Chemistry (9701) | Nitrogen and sulfur

Study Notes: Inorganic Chemistry - Nitrogen and Sulfur (AS Level, Topic 12)

Hello future Chemists! This chapter takes us into the world of two incredibly important elements, Nitrogen (N) and Sulfur (S). Although they are non-metals sitting relatively far apart on the Periodic Table, their chemistry has huge implications for industrial processes, agriculture, and especially, the environment (think air pollution and acid rain!).

Don't worry if some concepts seem complex—we'll break down the structure of molecules like $\text{N}_2$ to understand its mysterious inactivity, and then explore how human activities transform harmless elements into dangerous pollutants. Let's dive in!

1. The Chemistry of Nitrogen ($\text{N}$) and Ammonia ($\text{NH}_3$)

1.1 The Lack of Reactivity of Nitrogen Gas ($\text{N}_2$)

Nitrogen gas makes up about 78% of the air we breathe. Despite being everywhere, it is surprisingly unreactive. Why? The answer lies in its structure and bonding.

• Bond Strength: The nitrogen molecule ($\text{N}_2$) contains a triple covalent bond ($\text{N} \equiv \text{N}$). This bond is one of the strongest known bonds in Chemistry.
  (Analogy: Imagine trying to break three heavy-duty chains connecting two objects—it requires huge effort!)
• Energy Required: Because the triple bond is so strong, a very high amount of energy (a very high activation energy) is required to break it so that $\text{N}$ atoms can react.
• Lack of Polarity: The $\text{N}_2$ molecule is non-polar. This means it is generally unreactive towards polar reagents (like water or acids) at normal temperatures.

Key Takeaway: Reacting Nitrogen is Expensive!

This lack of reactivity is why high temperatures and pressures are needed in industrial processes, such as the Haber process (used to make ammonia), to force the $\text{N} \equiv \text{N}$ bond to break.

1.2 Basicity of Ammonia ($\text{NH}_3$)

Ammonia is famous for being a base, but we need to explain this using the Brønsted-Lowry theory.

Definition Reminder: Brønsted-Lowry Theory

An acid is a proton donor ($\text{H}^+$ donor). A base is a proton acceptor ($\text{H}^+$ acceptor).

• Mechanism: Ammonia acts as a base because the Nitrogen atom in $\text{NH}_3$ has a lone pair of electrons. This lone pair can easily accept a proton ($\text{H}^+$) from an acid (or water).
• Reaction with Water (Aqueous Solution): $$ \text{NH}_3(\text{aq}) + \text{H}_2\text{O}(\text{l}) \rightleftharpoons \text{NH}_4^+(\text{aq}) + \text{OH}^-(\text{aq}) $$
  In this reaction, $\text{NH}_3$ accepts a proton from water, forming the ammonium ion ($\text{NH}_4^+$) and hydroxide ions ($\text{OH}^-$), which makes the solution alkaline (basic).

1.3 The Ammonium Ion ($\text{NH}_4^+$)

When $\text{NH}_3$ accepts an $\text{H}^+$, it forms the ammonium ion.

• Structure: The $\text{NH}_4^+$ ion has a tetrahedral shape with a bond angle of $109.5^\circ$.
• Dative Covalent Bonding: The bond formed between the $\text{NH}_3$ molecule and the incoming $\text{H}^+$ ion is a special type of covalent bond called a coordinate bond (or dative covalent bond). The lone pair from the nitrogen atom provides both electrons for the bond.

1.4 Displacement of Ammonia from Ammonium Salts

Ammonia can be easily released from its salts when reacted with a strong base (alkali). This is a common laboratory test for ammonium ions ($\text{NH}_4^+$).

• The Reaction: The reaction is an acid-base reaction where a strong base (like $\text{NaOH}$ or $\text{KOH}$) displaces the weak base ($\text{NH}_3$) from its salt.
• Example (Ammonium Chloride): $$ \text{NH}_4\text{Cl}(\text{s}) + \text{NaOH}(\text{aq}) \longrightarrow \text{NaCl}(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{NH}_3(\text{g}) $$
• Observation: Heating an ammonium salt with a base produces ammonia gas, which can be identified by its distinctive pungent smell or by turning damp red litmus paper blue.

---

---

2. Environmental Impact of Nitrogen and Sulfur Oxides

The reactions involving nitrogen and sulfur oxides are key examples of how chemistry connects directly to environmental science. Both are major contributors to acid rain and photochemical smog.

2.1 Sources of Oxides of Nitrogen ($\text{NO}_x$)

Oxides of nitrogen, primarily $\text{NO}$ (nitrogen monoxide) and $\text{NO}_2$ (nitrogen dioxide), are formed under high-energy conditions.

• Natural Occurrence:
  Example: Lightning provides the extremely high energy needed to break the $\text{N} \equiv \text{N}$ bond, allowing $\text{N}_2$ to react with $\text{O}_2$ in the atmosphere. $$ \text{N}_2(\text{g}) + \text{O}_2(\text{g}) \longrightarrow 2\text{NO}(\text{g}) $$
• Man-Made Occurrence (Major Source):
  Example: Internal combustion engines (car engines). The high temperatures and pressures inside the engine cylinder provide the energy needed for $\text{N}_2$ and $\text{O}_2$ from the air intake to react, producing $\text{NO}$.

