Ch 1 · Nitrogen and sulfur · AS & A Level Chemistry 9701

Topic 1 of 1 in Nitrogen and sulfur — AS & A Level Chemistry 9701 — 68 lessons.

Introduction

Nitrogen is famously unreactive, yet its compounds are central to two of the most important applied topics in AS Chemistry: the base chemistry of ammonia and ammonium salts, and the environmental chemistry of nitrogen oxides. This lesson links molecular bonding directly to macroscopic behaviour, from a Bunsen flame test for ammonium salts to the causes of photochemical smog and acid rain.

Key ideas — Why nitrogen is unreactive

Nitrogen exists as the diatomic molecule N_2, in which the two atoms are joined by a triple bond (one σ bond and two π bonds). This triple bond has a very high bond enthalpy (945\ mathrmkJ mol^-1), so a very large amount of energy is needed to break it before nitrogen can react; nitrogen is also a non-polar molecule (two identical atoms), so it has no permanent dipole to attract other reagents or make it susceptible to nucleophilic or electrophilic attack. Together, the strong triple bond and the lack of polarity mean nitrogen reacts only under extreme conditions (very high temperature, pressure or with a catalyst, as in the Haber process), despite being thermodynamically favourable to react with many substances.

🧑‍🏫 Teacher's explanation

Imagine trying to break a heavy-duty steel chain compared to a single thin thread. The nitrogen molecule, N_2, is like that steel chain. Even though the atoms 'want' to bond with other things to become more stable, the three bonds holding them together are so incredibly strong that you need a massive amount of energy just to get them to start reacting.

📖 Definitions

🔍 Worked example — Easy

Calculate the energy required to break 0.5 moles of NequivN bonds, given the bond enthalpy is 945 kJ mol^-1.

🔍 Worked example — Standard

Explain why nitrogen gas (N_2) is chemically inert under standard conditions despite the reaction N_2(g) + 3H_2(g) arrow 2NH_3(g) being exothermic.

🔍 Worked example — Exam-style

A sample of nitrogen gas contains 3.01 × 10^23 molecules of N_2. Calculate the total energy required to break all the triple bonds in this sample.

⚠️ Common errors

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🎯 Exam question

(a) Explain this lack of reactivity in terms of its bonding and molecular structure. [3] (b) State the type of bond found in the N_2 molecule. [1] [4 marks]

Key ideas — Basicity of ammonia

Ammonia, NH_3, has a lone pair of electrons on the nitrogen atom. Using the Brønsted–Lowry theory (a base is a proton, H^+, acceptor), ammonia acts as a base by using this lone pair to accept a proton from an acid:

🧑‍🏫 Teacher's explanation

Think of ammonia as a molecule with a 'spare' part. The nitrogen atom has three bonds holding it to hydrogen atoms, but it also has one pair of electrons that isn't being used for bonding. This lone pair is like a magnet looking for a positive charge to latch onto.

📖 Definitions

🔍 Worked example — Easy

Write the equation for the reaction between ammonia and hydrochloric acid, NH_3(aq) + extHCl(aq) arrow dots

🔍 Worked example — Standard

Calculate the concentration of OH^- ions in a 0.10 mol dm^-3 solution of ammonia, given the equilibrium constant K_b = 1.8 × 10^-5.

🔍 Worked example — Exam-style

An ammonium salt, NH_4Cl(s), is heated strongly. Describe the products formed and explain why the reaction occurs.

⚠️ Common errors

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🎯 Exam question

Key ideas — The ammonium ion and its formation

When ammonia accepts a proton, the nitrogen atom's lone pair forms a new N–H bond (a dative covalent (coordinate) bond, in which both electrons are donated by nitrogen) with the incoming H^+, giving the ammonium ion, NH_4^+. Once formed, all four N–H bonds in NH_4^+ are identical and indistinguishable in length and strength, since the electron pair is now shared equally regardless of its origin. This is a straightforward Brønsted–Lowry acid–base reaction: ammonia (the base) accepts a proton from the acid.

🧑‍🏫 Teacher's explanation

Imagine you are playing a game of catch. Ammonia (NH_3) is like a player holding a ball (a lone pair of electrons) who is looking for someone to catch it. When a proton (H^+) comes along, it is like a player with empty hands. Ammonia 'throws' its electron pair to the proton, creating a permanent connection. This is a dative covalent bond, where one atom provides both electrons for the bond.

📖 Definitions

🔍 Worked example — Easy

Calculate the molar mass of the ammonium ion, NH_4^+, given A_r values of H = 1 and N = 14.

🔍 Worked example — Standard

A sample of 0.500 mol of ammonia reacts completely with excess hydrochloric acid (HCl) to form ammonium chloride (NH_4Cl). Calculate the mass of NH_4Cl produced. (A_r: N=14, H=1, Cl=35.5)

🔍 Worked example — Exam-style

Explain why the NH_4^+ ion is more stable than the NH_3 molecule in terms of its bonding and geometry.

