Question

What is the change in oxidation number for the following reaction? $$ 3 \mathrm{O}_{2} \longrightarrow 2 \mathrm{O}_{3} $$

Short answer

No change in oxidation number for oxygen atoms.

Step-by-step solution

Determine the oxidation number of oxygen in each molecule

In \(\mathrm{O}_2\), oxygen is in its elemental form, so its oxidation number is \(0\). In ozone \(\mathrm{O}_3\), the average oxidation number of oxygen atoms is still \(0\) because the molecule is made only of oxygen atoms.

Consider the change in structure

Although there is no numerical change in oxidation number when considering the whole molecule, the transformation involves breaking the \(\mathrm{O}_2\) double bonds and forming \(\mathrm{O}_3\) bonds, which is indicative of the rearrangement of electrons.

Key concepts

Redox Reactions

Redox reactions are a type of chemical reaction involving the transfer of electrons between two species. The term 'redox' is short for reduction-oxidation, where reduction refers to the gain of electrons, and oxidation denotes the loss of electrons. These reactions always happen simultaneously, meaning when one substance is oxidized, another is reduced.

In the reaction under study, two oxygen molecules (\(3\ \mathrm{O}_2\)) react to form ozone (\(2\ \mathrm{O}_3\)). Although the elemental oxidation state of oxygen does not change, the reaction still involves electron rearrangement, which is the hallmark of redox reactions. The focus here is on the structural transformation, rather than a simple numerical change in oxidation numbers, to understand this specific electron movement.

Elemental Form Oxidation State

The oxidation state of an element in its pure form is always zero. This principle applies to diatomic molecules like oxygen gas (\(\mathrm{O}_2\)). In its natural state, oxygen consists of two atoms, sharing electrons equally, hence each atom bears no net charge or oxidation number. Understanding this is crucial, as it forms the basis for determining changes in oxidation state during a reaction.

In our particular example, oxygen starts in its elemental form with an oxidation state of zero. This unchanged oxidation state often circumscribes reactions involving only one element, particularly when examining the electron distributions in a molecule.

Electron Rearrangement

Electron rearrangement refers to the distribution or redistribution of electrons among atoms in a molecule. Even if there is no visible change in the oxidation number, a rearrangement occurs when the molecular structure changes.

• In the case where oxygen transforms from \(\mathrm{O}_2\) to \(\mathrm{O}_3\), oxygen atoms reorganize from double-bonded pairs into a more complex tri-atomic structure.
• Electrons primarily associated with the double bonds in \(\mathrm{O}_2\) get redistributed among new bonds in \(\mathrm{O}_3\).

These movements suggest that even without a change in numerical oxidation state, the physical dance of electrons influences the reaction dynamics.

Average Oxidation Number

The average oxidation number represents a mean value of oxidation states for a particular element within a molecule composed of identical atoms. Even when molecules undergo transformation, this average can remain constant, as observed in the formation of ozone from oxygen.

• In \(\mathrm{O}_3\), while individual oxygen atoms might share electrons differently due to structural rearrangement, the average oxidation state of each oxygen atom remains zero.
• This outcome underscores the concept that oxidation states offer a simplified electron accounting method amidst complex chemical interactions.

Understanding the average oxidation number provides insights into the stability or reactivity offered by the chemical environment described by the molecule.