2.2 Catalytic Removal of $\text{NO}_x$ (Car Exhausts)

To combat this pollution, modern cars use catalytic converters. These converters reduce harmful emissions before they leave the exhaust pipe.

• Function: They convert harmful oxides of nitrogen ($\text{NO}$ and $\text{NO}_2$) back into harmless nitrogen gas ($\text{N}_2$) and oxygen ($\text{O}_2$).
• Catalyst: The converters use metals like Palladium, Platinum, and Rhodium. These act as heterogeneous catalysts (different state from reactants).
• Key Reaction: $$ 2\text{NO}(\text{g}) + 2\text{CO}(\text{g}) \xrightarrow{\text{catalyst}} \text{N}_2(\text{g}) + 2\text{CO}_2(\text{g}) $$ (Note: They also oxidize unburnt hydrocarbons and carbon monoxide, $\text{CO}$, into $\text{CO}_2$ and $\text{H}_2\text{O}$).

2.3 Photochemical Smog and PAN

When $\text{NO}_x$ pollutants mix with unburned hydrocarbons (also from vehicle exhausts) in the presence of sunlight, they undergo complex reactions to form secondary pollutants, leading to photochemical smog.

• What is PAN? One key component of this smog is Peroxyacetyl Nitrate (PAN).
• Effect: PAN is a powerful irritant, damaging plant life and irritating human eyes and respiratory systems. It is a sign of severe air quality issues, especially in sunny urban areas.

2.4 The Role of Nitrogen and Sulfur Oxides in Acid Rain

Acid rain is primarily caused by two pollutants: Sulfur Dioxide ($\text{SO}_2$) and Oxides of Nitrogen ($\text{NO}_x$).

Sulfur Dioxide ($\text{SO}_2$)

Sulfur impurities in fossil fuels (especially coal) burn to produce $\text{SO}_2$.

$$ \text{S}(\text{in fuel}) + \text{O}_2(\text{g}) \longrightarrow \text{SO}_2(\text{g}) $$

$\text{SO}_2$ reacts with oxygen and water in the atmosphere to form sulfuric acid ($\text{H}_2\text{SO}_4$).

The Role of $\text{NO}$ and $\text{NO}_2$ (The Catalyst)

This is a critical syllabus point! $\text{NO}$ and $\text{NO}_2$ contribute to acid rain directly by forming nitric acid, and catalytically by speeding up the oxidation of $\text{SO}_2$.

1. Direct Contribution (Nitric Acid): $\text{NO}_2$ reacts with water and oxygen to form nitric acid. $$ 4\text{NO}_2(\text{g}) + 2\text{H}_2\text{O}(\text{l}) + \text{O}_2(\text{g}) \longrightarrow 4\text{HNO}_3(\text{aq}) $$

2. Catalytic Role (Oxidizing $\text{SO}_2$): $\text{NO}_2$ is a strong oxidizing agent.

Step 1: Oxidation of $\text{SO}_2$ $$ \text{SO}_2(\text{g}) + \text{NO}_2(\text{g}) \longrightarrow \text{SO}_3(\text{g}) + \text{NO}(\text{g}) $$ (The $\text{NO}_2$ is reduced to $\text{NO}$)

Step 2: Regeneration of Catalyst $$ 2\text{NO}(\text{g}) + \text{O}_2(\text{g}) \longrightarrow 2\text{NO}_2(\text{g}) $$ (The $\text{NO}$ is immediately re-oxidized back to $\text{NO}_2$ by atmospheric oxygen)

Because $\text{NO}_2$ is reformed in the second step, it acts as a homogeneous catalyst (all components are gases) for the overall reaction: $\text{SO}_2 \rightarrow \text{SO}_3$. The $\text{SO}_3$ then dissolves in rain to form $\text{H}_2\text{SO}_4$.

---

---

3. Summary of Environmental Consequences

3.1 Quick Review: Pollutants and Effects

The key atmospheric reactions involving nitrogen and sulfur are summarized below:

Oxides of Nitrogen ($\text{NO}, \text{NO}_2$)

• Source: High temperature combustion (vehicle engines).
• Main Effect 1: Photochemical Smog (through formation of PAN).
• Main Effect 2: Acid Rain (by forming $\text{HNO}_3$ and catalysing the conversion of $\text{SO}_2$ to $\text{SO}_3$).
• Solution: Catalytic converters (reducing $\text{NO}$ to $\text{N}_2$).

Sulfur Dioxide ($\text{SO}_2$)

• Source: Burning of sulfur-containing fossil fuels (coal, oil).
• Main Effect: Acid Rain (by oxidizing to $\text{SO}_3$ which forms $\text{H}_2\text{SO}_4$).
• Solution: Flue gas desulfurisation (removing $\text{SO}_2$ from power station exhaust using bases like calcium oxide or calcium carbonate).

---