⚠️ Common errors

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🎯 Exam question

Key ideas — Displacement of ammonia from ammonium salts

An ammonium salt reacts with a base stronger than ammonia (such as aqueous sodium hydroxide) in an acid–base reaction that displaces (releases) ammonia gas, since hydroxide ions remove a proton from the ammonium ion:

🧑‍🏫 Teacher's explanation

Think of the ammonium ion, NH_4^+, as a crowded taxi. The nitrogen atom is holding onto four hydrogen atoms, but it only really wants to hold onto three. Because it has a positive charge, it is 'unstable' in a sense—it is looking for a way to get rid of that extra proton (H^+) to reach a more stable, neutral state as ammonia ($ ext{NH}_3$).

📖 Definitions

🔍 Worked example — Easy

Predict the products for the reaction between ammonium sulfate and sodium hydroxide: NH_4SO_4(s) + NaOH(aq) arrow dots

🔍 Worked example — Standard

Calculate the mass of NH_4 extCl required to produce 1.7 ext g of NH_3 gas when reacted with excess NaOH.

🔍 Worked example — Exam-style

A student tests an unknown salt for the presence of NH_4^+ by adding NaOH(aq) and warming. The gas evolved turns damp red litmus paper blue. Write the balanced equation for the reaction between the salt and NaOH if the salt is NH_4NO_3.

⚠️ Common errors

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🎯 Exam question

Key ideas — Oxides of nitrogen: sources and catalytic removal

Oxides of nitrogen (NO and NO_2, collectively NO_x) form naturally when lightning provides enough energy to overcome nitrogen's triple bond, allowing N_2 and O_2 in the air to combine: N_2(g) + O_2(g) arrow 2NO(g). The same reaction occurs, man-made, inside the high-temperature, high-pressure cylinders of internal combustion engines, where N_2 and O_2 drawn in with the air react; the NO formed is then further oxidised in air to NO_2. Modern vehicles fit a catalytic converter, containing a platinum/rhodium catalyst, which converts NO_x and unburned hydrocarbons and carbon monoxide into less harmful nitrogen, carbon dioxide and water before they leave the exhaust, for example 2CO(g) + 2NO(g) arrow 2CO_2(g) + N_2(g).

🧑‍🏫 Teacher's explanation

Think of nitrogen gas (N_2) as a very strong, locked vault. The two nitrogen atoms are held together by a triple bond, which is one of the strongest bonds in nature. Because this bond is so tough and the molecule is perfectly symmetrical (non-polar), nitrogen is incredibly lazy and doesn't want to react with anything under normal conditions. It's like trying to break into a high-security vault with a toothpick; you need a massive amount of energy to crack it open.

📖 Definitions

🔍 Worked example — Easy

Write a balanced chemical equation for the reaction between nitrogen and oxygen during lightning, and calculate the mass of NO produced from 1.4 g of N_2. (A_r: N=14, O=16)

🔍 Worked example — Standard

A catalytic converter facilitates the reaction 2CO(g) + 2NO(g) arrow 2CO_2(g) + N_2(g). If 0.5 moles of NO are reacted, how many moles of CO_2 are produced?

🔍 Worked example — Exam-style

In the atmosphere, NO_2 reacts with water to form nitric acid. Write the balanced equation for the reaction: NO_2(g) + H_2O(l) arrow HNO_3(aq) + HNO_2(aq) and identify the role of NO_2 in acid rain formation.

⚠️ Common errors

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🎯 Exam question

(a) Write a balanced equation for the reaction between NO and CO that occurs in a catalytic converter. [1] (b) Explain how NO_2 contributes to the formation of acid rain. [2] [3 marks]

Key ideas — Photochemical smog and acid rain

Sunlight provides the energy for NO_2 to react with unburned hydrocarbons released from vehicle exhausts and other sources, producing a complex mixture of secondary pollutants, including ozone and other oxidising and irritant compounds, known as photochemical smog; this haze is most severe in sunny, traffic-congested cities and worsens respiratory conditions. Oxides of nitrogen also contribute to acid rain directly, since NO_2 dissolves in atmospheric water and disproportionates to a mixture of nitric and nitrous acids: 3NO_2(g) + H_2O(l) arrow 2HNO_3(aq) + NO(g). They also contribute indirectly, because NO_2 acts as a catalyst that oxidises atmospheric sulfur dioxide to sulfur trioxide, which then dissolves in rainwater to form sulfuric acid: SO_2(g) + NO_2(g) arrow SO_3(g) + NO(g), followed by SO_3(g) + H_2O(l) arrow H_2SO_4(aq).

🧑‍🏫 Teacher's explanation

Think of the atmosphere as a giant chemical reactor. Usually, gases like nitrogen and oxygen just hang out, but when we add energy (sunlight) and pollutants (car exhaust), the rules change. Photochemical smog is like a 'chemical haze' created when sunlight breaks apart nitrogen dioxide, which then attacks unburned fuel to create ozone and other irritants.

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Nitrogen and sulfur