Chemical Reactions - Redox

This section explores redox reactions, a fundamental type of chemical reaction involving the transfer of electrons between species.

Oxidation and Reduction

In a redox reaction, one species loses electrons (oxidation) while another species gains electrons (reduction). These two processes always occur together.

Oxidation

Oxidation is the loss of electrons. It can be remembered using the mnemonic OIL RIG: Oxidation Is Loss of electrons.

The species that undergoes oxidation is called the reducing agent because it causes another species to be reduced.

In a redox reaction, the oxidation state of the species that is oxidized increases.

Reduction

Reduction is the gain of electrons. It can be remembered using the mnemonic OIL RIG: Oxidation Is Loss of electrons.

The species that undergoes reduction is called the oxidizing agent because it causes another species to be oxidized.

In a redox reaction, the oxidation state of the species that is reduced decreases.

Identifying Oxidation and Reduction

To identify oxidation and reduction in a chemical reaction, we need to look at the change in oxidation states of the elements involved.

The oxidation state is a way of assigning a charge to an atom in a molecule or ion. It is based on a set of rules.

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

Rule	Description
1	The oxidation state of an atom in its elemental form is 0.
2	The oxidation state of a monatomic ion is equal to its charge.
3	The oxidation state of oxygen is usually -2.	Exceptions: In peroxides (e.g., $H_2O_2$), oxygen is -1. In compounds with fluorine (e.g., $OF_2$), oxygen is positive.
4	The oxidation state of hydrogen is usually +1.	Exception: In hydrides (e.g., $NaH$), hydrogen is -1.
5	The oxidation state of metals is usually positive.	The charge of the metal ion is equal to its oxidation state.
6	The sum of the oxidation states in a neutral compound is 0.	The sum of the oxidation states in a polyatomic ion is equal to the charge of the ion.

Example Redox Reactions

1. $2Mg(s) + O_2(g) \rightarrow 2MgO(s)$

   Magnesium (Mg) is oxidized (loses electrons) because its oxidation state increases from 0 to +2. It is the reducing agent.

   Oxygen (O₂) is reduced (gains electrons) because its oxidation state decreases from 0 to -2. It is the oxidizing agent.

2. $Zn(s) + Cu^2+(aq) \rightarrow Zn^2+(aq) + Cu(s)$

   Zinc (Zn) is oxidized (loses electrons) because its oxidation state increases from 0 to +2. It is the reducing agent.

   Copper(II) ion ($Cu^{2+}$) is reduced (gains electrons) because its oxidation state decreases from +2 to 0. It is the oxidizing agent.

Oxidation Number Changes

|Oxidation Number Change| | Process |

Oxidation Number Change	Process
Increase	Oxidation
Decrease	Reduction

Key Terms

• Oxidation: Loss of electrons.
• Reduction: Gain of electrons.
• Reducing Agent: Species that causes reduction (gets oxidized).
• Oxidizing Agent: Species that causes oxidation (gets reduced).
• Oxidation State: A hypothetical charge assigned to an atom in a molecule or ion.