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Date	May 2022	Marks available	1	Reference code	22M.2.hl.TZ2.7
Level	HL	Paper	2	Time zone	TZ2
Command term	Identify	Question number	7	Adapted from	N/A

Question

The overall equation for the production of hydrogen cyanide, HCN, is shown below.

CH₄ (g) + NH₃ (g) + $\frac{3}{2}$ O₂ (g) → HCN (g) + 3H₂O (g)

State why NH₃ is a Lewis base.

[1]

a(i).

Calculate the pH of a 1.00 × 10⁻² mol dm⁻³ aqueous solution of ammonia.

pK_b = 4.75 at 298 K.

[3]

a(ii).

Justify whether a 1.0 dm³ solution made from 0.10 mol NH³ and 0.20 mol HCl will form a buffer solution.

[1]

a(iii).

Sketch the shape of one sigma ( $σ$ ) and one pi ( $π$ ) bond.

[2]

b(i).

Identify the number of sigma and pi bonds in HCN.

[1]

b(ii).

State the hybridization of the carbon atom in HCN.

[1]

b(iii).

Suggest why hydrogen chloride, HCl, has a lower boiling point than hydrogen cyanide, HCN.

[1]

Explain why transition metal cyanide complexes are coloured.

[3]

Markscheme

donates «lone/non-bonding» pair of electrons ✔

a(i).

Kb = 10^-4.75 /1.78 x 10^-5
OR
Kb = $\frac{{[{OH}^{-}]}^{2}}{[{NH}_{3}]}$ ✔

[OH^–] = « $\sqrt{(1.00 \times 10^{- 2} \times 10^{- 4.75})}$ =» 4.22 × 10^–4 «(mol dm^–3)» ✔

pOH« = –log₁₀ (4.22 × 10^–4)» = 3.37
AND
pH = «14 – 3.37» = 10.6

OR

[H⁺]« = $\frac{1.00 \times 10^{- 14}}{4.22 \times 10^{- 4}}$ » = 2.37 × 10^–11
AND
pH« = –log₁₀ 2.37 × 10^–11» = 10.6 ✔

Award [3] for correct final answer.

a(ii).

no AND is not a weak acid conjugate base system

no AND weak base «totally» neutralized/ weak base is not in excess

no AND will not neutralize small amount of acid ✔

Accept “no AND contains 0.10 mol NH₄Cl + 0.10 mol HCl”.

a(iii).

Sigma ( $σ$ ):

Pi ( $π$ ):

Accept overlapping p-orbital(s) with both lobes of equal size/shape.

Shaded areas are not required in either diagram.

b(i).

Sigma ( $σ$ ): 2 AND Pi ( $π$ ): 2 ✔

b(ii).

sp ✔

b(iii).

HCN has stronger dipole–dipole attraction ✔

Do not accept reference to H-bonds.

Any three from:

partially filled d-orbitals ✔

«CN- causes» d-orbitals «to» split ✔

light is absorbed as electrons transit to a higher energy level «in d–d transitions»
OR
light is absorbed as electrons are promoted ✔

energy gap corresponds to light in the visible region of the spectrum ✔

Do not accept “colour observed is the complementary colour” for M4.

Examiners report

The main error was the omission of lone electron "pair", though there was also a worrying amount of very confused answers for a very basic chemistry concept where 40% provided very incorrect answers.

a(i).

Rather surprisingly, many students got full marks for this multi-step calculation; others went straight to the pH/pKa acid/base equation so lost at least one of the marks: students often seem less prepared for base calculations, as opposed to acid calculations.

a(ii).

Poorly answered revealing little understanding of buffering mechanisms, which is admittedly a difficult topic.

a(iii).

[N/A]

b(i).

[N/A]

b(ii).

[N/A]

b(iii).

This proved to be the most challenging question (10%). It was a good question, where candidates had to explain a huge difference in boiling point of two covalent compounds, requiring solid understanding of change of state where breaking bonds cannot be involved). Yet most considered the triple bonds in HCN as the cause, suggesting covalent bonds break when substance boil, which is very worrying. Others considered H-bonds which at least is an intermolecular force, but shows they are not too familiar with the conditions necessary for H-bonding.

This question appears frequently in exams but with slightly different approaches. In general candidates ignore the specific question and give the same answers, failing in this case to describe why complexes are coloured rather than what colour is seen. These answers appear to reveal that many candidates don't really understand this phenomenon, but learn the answer by heart and make mistakes when repeating it, for example, stating that the ‘d-orbitals of the ligands were split’- an obvious misconception. The average mark was 1.6/3, with a MS providing 4 ideas that would merit a mark

Syllabus sections

Additional higher level (AHL) » Topic 14: Chemical bonding and structure » 14.1 Covalent bonding and electron domain and molecular geometries

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Additional higher level (AHL) » Topic 14: Chemical bonding and structure

Additional higher level (AHL)

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