Nitrogen and sulfur

• Strong, non-polar bond: inert. Lone pair: base. Air plus heat: NOₓ. NO₂ used and given back: catalyst.
• 944 and non-polar. Lone pair takes the proton; tetrahedral, 109.5°. Air nitrogen, engine heat. NO₂ used, NO₂ given back.

• 12.1.1 I can explain — explain the lack of reactivity of nitrogen, with reference to triple bond strength and lack of polarity
• 12.1.2 I can describe — describe and explain: (a) the basicity of ammonia, using the Brønsted–Lowry theory (b) the structure of the ammonium ion and its formation by an acid–base reaction (c) the displacement of ammonia from ammonium salts by an acid–base reaction
• 12.1.3 I can state — state and explain the natural and man-made occurrences of oxides of nitrogen and their catalytic removal from the exhaust gases of internal combustion engines
• 12.1.4 I can understand — understand that atmospheric oxides of nitrogen (NO and NO2) can react with unburned hydrocarbons to form peroxyacetyl nitrate, PAN, which is a component of photochemical smog
• 12.1.5 I can describe — describe the role of NO and NO2 in the formation of acid rain both directly and in their catalytic role in the oxidation of atmospheric sulfur dioxide

Units and significant figures are marked. The syllabus states that failure to quote units, the inclusion of units in quantities defined as ratios, and answers given to an inappropriate number of significant figures are all liable to be penalised. Give a calculated answer to the same number of significant figures as the least precise data, or one more; keep full precision in the working and round only at the end. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so the practical work in this chapter is set out as procedure, recording and evaluation rather than as theory.

• “NH₄⁺ is pyramidal, like ammonia, with a lone pair.” Correct NH₄⁺ is tetrahedral; the lone pair is gone. It became the fourth N–H bond when it accepted the proton, so there are four bonding pairs, no lone pair, and every bond angle is 109.5°.
• “Nitrogen is unreactive because it has a full outer shell.” Correct A nitrogen atom has five outer electrons. It is the N₂ molecule that is unreactive: the N≡N bond is very strong (944 kJ mol⁻¹) and non-polar.
• “Ammonia is a base because it makes OH⁻ ions.” Correct By the Brønsted–Lowry theory a base is a proton acceptor. Ammonia accepts a proton onto its lone pair, even with no water present: NH₃(g) + HCl(g) → NH₄Cl(s).
• “In NH₄⁺ + OH⁻ → NH₃ + H₂O the ammonium ion is the base.” Correct NH₄⁺ gives up a proton, so it is the acid; OH⁻ accepts it, so it is the base.
• “Oxides of nitrogen come from the nitrogen in petrol.” Correct The nitrogen comes from the air drawn into the engine. At the high temperature in the cylinder, N₂ and O₂ from the air combine.
• “SO₂ dissolves in rain to give sulfuric acid.” Correct SO₂ gives sulfurous acid, H₂SO₃, a weak acid. Sulfuric acid needs SO₃, and it is the oxidation of SO₂ to SO₃ that NO₂ catalyses.
• “Nitrogen is unreactive because it has a full outer shell.” Repair Each nitrogen atom has five outer electrons. The molecule is unreactive because the N≡N bond is very strong (944 kJ mol⁻¹), giving a very high activation energy, and non-polar, giving no δ+ or δ− site.
• “Nitrogen is unreactive because its reaction with hydrogen is endothermic.” Repair It is exothermic, about −88 kJ mol⁻¹ from bond energies. The barrier is the activation energy of breaking the triple bond, not the enthalpy change.
• “Ammonia is a base because it produces OH⁻ ions.” Repair By Brønsted–Lowry, ammonia is a base because the lone pair on its nitrogen accepts a proton; it does so even with no water present, in NH₃(g) + HCl(g) → NH₄Cl(s).
• “NH₄⁺ is pyramidal with a lone pair, angle 107°.” Repair The lone pair became the fourth bond. NH₄⁺ has four bonding pairs and no lone pair: tetrahedral, 109.5°, all four bonds identical.
• “In the dot-and-cross diagram of NH₄⁺, every hydrogen has a cross.” Repair The fourth hydrogen arrived as H⁺ with no electron, so its shared pair is two dots from nitrogen: five dots and three crosses, eight electrons around N.
• “In NH₄⁺ + OH⁻ → NH₃ + H₂O the ammonium ion is the base.” Repair NH₄⁺ is the acid (proton donor); OH⁻ is the base. Ammonia is displaced because hydroxide is the stronger base.
• “Oxides of nitrogen come from the nitrogen in petrol.” Repair They come from the nitrogen in the air reacting with oxygen at the high temperature inside the engine: N₂ + O₂ → 2NO.
• “In the converter the catalyst converts NO into CO₂.” Repair NO is reduced to N₂ and CO is oxidised to CO₂: 2NO + 2CO → N₂ + 2CO₂. The platinum, palladium and rhodium catalyst is not consumed.
• “PAN is made in the engine.” Repair PAN forms in the atmosphere, from NO/NO₂ and unburned hydrocarbons, in sunlight.
• “SO₂ dissolves in rain to give sulfuric acid.” Repair SO₂ gives the weak sulfurous acid, H₂SO₃. Sulfuric acid needs the oxidation of SO₂ to SO₃ first, which NO₂ catalyses.
• “NO₂ is a reactant in the oxidation of SO₂.” Repair It is a catalyst: used in SO₂ + NO₂ → SO₃ + NO and regenerated in 2NO + O₂ → 2NO₂, so it does not appear in the overall 2SO₂ + O₂ → 2SO₃.
• “2NO₂ + H₂O → 2HNO₃.” Repair That does not balance (O: 5 on the left, 6 on the right). 2NO₂ + H₂O → HNO₂ + HNO₃, a disproportionation (+4 → +3 and +5); nitric acid alone needs oxygen: 4NO₂ + O₂ + 2H₂O → 4HNO₃.
• “Use thymolphthalein for the ammonia titration.” Repair The equivalence point of a strong acid – weak base titration is below pH 7; thymolphthalein (9.3–10.5) changes long before it. Use methyl orange or bromophenol blue.

• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.
• Two reasons, each with its consequence. An explanation that gives only “strong triple bond” has answered half the outcome. The syllabus asks for bond strength and lack of polarity, and each needs its consequence: high activation energy, and no site for attack.
• Say where the proton goes. “Ammonia is a base because it accepts a proton” is the definition. “…because the lone pair on nitrogen accepts a proton” is the explanation the outcome asks for.
• Dative going in, identical once there. The dative bond describes where the electrons came from. It does not make the fourth bond different, so the ion is a regular tetrahedron, and a diagram that draws one bond longer or at a different angle is wrong.
• Interleave with the chapters that use this one. Topic 12 comes back in topic 25 (acid–base equilibria: return to why ammonia is a weak base and why an ammonium salt solution is acidic), topic 26 (reaction kinetics: re-answer how NO₂ acts as a catalyst in the oxidation of SO₂), and in the organic chapters on amines, where the lone pair on nitrogen makes them bases for the same reason as ammonia. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Chemistry 9701 has five components. Topic 12 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E.
• A Paper 1 item on topic 12 can turn on a single distinction: the shape of NH₄⁺ against NH₃, which species is the acid in a proton transfer, which oxide is reduced in the converter, or the oxidation-number change of sulfur. Paper 2 can ask you to explain the inertness of nitrogen with both reasons, to describe the ammonium ion and its formation, and to write balanced equations with state symbols for the formation and removal of NOₓ and for both routes to acid rain.
• The numbers come from other topics used in this setting: an enthalpy change from bond energies (topic 5), gas volumes from ammonium salts and masses from sulfur dioxide (topic 2), and electron counts from oxidation numbers (topic 6). The bond energies, electronegativities, the molar gas volume and the Ar values are supplied in the Data section and Periodic Table; the equations and the reasons are not.
• Topic 12 supplies the titration of aqueous ammonia with hydrochloric acid: burette readings to 0.05 cm³, concordant titres within 0.10 cm³, the choice of methyl orange or bromophenol blue, and the loss of volatile ammonia as a systematic error. It also supplies the ammonium and nitrate tests from the qualitative-analysis notes, including why the nitrate test cannot be trusted while ammonium is present.
• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.