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Environmental Chemistry — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistryEnvironmental Chemistry5 Marks
Question
Write down the reactions involved during the formation of photochemical smog.
✓

Answer

Photochemical smog is produced by action of sunlight on hydrocarbons and nitrogen oxides. NO is converted to $\mathrm{NO}_2$ by the effect of sunlight. This $\mathrm{NO}_2$ further absorbs sunlight and disintegrates into NO and $\mathrm{O}_2$. Thus, a chain reaction begins.
$\text{NO}_{\text{(g)}}+\frac{1}{2}\text{O}_{2(\text{g})}\xrightarrow{ \ \ \ \ \text{h}v \\ \ \ \ \\\ \ }\text{NO}_{2(\text{g})}$
$\text{NO}_2\xrightarrow{ \ \ \ \ \text{h}v \ \ \\ \ }\text{NO}_{(\text{g})}+\text{O}_{(\text{g})}$
These oxygen atoms react with $\mathrm{O}_2$ to form ozone as:
$\text{O}_{2(\text{g})}+\text{O}_{(\text{g})}\leftrightharpoons\text{O}_{3(\text{g})}$
$\text{O}_{3(\text{g})}+\text{NO}_{(\text{g})}\leftrightharpoons\text{}\text{NO}_{2(\text{g})}+\text{O}_{2(\text{g})}$
These two gases formed, react with hydrocarbons to produce other chemicals.
$3\text{CH}_4 + 2\text{O}_3 \xrightarrow{\ \ \ \ \ \ } 3\text{CH}_2 = \text{O} + 3\text{H}_2\text{O}\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{formaldehyde}$
Other chemicals produced are:
$\text{CH}_2 = \text{CH} - \text{CH} = \text{O}\\ \ \ \ \ \ \ \ \text{Acrolein} $
$\text{CH}_3 - \text{C} - \text{O} - \text{O} - \text{NO}_2\\ \ \ \ \ \ \ \ \ \ \ \ \ \ {||}\\ \ \ \ \ \ \ \ \ \ \ \ \ \text{O}\\\text{Peroxyacetyl nitrate (PAN)}$

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