Problem 152 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following compounds the oxidation state of oxygen is other than \(-2 ?\) (a) \(\mathrm{H}_{2} \mathrm{O}_{2}\) (b) \(\mathrm{O}_{2}\) (c) \(\mathrm{O}_{2} \mathrm{~F}_{2}\) (d) \(\mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

Oxygen has an oxidation state other than -2 in \\(\mathrm{H}_{2}\mathrm{O}_{2}\\), \\(\mathrm{O}_{2}\\), and \\(\mathrm{O}_{2}\mathrm{~F}_{2}\\).

Step 1: Identify Typical Oxidation State

Typically, the oxidation state of oxygen in most compounds is -2. However, there are exceptions to this rule, particularly in peroxides, superoxides, and compounds containing oxygen with fluorine.

Step 2: Examine Each Compound

Let's examine the oxidation state of oxygen in each given compound:(a) In \(\mathrm{H}_{2}\mathrm{O}_{2}\), hydrogen peroxide, the structure is \(\mathrm{H}-\mathrm{O}-\mathrm{O}-\mathrm{H}\). The oxidation state of oxygen in peroxides is -1.(b) In \(\mathrm{O}_{2}\), molecular oxygen, each oxygen is in its elemental form, and therefore its oxidation state is 0.(c) In \(\mathrm{O}_{2}\mathrm{~F}_{2}\), dioxygen difluoride, oxygen is bound to a more electronegative element (fluorine). Here, each oxygen has an oxidation state of +1.(d) In \(\mathrm{H}_{2}\mathrm{O}\), water, the oxidation state of oxygen is the typical -2.

Step 3: Identify the Exception

From the examination of the compounds:- \(\mathrm{H}_{2}\mathrm{O}_{2}\) has oxygen with -1;- \(\mathrm{O}_{2}\) has oxygen with 0;- \(\mathrm{O}_{2}\mathrm{~F}_{2}\) has oxygen with +1;- \(\mathrm{H}_{2}\mathrm{O}\) has oxygen with the typical -2.Therefore, the compounds where oxygen has an oxidation state other than -2 are \(\mathrm{H}_{2}\mathrm{O}_{2}\), \(\mathrm{O}_{2}\), and \(\mathrm{O}_{2}\mathrm{~F}_{2}\). However, since all of them are other than -2, the non-typical scenarios are options (a), (b), and (c).

Key Concepts

Peroxides

In chemistry, peroxides are interesting compounds where the oxidation state of oxygen is not the typical -2. Instead, oxygen in peroxides has an oxidation state of -1. This is because peroxides contain an oxygen-oxygen single bond which distributes the electrons differently.

The most well-known peroxide is hydrogen peroxide (\(\text{H}_2\text{O}_2\)), which consists of two hydrogen atoms and two oxygen atoms. The structure is represented as \(\text{H}-\text{O}-\text{O}-\text{H}\), where the oxygen-oxygen bond equalizes charge distribution between the two oxygen atoms.

• In peroxides, the electron sharing between the two oxygen atoms results in each oxygen atom being assigned an oxidation state of -1.
• This bonding is unique and crucial for properties such as bleaching and disinfectant abilities of hydrogen peroxide.

Peroxides illustrate how changes in bonding can impact chemical properties and reactivity significantly.

Superoxides

Superoxides are another special category of oxygen-containing compounds, distinguished by their unusual oxidation states. Unlike peroxides, superoxides assign an oxidation state of -\(\frac{1}{2}\) to each oxygen atom due to their bonding structure.

In superoxides, a single extra electron is shared between the oxygen atoms, leading to distinct properties and reactivity. Potassium superoxide (\(\text{KO}_2\)) is a common example that utilizes this behavior to advantage, especially in applications that require oxygen generation or carbon dioxide absorption.

• In the structure of a superoxide, the bond between oxygen atoms is stronger than in peroxides, allowing for only a half-charge per oxygen.
• This structure enables superoxides to act as excellent oxidizing agents, often used in safety equipment like life-support systems.

Superoxides are fundamental in studying the effects of bonding asymmetry in oxygen chemistry.

Oxygen-Fluorine Compounds

When it comes to compounds where oxygen is bound to fluorine, such as in dioxygen difluoride (\(\text{O}_2\text{F}_2\)), things get really exciting and unusual. Fluorine's exceptional electronegativity pulls electrons away from the oxygen, resulting in a positive oxidation state for oxygen.

In \(\text{O}_2\text{F}_2\), each oxygen atom possesses an oxidation state of +1. This condition arises due to fluorine's greater ability to attract electrons.

• Fluorine is the most electronegative element, overpowering other elements’ tendencies toward oxygen to hold electrons.
• As a result, oxygen acts unusually in these compounds, accommodating a higher positive oxidation number than we'd expect.

These compounds showcase fascinating behavior and are largely explored due to their potent oxidizing capacity. In environments where unique reactivity of oxygen is desired, the study of oxygen-fluorine compounds is essential.
This highlights the extraordinary flexibility of oxidation states oxygen can possess depending on its bonding partners.