Dioxygen and Ozone — Revision Notes

⚡ 30-Second Revision

Dioxygen ($O_2$) — Colorless, odorless, tasteless. Paramagnetic. Linear structure. Essential for respiration/combustion. Lab prep: $2KClO_3 \xrightarrow{MnO_2} 2KCl + 3O_2$; $2H_2O_2 \xrightarrow{MnO_2} 2H_2O + O_2$. Industrial prep: Fractional distillation of liquid air.
Ozone ($O_3$) — Pale blue gas, pungent smell. Diamagnetic. Bent structure ($116.8^circ$), resonance, identical $O-O$ bond lengths ($128,\text{pm}$). Strong oxidizing agent ($O_3 \rightarrow O_2 + O$). Lab prep: $3O_2 \xrightarrow{\text{silent electric discharge}} 2O_3$. Thermodynamically unstable.
Key Reactions of $O_3$ — $2KI + H_2O + O_3 \rightarrow 2KOH + I_2 + O_2$; $PbS + 4O_3 \rightarrow PbSO_4 + 4O_2$; $2Hg + O_3 \rightarrow Hg_2O + O_2$.
Environmental — Stratospheric $O_3$ (UV shield), Tropospheric $O_3$ (pollutant).

2-Minute Revision

Dioxygen ($O_2$) and ozone ($O_3$) are the two main allotropes of oxygen. Dioxygen is a stable, colorless, odorless, and tasteless gas, crucial for life. Its most notable property is paramagnetism, explained by two unpaired electrons in its molecular orbitals. It's prepared in labs by decomposing compounds like $KClO_3$ or $H_2O_2$ and industrially by fractional distillation of liquid air.

Ozone ($O_3$) is a less stable, pale blue gas with a pungent smell. It's diamagnetic, meaning all its electrons are paired. Its structure is bent, with a bond angle of about $116.8^circ$, and it exhibits resonance, leading to two identical $O-O$ bond lengths.

Ozone is a very powerful oxidizing agent because it readily decomposes to release nascent oxygen. It's prepared by passing a silent electric discharge through dry oxygen to prevent decomposition. Key reactions include oxidizing $KI$ to $I_2$, $PbS$ to $PbSO_4$, and $NO$ to $NO_2$.

Environmentally, stratospheric ozone protects from UV radiation, while tropospheric ozone is a pollutant.

5-Minute Revision

Let's quickly review the essential aspects of Dioxygen ($O_2$) and Ozone ($O_3$) for NEET. These are the two primary allotropes of oxygen, differing significantly in their properties and roles.

**Dioxygen ($O_2$): This is the common form of oxygen, a colorless, odorless, tasteless gas. It's vital for respiration and combustion. A critical point for NEET is its paramagnetic** nature, arising from two unpaired electrons in its antibonding $pi^*$ molecular orbitals, as explained by Molecular Orbital Theory.

Its structure is linear. Laboratory preparation involves heating potassium chlorate with manganese dioxide ($2KClO_3 \xrightarrow{MnO_2, Delta} 2KCl + 3O_2$) or decomposing hydrogen peroxide ($2H_2O_2 \xrightarrow{MnO_2} 2H_2O + O_2$).

Industrially, it's obtained by fractional distillation of liquid air.

**Ozone ($O_3$)**: This is a pale blue gas with a characteristic pungent smell. Unlike $O_2$, it is diamagnetic (all electrons paired). Its molecular structure is bent, not linear, with a bond angle of approximately $116.

8^circ$. The central oxygen atom is$sp^2$hybridized, and it exhibits resonance, resulting in two identical$O-O$bond lengths ($128, ext{pm}$), intermediate between single and double bonds. Ozone is thermodynamically unstable and readily decomposes to$O_2$, making it a **very powerful oxidizing agent** due to the release of nascent oxygen ($O_3 \rightarrow O_2 + O$).

Preparation of Ozone: It's prepared by passing a silent electric discharge through dry oxygen in an ozonizer ($3O_2(g) \xrightarrow{\text{silent electric discharge}} 2O_3(g)$). The 'silent' discharge is crucial to prevent the decomposition of the unstable ozone due to excessive heat.

