Fix the mistake: Redox reactions

Definitions of Oxidation and Reduction and Transfer of Electrons

What does it mean for something to be “oxidized” or “reduced”? We will learn the classical definitions and the modern definitions based on the transfer of electrons.

Basic Knowledge

Historically, oxidation and reduction have been defined as follows.
① Exchange of oxygen: Bonding with oxygen → oxidation, losing oxygen → reduction
② Exchange of hydrogen: Losing hydrogen → oxidation, bonding with hydrogen → reduction
③ Exchange of electrons (most general): Losing electrons → oxidation, receiving electrons → reduction
Example: In 2Mg + O2 → 2MgO, Mg loses electrons (oxidation), and O2 receives electrons (reduction).
Oxidation and reduction always occur simultaneously. This is called a redox reaction.

Important Terms
Oxidation（A change involving the loss of electrons, bonding with oxygen, or loss of hydrogen）
Reduction（A change involving the receipt of electrons, loss of oxygen, or bonding with hydrogen）
Redox reaction（A reaction in which oxidation and reduction occur simultaneously）
Oxidizing agent（Oxidizes the other substance = receives electrons and is itself reduced）
Reducing agent（Reduces the other substance = loses electrons and is itself oxidized）
Electron e-（The key participant in redox reactions. The amount transferred determines the reaction amount）

Further Study (Background・Significance)

Originally, “oxidation” literally referred to bonding with oxygen. However, people also wanted to explain the exchange of hydrogen and reactions that contain neither oxygen nor hydrogen (for example, 2Na + Cl2 → 2NaCl) using the same mechanism. Therefore, the most general definition, “transfer of electrons,” was adopted.
This makes it possible to understand ionic reactions (for example, Cu2+ + Zn → Cu + Zn2+), batteries, and electrolysis uniformly as redox processes.
The relationship “an oxidizing agent is reduced, and a reducing agent is oxidized” is easy to confuse, but it is useful to remember that “the action and the change undergone by the substance itself are opposite.”

Key Points
  • Oxidation = losing electrons
  • Reduction = receiving electrons
  • Oxidation and reduction always occur simultaneously
  • Oxidizing agent = oxidizes the other substance and is itself reduced
  • Reducing agent = reduces the other substance and is itself oxidized
  • In “Mg + 1/2 O2 → MgO,” Mg is oxidized and O2 is reduced
  • The essence of electron transfer is the two opposite sides of the same reaction

Points to Note (Easily Confused)

① An oxidizing agent is reduced, and a reducing agent is oxidized. Their actions and changes are opposite. ② “Bonding with oxygen = oxidation” is an old definition that is still valid, but electron transfer is the broadest definition. ③ Both always occur simultaneously. A reaction in which only one occurs does not exist. ④ Combustion, corrosion, respiration, and photosynthesis are all examples of redox reactions.

Practice

  1. State the definitions of “oxidation” and “reduction” based on the transfer of electrons.
  2. In 2Mg + O2 → 2MgO, which substance was oxidized?
  3. Explain what kind of substance an “oxidizing agent” is.

Chemistry (High School Basic): related lessons