7.1.3 Oxidation and Reduction in Terms of Transfer of Electrons

Oxidation is the loss of electrons. Reduction is the gain of electrons.

Simple explanation: When an atom loses electrons, it is oxidized; when it gains electrons, it is reduced.

Example: Consider the reaction between calcium and oxygen:

2Ca + O₂ → 2CaO

Problem Solving Strategy: Ca being a metal forms a cation by losing electrons (oxidation), and oxygen being a non-metal gains electrons (reduction) to form an anion.

Solution:

Calcium loses electrons:

2Ca → 2Ca²⁺ + 4e^- (oxidation)

Oxygen gains electrons:

O₂ + 4e^- → 2O^2- (reduction)

Simple explanation: Calcium loses electrons (oxidized); oxygen gains electrons (reduced).

Example 7.2: Identifying the element oxidized or reduced

In the following reaction, identify which element is oxidized and which is reduced:

2Ca + O₂ → 2CaO

Problem Solving Strategy: Ca being a metal forms a cation by losing electrons (oxidation), and oxygen being a non-metal gains electrons (reduction) to form an anion.

Solution: Remember that Group IIA metals form M²⁺ cations, and Group VIA non-metals form X²⁻ anions.

• Each Ca atom loses two electrons to form Ca²⁺ → oxidized.
• Each oxygen atom gains two electrons to form O²⁻ → reduced.

2Ca → 2Ca²⁺ + 4e^- (oxidation)

O₂ + 4e^- → 2O^2- (reduction)

Concept Assessment Exercise 7.1 (from the book)

In the following reactions, identify which element is oxidized and which is reduced in terms of electron transfer:

1. 4Na + O₂ → 2Na₂O

• Na loses electrons → oxidized
• O gains electrons → reduced

2. 2Al + 3Cl₂ → 2AlCl₃

• Al loses electrons → oxidized
• Cl gains electrons → reduced

3. Mg + Cl₂ → MgCl₂

• Mg loses electrons → oxidized
• Cl gains electrons → reduced

Concept Assessment Exercise 7.2 (from the book)

1. N₂ + 3H₂ → 2NH₃

• N gains electrons → reduced
• H loses electrons → oxidized

2. 2H₂ + O₂ → 2H₂O

• H loses electrons → oxidized
• O gains electrons → reduced

3. Fe₂O₃ + 3CO → 2Fe + 3CO₂

• Fe gains electrons → reduced
• C loses electrons → oxidized

4. 4Al + 3O₂ → 2Al₂O₃

• Al loses electrons → oxidized
• O gains electrons → reduced

Intermediate Level

Oxidation and Reduction Always Occur Together

In a redox reaction, oxidation and reduction always occur together. One substance loses electrons (or hydrogen, or gains oxygen), and another gains electrons (or hydrogen, or loses oxygen).

Simple explanation: You can't have oxidation without reduction — they happen at the same time.

Electron Transfer in Redox Reactions

Oxidation: Loss of electrons → increase in positive charge (or decrease in negative charge)

Example: Na → Na⁺ + e⁻

Reduction: Gain of electrons → decrease in positive charge (or increase in negative charge)

Example: Cl + e⁻ → Cl⁻

Identifying Oxidized and Reduced Elements

• Write the balanced equation.
• Identify the oxidation state of each element on both sides.
• If oxidation state increases → oxidized.
• If oxidation state decreases → reduced.

Note: This is covered in more detail in Section 7.2 (Oxidation States).

Why Redox Reactions Are Important

• Combustion — burning fuels
• Corrosion — rusting of iron
• Respiration — energy production in cells
• Photosynthesis — plants making food
• Batteries — producing electricity
• Metallurgy — extracting metals from ores

Simple explanation: Redox reactions are everywhere — from your car engine to your own body.

Important Points

• - Redox reactions: simultaneous oxidation and reduction.
• - Oxidation (oxygen): gain of oxygen.
• - Reduction (oxygen): loss of oxygen.
• - Oxidation (hydrogen): loss of hydrogen.
• - Reduction (hydrogen): gain of hydrogen.
• - Oxidation (electrons): loss of electrons.
• - Reduction (electrons): gain of electrons.
• - Memory trick: OIL RIG (Oxidation Is Loss, Reduction Is Gain).
• - Examples:
• - 2Mg + O₂ → 2MgO (Mg oxidized, O reduced)
• - 2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O (C₂H₂ oxidized, O₂ reduced)
• - 2Ca + O₂ → 2CaO (Ca oxidized, O reduced)
• - N₂ + 3H₂ → 2NH₃ (H₂ oxidized, N₂ reduced)
• - Oxidation and reduction always occur together.

Remember

• - Redox = oxidation + reduction.
• - OIL RIG: Oxidation Is Loss, Reduction Is Gain (electrons).
• - Oxygen: oxidation = gain; reduction = loss.
• - Hydrogen: oxidation = loss; reduction = gain.
• - Electrons: oxidation = loss; reduction = gain.
• - Ca → Ca²⁺ + 2e⁻ (oxidation).
• - O₂ + 4e⁻ → 2O²⁻ (reduction).
• - Always occur together.

Common Mistakes

• 1. Thinking oxidation and reduction can occur separately. They always occur together in a redox reaction.
• 2. Confusing "loss" and "gain" for oxygen and hydrogen.
• - Oxygen: oxidation = gain of oxygen.
• - Hydrogen: oxidation = loss of hydrogen.
• 3. Forgetting OIL RIG. Oxidation Is Loss, Reduction Is Gain (of electrons).
• 4. Thinking oxidation always involves oxygen. In modern chemistry, oxidation is defined as loss of electrons, not necessarily gain of oxygen.
• 5. Thinking reduction always involves hydrogen. Reduction can also be defined as gain of electrons.
• 6. Forgetting the electron transfer. In redox reactions, electrons are transferred from the oxidized substance to the reduced substance.

Quick Revision

• Redox reactions: simultaneous oxidation and reduction.
• Oxidation can be defined as:
• Gain of oxygen
• Loss of hydrogen
• Loss of electrons
• Reduction can be defined as:
• Loss of oxygen
• Gain of hydrogen
• Gain of electrons
• Memory trick: OIL RIG — Oxidation Is Loss, Reduction Is Gain.
• Examples:
• 2Mg + O₂ → 2MgO (Mg oxidized, O reduced)
• 2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O (C₂H₂ oxidized, O₂ reduced)
• 2Ca + O₂ → 2CaO (Ca oxidized, O reduced)
• N₂ + 3H₂ → 2NH₃ (H₂ oxidized, N₂ reduced)
• Oxidation and reduction always occur together.

60-Second Revision

• Redox = oxidation + reduction.
• Oxidation: gain of O, loss of H, loss of e⁻.
• Reduction: loss of O, gain of H, gain of e⁻.
• OIL RIG — Oxidation Is Loss, Reduction Is Gain.
• Ca → Ca²⁺ + 2e⁻ (oxidation).
• O₂ + 4e⁻ → 2O²⁻ (reduction).
• Warning: oxidation and reduction always occur together.