Oxidation and Reduction Introduction

Oxidation And Reduction

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Oxidation and Reduction (Redox): Oxidation and reduction are fundamental types of chemical reactions involving the transfer of electrons between chemical species. These processes are crucial in many chemical and biological systems, including combustion, corrosion, respiration, and electrochemistry.

Oxidation And Reduction

There are several ways to define oxidation and reduction:

1. Based on Oxygen Gain/Loss (Historical Definition):

• Oxidation: Gain of oxygen or loss of hydrogen.
• Reduction: Loss of oxygen or gain of hydrogen.

Example: The reaction of copper oxide with hydrogen:

$$CuO(s) + H_2(g) \rightarrow Cu(s) + H_2O(g)$$

• Copper oxide ($CuO$) loses oxygen, so it is reduced to copper ($Cu$).
• Hydrogen ($H_2$) gains oxygen, so it is oxidized to water ($H_2O$).

2. Based on Electron Transfer (Modern and Most Common Definition):

• Oxidation: Loss of electrons. A species that loses electrons is said to be oxidized.
• Reduction: Gain of electrons. A species that gains electrons is said to be reduced.

Oxidizing Agent (Oxidant): An oxidizing agent is a substance that causes oxidation in another substance by accepting its electrons. The oxidizing agent itself gets reduced in the process.

Reducing Agent (Reductant): A reducing agent is a substance that causes reduction in another substance by donating its electrons. The reducing agent itself gets oxidized in the process.

Example: The reaction between sodium and chlorine:

$$2Na(s) + Cl_2(g) \rightarrow 2NaCl(s)$$

• Sodium ($Na$) loses an electron to become $Na^+$ ion. So, Sodium is oxidized. It acts as the reducing agent.
• Chlorine ($Cl_2$) gains electrons to become chloride ions ($Cl^-$). So, Chlorine is reduced. It acts as the oxidizing agent.

The overall reaction can be written as two half-reactions:

• Oxidation half-reaction: $2Na \rightarrow 2Na^+ + 2e^-$
• Reduction half-reaction: $Cl_2 + 2e^- \rightarrow 2Cl^-$

3. Based on Change in Oxidation State:

• Oxidation: An increase in the oxidation state of an atom.
• Reduction: A decrease in the oxidation state of an atom.

Oxidation State: The oxidation state (or oxidation number) is a hypothetical charge that an atom would have if all bonds to atoms of different elements were $100\%$ ionic. It's a bookkeeping tool to track electron transfer.

Example (using the Na and Cl reaction):

$$2Na(s) + Cl_2(g) \rightarrow 2NaCl(s)$$

• Initial oxidation state of Na: 0 (elemental form)
• Final oxidation state of Na: +1 (in $Na^+$)
• Change in oxidation state for Na: 0 to +1 (increase). Na is oxidized.
• Initial oxidation state of Cl: 0 (elemental form)
• Final oxidation state of Cl: -1 (in $Cl^-$)
• Change in oxidation state for Cl: 0 to -1 (decrease). Cl is reduced.

Key Principle: Oxidation and reduction always occur simultaneously in a redox reaction. One cannot happen without the other.