Key Reactions of Ozone: Focus on its oxidizing reactions:

Oxidation of potassium iodide: $2KI(aq) + H_2O(l) + O_3(g) \rightarrow 2KOH(aq) + I_2(s) + O_2(g)$ (used for quantitative estimation).
Oxidation of lead sulfide: $PbS(s) + 4O_3(g) \rightarrow PbSO_4(s) + 4O_2(g)$ (black $PbS$ to white $PbSO_4$).
Oxidation of mercury ('tailing of mercury'): $2Hg(l) + O_3(g) \rightarrow Hg_2O(s) + O_2(g)$.
Oxidation of nitrogen monoxide: $NO(g) + O_3(g) \rightarrow NO_2(g) + O_2(g)$.

Environmental Role: Stratospheric ozone forms a protective layer, absorbing harmful UV radiation. Tropospheric ozone, however, is a harmful pollutant contributing to smog.

Common Traps: Don't confuse the magnetic properties or stability. Remember $O_3$ is less stable but a stronger oxidizer. Be clear on the specific products of ozone's reactions.

Prelims Revision Notes

Oxygen Allotropes — Dioxygen ($O_2$) and Ozone ($O_3$).
Dioxygen ($O_2$) Properties

* Colorless, odorless, tasteless gas. * Paramagnetic (due to 2 unpaired electrons in $pi^*$ MOs). * Linear molecule. * Essential for respiration and combustion. * Oxidation state typically $-2$, but $-1$ in peroxides ($H_2O_2$) and $-1/2$ in superoxides ($KO_2$). With fluorine, $+1$ ($O_2F_2$) and $+2$ ($OF_2$).

Dioxygen Preparation

* Lab: Thermal decomposition of $KClO_3$ (with $MnO_2$ catalyst), $H_2O_2$ (with $MnO_2$ catalyst), $HgO$, $Ag_2O$. * $2KClO_3(s) \xrightarrow{MnO_2, Delta} 2KCl(s) + 3O_2(g)$ * $2H_2O_2(aq) \xrightarrow{MnO_2} 2H_2O(l) + O_2(g)$ * Industrial: Fractional distillation of liquid air.

Ozone ($O_3$) Properties

* Pale blue gas, pungent 'electrical' smell. * Diamagnetic (all electrons paired). * Bent structure, $sp^2$ hybridized central O, resonance stabilized. * Bond angle $approx 116.8^circ$, identical $O-O$ bond lengths ($128,\text{pm}$). * Thermodynamically unstable (decomposes to $O_2$), endothermic formation ($Delta H = +142,\text{kJ/mol}$). * Very strong oxidizing agent (releases nascent oxygen: $O_3 \rightarrow O_2 + O$).

Ozone Preparation

* Lab/Industrial: Passing dry oxygen through a silent electric discharge in an ozonizer. * $3O_2(g) \xrightarrow{\text{silent electric discharge}} 2O_3(g)$ * Silent discharge prevents decomposition due to heat.

Key Reactions of Ozone (Oxidizing Action)

* Oxidizes $KI$ to $I_2$: $2KI + H_2O + O_3 \rightarrow 2KOH + I_2 + O_2$ * Oxidizes $PbS$ to $PbSO_4$: $PbS + 4O_3 \rightarrow PbSO_4 + 4O_2$ * Oxidizes $Hg$ ('tailing of mercury'): $2Hg + O_3 \rightarrow Hg_2O + O_2$ * Oxidizes $NO$ to $NO_2$: $NO + O_3 \rightarrow NO_2 + O_2$

Uses — Germicide, disinfectant, bleaching agent, air purifier.
Environmental Role — Stratospheric ozone (protective UV shield), Tropospheric ozone (harmful pollutant, smog component).

Vyyuha Quick Recall

OZONE's Properties: Pungent smell, Pale blue, Powerful oxidizer, Prepped by silent discharge. Bent structure, Bond lengths equal, But Bad at ground level. Diamagnetic, Decomposes easily.