A-LevelChemistryNitrogen and sulfur9701/22 · Feb/Mar 2019 · Q113 Marks

Nitrogen, N2, is the most abundant gas in the Earth's atmosphere and is very unreactive.

(a)State why N₂ is very unreactive.[1]
(b)(i)Write an equation for the reaction of magnesium with N₂ to form magnesium nitride.[1]
(b)(ii)Solid lithium nitride, Li3N, reacts with water according to the following equation. Li3N(s) + 3H2O(l) → 3LiOH(aq) + NH3(aq) State one observation you would make during this reaction.[1]
(c)(i)State the industrial importance of ammonia.[1]
(c)(ii)One method of producing NH3 is by heating ammonium chloride, NH4Cl, with CaO. 2NH4Cl + CaO → 2NH3 + CaCl2 + H2O Explain why the reaction of NH4Cl with CaO produces ammonia.[2]
(d)(i)Three oxides of nitrogen, NO, NO2 and N₂O, can be formed under different conditions. Complete the table to give the oxidation numbers of nitrogen in NO and NO2. compound | NO | NO2 --- | --- | --- oxidation number of N | | [1]
(d)(ii)NO2 can be formed by different chemical reactions. Write equations for the formation of NO₂ by: • the reaction of N₂ with O2 • the thermal decomposition of magnesium nitrate.[2]
(d)(iii)Molecules of N₂O can be formed by the reaction between N₂ and O2. The bond between the N and O atoms (N=O) is a co-ordinate (dative covalent) bond. 2N2(g) + O2(g) → 2N=N→O(g) The enthalpy change of reaction for this reaction is +82kJ mol-1. Calculate the bond enthalpy, in kJ mol⁻¹, of the N→O bond. Use relevant data from the Data Booklet to answer this question. bond enthalpy of the N→O bond = ............................... kJ mol⁻¹[2]
(e)Nitrosyl chloride, NOCl, is a reactive gas that is sometimes formed when NO reacts with Cl2. nitrosyl chloride [Figure 1.1: Structure of nitrosyl chloride O=Nδ+Clδ-] NOCl is a strong electrophile and readily undergoes an addition reaction with alkenes. Complete the diagram to show the mechanism of the electrophilic addition reaction of NOCl with ethene. Include all necessary charges, lone pairs and curly arrows, and the structure of the organic intermediate. [Figure 1.2: Incomplete reaction mechanism diagram showing ethene reacting with NOCl and intermediate structure][2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2019). The mark or mark type is shown in brackets.

[1] strong triple bond / strong N≡N OR high activation energy / Ea OR non-polar
[1] 3Mg + N2 → Mg3N2
[1] solid disappears
[1] (it is used to make) fertilisers
[2] M1 CaO displaces NH3 (from its salt / NH4+) M2 CaO is a stronger base / more basic (than NH3)
[1] NO: (+)2/(+)II NO2: (+)4/(+)IV
[2] M1 ½N2 + O2 → NO2 M2 Mg(NO3)2 → MgO + 2NO2 + ½O2
[2] M1 +82 (= EN≡O − 2EN→O) = (+)496 – 2 × EN→O M2 EN→O = ½ × (496–82) = ½ × 414 = 207 (kJ mol-1)
[2] M1 curly arrow from C=C to Nδ+ AND curly arrow from N—Cl to Clδ- M2 intermediate AND curly arrow from lone pair on Cl- to C(+)