Introduction to REDOX reactions and oxidation numbers

Redox (reduction-oxidation) reactions are chemical reactions that involve the transfer of electrons between species. These reactions are fundamental in chemistry, occurring in various natural and artificial processes, including corrosion, combustion, photosynthesis, and cellular respiration.
The key players in redox reactions are:
Oxidation: The process in which a substance loses electrons. It leads to an increase in the oxidation state of an element. For example:
Zn→Zn2++2e−
Zn→Zn 2+ +2e −
Here, zinc (Zn) is oxidized to zinc ions (Zn²⁺) by losing two electrons.
Reduction: The process in which a substance gains electrons. It results in a decrease in the oxidation state of an element. For example:
Cu2++2e−→CuCu 2+ +2e −
→Cu
Copper ions (Cu²⁺) gain two electrons to form neutral copper (Cu) atoms.
Oxidation and reduction always occur together in a redox reaction. The substance being oxidized is called the reducing agent because it causes the reduction of another substance by donating electrons. Conversely, the substance being reduced is called the oxidizing agent because it causes the oxidation of another substance by accepting electrons.
Oxidation Numbers:
Oxidation numbers (or oxidation states) are hypothetical charges assigned to atoms in a compound or ion to keep track of electron transfer in chemical reactions. These numbers help identify which element is oxidized or reduced during a redox process.
Here are some rules to determine oxidation numbers:
Free elements: In their natural elemental state, elements have an oxidation number of zero. For example, oxygen (O₂), hydrogen (H₂), and metals like iron (Fe) have an oxidation number of 0.
Monoatomic ions: The oxidation number of a monatomic ion is equal to its charge. For example, sodium ion (Na⁺) has an oxidation number of +1, while chloride ion (Cl⁻) has an oxidation number of -1.
Oxygen: In most compounds, oxygen has an oxidation number of -2. However, there are exceptions, like in peroxides (e.g., H₂O₂), where oxygen has an oxidation number of -1.
Hydrogen: In most compounds, hydrogen has an oxidation number of +1. However, in metal hydrides (e.g., NaH), hydrogen has an oxidation number of -1.
Sum of oxidation numbers: The sum of oxidation numbers in a neutral compound is zero, and in a polyatomic ion, it equals the ion's charge.
By identifying the changes in oxidation numbers for elements in a chemical reaction, you can determine the electrons' transfer, which defines the redox process.
Understanding redox reactions and oxidation numbers is crucial in various fields of chemistry, including analytical chemistry, environmental science, biochemistry, and industrial processes.