DP Chemistry (last assessment 2024)

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Question 22M.2.hl.TZ2.c

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Date May 2022 Marks available [Maximum mark: 1] Reference code 22M.2.hl.TZ2.c
Level hl Paper 2 Time zone TZ2
Command term Suggest Question number c Adapted from N/A
c.
[Maximum mark: 1]
22M.2.hl.TZ2.c

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

CH4 (g) + NH3 (g) +32O2 (g) → HCN (g) + 3H2O (g)

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

[1]

Markscheme

HCN has stronger dipole–dipole attraction ✔

 

Do not accept reference to H-bonds.

Examiners report

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.

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