Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 8.5 Oxidation and reduction-Study Notes - New Syllabus

8.5 Oxidation and Reduction 

Oxidation and reduction (redox) reactions involve electron transfer and corresponding changes in oxidation number. These concepts are essential for understanding reactions of s-block and p-block elements.

Definitions (Electron Transfer)

• Oxidation: loss of electrons
• Reduction: gain of electrons

Remember: OIL RIG (Oxidation Is Loss, Reduction Is Gain)

Definitions (Oxidation Number)

• Oxidation: increase in oxidation number
• Reduction: decrease in oxidation number

• Oxidation and reduction always occur together.
• Electrons lost = electrons gained.

Example 1: Reaction of Sodium with Chlorine (s-block)

\( \mathrm{2Na + Cl_2 \rightarrow 2NaCl} \)

• Na: \( \mathrm{0 \rightarrow +1} \) → oxidation (loses 1 e⁻)
• Cl: \( \mathrm{0 \rightarrow -1} \) → reduction (gains 1 e⁻)

Half-equations

• \( \mathrm{Na \rightarrow Na^+ + e^-} \)
• \( \mathrm{Cl_2 + 2e^- \rightarrow 2Cl^-} \)

Therefore, sodium is oxidised and chlorine is reduced.

Example 2: Reaction of Magnesium with Oxygen (s-block)

\( \mathrm{2Mg + O_2 \rightarrow 2MgO} \)

• Mg: \( \mathrm{0 \rightarrow +2} \) → oxidation
• O: \( \mathrm{0 \rightarrow -2} \) → reduction

Example 3: Reaction of Chlorine with Water (p-block)

\( \mathrm{Cl_2 + H_2O \rightarrow HCl + HClO} \)

• Cl in \( \mathrm{Cl_2} \) = 0
• Cl in \( \mathrm{HCl} \) = −1 (reduction)
• Cl in \( \mathrm{HClO} \) = +1 (oxidation)

This is a disproportionation reaction (same element oxidised and reduced).

Application to s-block Elements

• s-block metals (Group 1 and 2) are easily oxidised.
• They lose electrons to form positive ions.
• Therefore, they are strong reducing agents.

Example: \( \mathrm{Na \rightarrow Na^+ + e^-} \)

Application to p-block Elements

• p-block elements show variable oxidation states.
• Can act as oxidising or reducing agents.
• Can undergo disproportionation reactions.

Key Points

• Oxidation = loss of electrons / increase in oxidation number.
• Reduction = gain of electrons / decrease in oxidation number.
• Redox reactions involve both processes simultaneously.
• s-block elements are easily oxidised.
• p-block elements show more varied redox behaviour.

Therefore, redox concepts explain the reactivity and behaviour of elements across the periodic table.