Concept of oxidation number, balancing redox reactions

1. Introduction to Redox Reactions

1. Redox reactions involve both oxidation (loss of electrons) and reduction (gain of electrons).
2. The term oxidation originally referred to the addition of oxygen, while reduction referred to the removal of oxygen.
3. Modern definitions are based on the transfer of electrons.

2. Concept of Oxidation Number

1. The oxidation number is the hypothetical charge an atom would have if all bonds were ionic.
2. Rules for determining oxidation numbers:
   • The oxidation number of a free element is always zero (e.g., O₂, N₂).
   • For ions, the oxidation number equals the charge (e.g., Na⁺: +1, Cl^-: -1).
   • Hydrogen usually has an oxidation number of +1, except in metal hydrides (e.g., NaH, where it is -1).
   • Oxygen usually has an oxidation number of -2, except in peroxides (-1) and superoxides (-1/2).
   • The sum of oxidation numbers in a neutral compound is zero, and in a polyatomic ion, it equals the ion’s charge.
3. Changes in oxidation number indicate oxidation (increase) or reduction (decrease).

3. Balancing Redox Reactions

1. Redox reactions can be balanced using the ion-electron method or the oxidation number method.
2. Steps for the oxidation number method:
   • Identify the elements undergoing oxidation and reduction.
   • Determine the changes in their oxidation numbers.
   • Balance the changes in oxidation numbers by adjusting coefficients.
   • Balance other elements and the overall charge.
3. Steps for the ion-electron method:
   • Split the reaction into two half-reactions: oxidation and reduction.
   • Balance atoms other than oxygen and hydrogen.
   • Balance oxygen atoms by adding H₂O.
   • Balance hydrogen atoms by adding H⁺.
   • Balance charges by adding electrons.
   • Combine the half-reactions, ensuring electrons cancel out.

4. Importance of Redox Reactions

1. Redox reactions are essential in biological processes, such as cellular respiration and photosynthesis.
2. They are the basis for electrochemical cells, including batteries and fuel cells.
3. Used in industrial processes, such as metal extraction and corrosion prevention.
4. Important in environmental chemistry, such as the oxidation of pollutants.

5. Key Points

1. Oxidation: Loss of electrons, increase in oxidation number.
2. Reduction: Gain of electrons, decrease in oxidation number.
3. Oxidation and reduction occur simultaneously in redox reactions.
4. Oxidation number is a useful tool for tracking electron transfer in reactions.
5. Balancing redox reactions is crucial for accurate chemical equations.
6. Redox processes are fundamental to energy production and industrial applications.