# 11.1 Nitrogen Nitrogen, a nonmetallic element in Group 15 (formerly Group V) of the periodic table, constitutes approximately 78% of the Earth's atmosphere. It primarily exists as diatomic molecules ($N_2$). Nitrogen gas is known for its low reactivity. ### Electronic Configuration and Bonding To understand the inert nature of nitrogen, we must examine its molecular bonding. - The electronic configuration of a nitrogen atom is $1s^2 2s^2 2p^3$. - Each nitrogen atom needs to gain three electrons to achieve a stable noble gas configuration (like Neon). - Two nitrogen atoms accomplish this by sharing three pairs of electrons, forming a **triple covalent bond** ($N \equiv N$). - This triple bond is extremely strong, with a bond energy of nearly $941 \, \text{kJ} \, \text{mol}^{-1}$ (often rounded to $1000 \, \text{kJ} \, \text{mol}^{-1}$). The large amount of energy required to break this bond is the reason for the chemical inertness of nitrogen gas ($N_2$). Consequently, nitrogen only reacts under extreme conditions of temperature and pressure. ### Ammonia and the Ammonium Ion #### Formation of the Ammonium Ion ($NH_4^+$) The ammonium ion is formed when ammonia ($NH_3$), an alkaline gas, reacts with an acid. Ammonia's alkaline nature is evident as it turns moist red litmus paper blue. The reaction with a hydrogen ion ($H^+$) is: $$ NH_3(aq) + H^+(aq) \rightarrow NH_4^+(aq) $$ **Mechanism (Lewis Acid-Base Theory):** - The nitrogen atom in ammonia has a lone pair of electrons. - It donates this lone pair to a hydrogen ion ($H^+$) to form a **dative (or coordinate) covalent bond**. - According to the Lewis concept, an electron-pair donor is a base. This is why ammonia is basic. #### Laboratory Preparation of Ammonia Gas In the laboratory, ammonia gas is prepared by heating an ammonium salt with a base. - **Reactants:** Ammonium chloride ($NH_4Cl$) and calcium hydroxide ($Ca(OH)_2$), both solids. - **Word Equation:** Ammonium Chloride + Calcium Hydroxide $\rightarrow$ Calcium chloride + Ammonia + Water - **Chemical Equation:** $$ 2NH_4Cl(s) + Ca(OH)_2(s) \xrightarrow{\Delta} CaCl_2(s) + 2NH_3(g) + 2H_2O(l) $$ #### The Haber Process: Industrial Manufacture of Ammonia Ammonia is manufactured on a large scale via the Haber process, which involves the direct combination of nitrogen and hydrogen gases. - **Main Equation:** $$ N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) $$ - **Conditions for the Haber Process:** - **Temperature:** $450^\circ \text{C} - 500^\circ \text{C}$ - **Pressure:** 200 atm - **Catalyst:** Iron (Fe) with promoters like $K_2O$ and $Al_2O_3$. - **Explanation of Conditions:** - The forward reaction is *exothermic*. According to Le Chatelier's principle, a lower temperature would favor a higher yield of ammonia. However, the reaction rate would be too slow. Therefore, a compromise temperature is used to achieve a reasonable rate and yield. - The forward reaction involves a decrease in the number of moles of gas (4 moles on the left, 2 moles on the right). High pressure shifts the equilibrium to the right, favoring the production of ammonia. ### Oxides of Nitrogen ($NO_x$) Nitrogen forms several oxides, including dinitrogen monoxide ($N_2O$), nitrogen monoxide ($NO$), and nitrogen dioxide ($NO_2$). #### Formation of Nitrogen Oxides 1. **Natural Formation:** During thunderstorms, the high energy from lightning provides the activation energy for nitrogen and oxygen in the air to react, forming nitrogen monoxide. $$ N_{2(g)} + O_{2(g)} \xrightarrow{\text{lightning}} 2NO_{(g)} $$ 2. **Internal Combustion Engines:** The high temperature and pressure inside an engine also cause nitrogen and oxygen from the air to react, producing $NO$. $$ N_{2(g)} + O_{2(g)} \xrightarrow{\text{high T, P}} 2NO_{(g)} $$ 3. **Oxidation to Nitrogen Dioxide:** The nitrogen monoxide formed is quickly oxidized by oxygen in the air to form brown nitrogen(IV) oxide, or nitrogen dioxide. $$ 2NO_{(g)} + O_{2(g)} \rightarrow 2NO_{2(g)} $$ #### Acid