Redox Reactions

Objective: Identify oxidation and reduction in redox reactions

What are Redox Reactions?

Redox reactions, short for reduction-oxidation reactions, are chemical reactions that involve the transfer of electrons between two species. These reactions always occur together; one species loses electrons (oxidation), and another species gains those electrons (reduction).

Key Terms

• Oxidation: The loss of electrons by a substance. Oxidation always results in an increase in oxidation state.
• Reduction: The gain of electrons by a substance. Reduction always results in a decrease in oxidation state.
• Reducing Agent: The substance that is oxidized and donates electrons.
• Oxidising Agent: The substance that is reduced and accepts electrons.

Oxidation States

Oxidation state (also known as oxidation number) is a way of keeping track of the electrons in an atom during a chemical reaction. It's a hypothetical charge an atom would have if all bonds were completely ionic. Rules for assigning oxidation states are important.

Here's a table summarizing the rules for assigning oxidation states:

Rule	Description
1. Elements in their elemental form	Oxidation state = 0 (e.g., Na(s), O2(g), Cu(s))
2. Monatomic ions	Oxidation state = Charge of the ion (e.g., Na+ = +1, Cl- = -1)
3. Fluorine	Oxidation state = -1 (in all compounds)
4. Oxygen	Usually -2. Exceptions: Peroxides (-1), in OF2 (+2)
5. Hydrogen	Usually +1. Exceptions: In hydrides (e.g., NaH, LiH) oxidation state is -1
6. Metals (in ionic compounds)	Oxidation state = Positive charge of the ion
7. Non-metals (in ionic compounds)	Oxidation state = Negative charge of the ion
8. Polyatomic ions	Sum of oxidation states of all atoms in the ion equals the charge of the ion.

Identifying Oxidation and Reduction

To identify oxidation and reduction in a redox reaction, you need to look at the change in oxidation states of the atoms involved.

Oxidation: An increase in oxidation state indicates oxidation. The species that undergoes oxidation is the reducing agent.

Reduction: A decrease in oxidation state indicates reduction. The species that undergoes reduction is the oxidising agent.

Example Reaction

$Zn(s) + Cu²⁺(aq) \rightarrow Zn²⁺(aq) + Cu(s)$

Let's determine the oxidation states of each element:

• Zn(s): Oxidation state = 0
• Cu²⁺(aq): Oxidation state = +2
• Zn²⁺(aq): Oxidation state = +2
• Cu(s): Oxidation state = 0

In this reaction:

• Zinc (Zn) goes from oxidation state 0 to +2. This is an oxidation (loss of electrons). Zinc is the reducing agent.
• Copper(II) ion (Cu²⁺) goes from oxidation state +2 to 0. This is a reduction (gain of electrons). Copper(II) is the oxidising agent.

Balancing Redox Reactions

Redox reactions are often balanced using the half-electron method. This involves separately balancing the oxidation and reduction half-reactions, and then combining them.