has an oxidation state of +2, sulfur (S) has an oxidation state of +6, and oxygen (O) has an oxidation state of -2.
• Write the half equations for oxidation and reduction:
  • Oxidation (loss of electrons): Zn(s) → Zn²⁺(aq) + 2e⁻
  • Reduction (gain of electrons): Cu²⁺(aq) + 2e⁻ → Cu(s)
• Balance the number of electrons transferred in each half equation. In this case, both half equations already have the same number of electrons (2e⁻), so there's no need for additional balancing.
• Combine the half equations to form the overall ionic equation. Make sure that the number of electrons lost in the oxidation half equation equals the number gained in the reduction half equation:
  • Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)
  • This is the ionic equation for the displacement reaction between zinc and copper sulfate.

The equation shows the transfer of electrons from zinc to copper ions, resulting in the formation of zinc ions and solid copper. It highlights the redox nature of the reaction, where zinc is oxidised (loses electrons) to form zinc ions, and copper ions are reduced (gain electrons) to form solid copper.

Identifying Oxidation and Reduction

To identify which species are oxidised and reduced in a given reaction, symbol equation, or half equation, follow these guidelines:

• Compare the oxidation states of each species before and after the reaction.
• The species that undergoes an increase in oxidation state is oxidised.
• The species that undergoes a decrease in oxidation state is reduced.

Conclusion

Oxidation and reduction are fundamental processes in chemistry that involve the transfer of electrons. Oxidation is the loss of electrons, while reduction is the gain of electrons. Redox reactions occur when there is a transfer of electrons between species.