Rain Formation Nitrogen dioxide dissolves in water droplets in the atmosphere to form nitric acid, which then falls as acid rain. This process is also a part of the natural nitrogen cycle. $$ 2NO_{2(g)} + H_2O_{(l)} + \frac{1}{2}O_{2(g)} \rightarrow 2HNO_{3(aq)} $$ Nitric acid in the soil is converted to nitrates, which are essential fertilizers for plant growth and protein synthesis. ### Catalytic Removal of Nitrogen Oxides from Exhaust Gases To reduce air pollution, vehicles are equipped with **catalytic converters** to remove harmful gases like nitrogen monoxide ($NO$) from the exhaust. - **Device:** A three-way catalytic converter. - **Catalysts:** A mixture of Platinum (Pt), Palladium (Pd), and Rhodium (Rh). - **Function:** The converter facilitates redox reactions that convert pollutants into less harmful substances. **Reactions in a Catalytic Converter:** 1. **Reduction of NO by Carbon Monoxide (CO):** $$ 2NO(g) + 2CO(g) \xrightarrow{\text{catalyst}} N_2(g) + 2CO_2(g) $$ 2. **Reduction of NO by unburned Hydrocarbons (HC):** $$ 2NO(g) + 2HC(g) \xrightarrow{\text{catalyst}} N_2(g) + 2H_2O(g) + CO_2(g) $$ These reactions convert toxic $NO$ and $CO$ into harmless nitrogen ($N_2$), water ($H_2O$), and carbon dioxide ($CO_2$). ### Photochemical Smog Photochemical smog is a type of air pollution, seen as a brown haze, that forms when pollutants react in the presence of sunlight (*photo* = light). - **Primary Pollutants:** Oxides of nitrogen ($NO_x$) and Volatile Organic Compounds (VOCs, or unburned hydrocarbons). - **Condition:** Strong sunlight (more common in summer). - **Secondary Pollutants:** These are formed from the reactions of primary pollutants. A major component is **Peroxyacetyl Nitrate (PAN)**, which is a powerful respiratory and eye irritant. **General Formation Equation:** $$ \text{Nitrogen Oxides} + \text{Hydrocarbons} + \text{Sunlight (UV)} \rightarrow \text{PAN} + O_3 \text{(Ozone)} $$ #### Mechanism of PAN Formation 1. **Formation of Peroxyacyl Radicals:** In sunlight, nitrogen dioxide ($NO_2$) undergoes photodissociation to form $NO$ and a highly reactive oxygen atom ($O$). This oxygen atom reacts with unburned hydrocarbons (VOCs) to form peroxyacyl radicals ($RCO_3^\cdot$). 2. **Reaction with Nitrogen Oxides:** These peroxyacyl radicals then react with nitrogen oxides to form Peroxyacetyl Nitrate (PAN). $$ (RCO_3) + NO \rightarrow R-CO_2ONO_2 \, (PAN) $$ 3. **Impact of PAN:** PAN is a stable secondary pollutant that can be transported over long distances. It acts as a reservoir for $NO_x$, which can be released later, prolonging the smog episode. ## Worked Examples / Questions & Answers *(Based on Concept Assessment Exercise 11.1)* - **Q1a:** Explain the lack of reactivity of nitrogen gas. **A:** Nitrogen gas ($N_2$) is unreactive due to the very strong **triple covalent bond** between the two nitrogen atoms. A large amount of energy (bond energy $\approx 941 \, \text{kJ} \, \text{mol}^{-1}$) is required to break this bond before nitrogen can react. - **Q1b:** Write a balanced chemical equation, including state symbols, for the reaction of ammonia solution with dilute nitric acid. **A:** This is an acid-base neutralization reaction. $$ NH_3(aq) + HNO_3(aq) \rightarrow NH_4NO_3(aq) $$ - **Q1c:** Write a balanced chemical equation, including state symbols, for the reaction of solid ammonium sulfate with solid sodium hydroxide. **A:** This reaction produces ammonia gas upon heating. $$ (NH_4)_2SO_4(s) + 2NaOH(s) \xrightarrow{\Delta} Na_2SO_4(s) + 2NH_3(g) + 2H_2O(l) $$ - **Q2a:** Ammonia is made in industry using the Haber process. Write the balanced equation to show the synthesis of ammonia from its elements, including the catalyst used. **A:** $$ N_2(g) + 3H_2(g) \rightleftharpoons[\text{Fe catalyst}] 2NH_3(g) $$ - **Q2b:** Write a balanced equation, including state symbols, for the formation of ammonium sulfate fertilizer from ammonia solution. **A:** $$ 2NH_3(aq) + H_2SO_4(aq) \rightarrow (NH_4)_2SO_4(aq) $$