Ozone ($O_3$)
Preparation, Thermodynamics, Oxidizing Power, and Depletion.
Ozone ($O_3$) is a highly reactive allotrope of oxygen. While too reactive to remain stably at sea level for long, it forms a crucial layer in the upper atmosphere (stratosphere) where it protects Earth from harmful ultraviolet (UV) radiation. Let's delve into the specific methods of preparing this unstable gas and its powerful oxidizing properties.
1. Preparation of Ozone
The Silent Electric Discharge Method
When a slow dry stream of oxygen is passed through a silent electric discharge, oxygen is partially converted (about 10%) to ozone. The resulting mixture is called ozonised oxygen.
Notice that the formation of ozone is an endothermic process ($\Delta H = +\text{ve}$). However, ozone is thermodynamically unstable and decomposes back to oxygen violently if heated. The discharge must be "silent" (sparkless) to prevent local heating, which would immediately decompose the newly formed ozone back into oxygen.
2. Physical Properties & Structure
Pure ozone is a pale blue gas with a strong, characteristic smell. When cooled, it forms a dark blue liquid and eventually a violet-black solid. In small concentrations it is harmless, but if the concentration rises above $\sim 100 \text{ ppm}$, it causes breathing discomfort, headache, and nausea.
The ozone molecule consists of three oxygen atoms and exhibits resonance. The central oxygen atom is $sp^2$ hybridized with one lone pair.
- Shape: Angular (Bent or V-shaped).
- Bond Angle: Approximately $117^\circ$.
- Bond Lengths: Due to resonance between a single and a double bond, both $O-O$ bond lengths are exactly identical ($128 \text{ pm}$).
Ozone is thermodynamically unstable with respect to oxygen. Its decomposition into oxygen ($2O_3 \rightarrow 3O_2$) is highly spontaneous because:
- It releases heat ($\Delta H$ is negative / Exothermic).
- It increases the number of gas molecules ($\Delta S$ is positive / Entropy increases).
Result: $\Delta G$ is highly negative. High concentrations of ozone can decompose explosively.
Figure 1: Ozone exists as a resonance hybrid of these two structures, explaining its identical bond lengths.
3. Chemical Properties: Powerful Oxidizing Agent
Because ozone decomposes so easily, it readily liberates atoms of nascent oxygen ($[O]$). This makes ozone a much more powerful oxidizing agent than regular dioxygen ($O_2$).
1. Oxidation of Lead Sulfide (Black to White)
Ozone oxidizes black lead sulfide to white lead sulfate. (Often asked in qualitative analysis).
2. Oxidation of Iodide Ions (Quantitative Estimation of Ozone)
When ozone reacts with an excess of Potassium Iodide ($KI$) solution buffered with a borate buffer (pH 9.2), iodine is liberated. This liberated iodine is then titrated against a standard solution of sodium thiosulfate ($Na_2S_2O_3$) to quantitatively estimate the amount of $O_3$ gas.
4. Depletion of the Ozone Layer
The ozone layer in the stratosphere protects the Earth from harmful UV radiations. However, it is highly susceptible to depletion by certain chemicals.
- Nitric Oxide ($NO$): Nitrogen oxides emitted from the exhaust systems of supersonic jet airplanes rapidly deplete the ozone layer. The NO acts as a catalyst.
$NO(g) + O_3(g) \rightarrow \mathbf{NO_2(g)} + O_2(g)$
- Freons (CFCs): Chlorofluorocarbons, heavily used in refrigerants and aerosols, release chlorine free radicals ($Cl^\bullet$) in the stratosphere under UV light. These radicals act as chain catalysts, destroying thousands of ozone molecules per radical.
Mastery Check: Ozone
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