Class 11 Chemistry – Chapter 11: Nitrogen and Sulphur (FBISE)
This section provides complete, exam-oriented notes for Class 11 Chemistry Chapter 11 – Nitrogen and Sulphur according to the Federal Board (FBISE) syllabus. Students will learn the chemical properties, reactions, and real-life applications of these important elements.
Key topics include properties of sulphur, uses of sulphur, and smog formation. The notes cover structural, chemical, and industrial aspects with examples, diagrams, and step-by-step explanations for easy understanding and exam preparation.
Students can also access video lectures, MCQs, solved numericals, test series, and live classes for this chapter on our official YouTube channel and stay updated through our WhatsApp channel.
Part 1: Photochemical Smog and Peroxyacetyl Nitrate (PAN)
1.1.4 Photochemical Smog
Photochemical smog is a mixture of pollutants formed when oxides of nitrogen ($NO_{x}$) and volatile organic compounds (VOCs) react with sunlight, creating a characteristic brown haze. It is most prevalent in summer due to high intensity sunlight.
A key reaction involves $NO$, $NO_{2}$, and unburned hydrocarbons, which leads to the formation of Peroxyacetyl Nitrate (PAN) and Ozone ($O_{3}$).
Equation: $Nitrogen\ oxides + hydrocarbons + UV \rightarrow PAN + O_{3}$
1.1.5 Mechanism of PAN Formation
- Formation of Peroxyacyl Radicals: $NO_{2}$ undergoes photodissociation in sunlight, splitting into $NO$ and an oxygen atom ($O$). The oxygen atom reacts with unburned hydrocarbons to form peroxyacyl radicals ($RCO_{3}^{\cdot}$).
- Reaction with NO: These radicals react with $NO$ to form various compounds, including PAN.
- Formation of PAN: The reaction is represented as: $(RCO_{3}^{\cdot}) + NO_{2} \rightarrow R-CO_{2}ONO_{2}\ (PAN)$.
Environmental Impact of PAN
- It is a secondary pollutant and a potent respiratory and eye irritant.
- It acts as a reservoir for $NO_{x}$, transporting it over long distances.
- It can undergo thermal decomposition in the lower atmosphere, releasing $NO_{2}$ and reactive radicals that further contribute to smog.
Part 2: Properties and Uses of Sulphur
11.3.1 Properties of Sulphur
Sulphur is a nonmetal element located below oxygen in Group VI of the periodic table. It is chemically versatile:
- Oxidising Agent: It accepts electrons to achieve a stable configuration (similar to noble gases), often reaching a $-2$ oxidation state.
- Reducing Agent: It can lose electrons, existing in $+4$ and $+6$ oxidation states.
11.3.4 Uses of Sulphur and its Compounds
| Industry | Application / Compound |
|---|---|
| Fertiliser | Sulphuric acid and ammonium sulphate provide sulphur for plant amino acids and vitamins. |
| Gunpowder | A mixture of sulphur, charcoal, and potassium nitrate used for combustion. |
| Rubber Industry | Used in the vulcanization of rubber to improve durability. |
| Synthetic Chemistry | Thiol ($-SH$) compounds and Sulfonyl chlorides (intermediates for sulfonamide antibiotics). |
| Food Preservation | $SO_{2}$ and sulphites ($Na_{2}SO_{3}$) act as reducing agents to inhibit food oxidation. |
Relevant Questions & Answers
Q1: Why is photochemical smog more common in the summer?
A: Photochemical smog requires sunlight to trigger the chemical reactions between nitrogen oxides and VOCs. Summer provides the maximum sunlight intensity needed for this process.
Q2: Define PAN and explain why it is classified as a secondary pollutant.
A: PAN stands for Peroxyacetyl Nitrate. It is a secondary pollutant because it is not emitted directly from a source (like a tailpipe) but is instead formed in the atmosphere through reactions involving primary pollutants like $NO_{x}$ and hydrocarbons in the presence of UV light.
Q3: How does sulphur act as a reducing agent?
A: Sulphur acts as a reducing agent when it loses electrons, typically resulting in oxidation states of $+4$ or $+6$.
Q4: What is the role of sulphites ($Na_{2}SO_{3}$) in food preservation?
A: Sulphites act as reducing agents. They prevent the spoilage of food by inhibiting the oxidation process.
Q5: What are the primary constituents of gunpowder?
A: Gunpowder typically consists of sulphur, charcoal, and potassium nitrate.