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Date	November 2020	Marks available	2	Reference code	20N.2.sl.TZ0.2
Level	SL	Paper	2	Time zone	TZ0
Command term	Predict	Question number	2	Adapted from	N/A

Question

Compound A is in equilibrium with compound B.

Predict the electron domain and molecular geometries around the oxygen atom of molecule A using VSEPR.

[2]

a.

The IR spectrum of one of the compounds is shown:

COBLENTZ SOCIETY. Collection © 2018 copyright by the U.S. Secretary of Commerce on behalf of the United States of America. All rights reserved.

Deduce, giving a reason, the compound producing this spectrum.

[1]

b.

Compound A and B are isomers. Draw two other structural isomers with the formula $C_{3} H_{6} O$ .

[2]

c.

The equilibrium constant, $K_{c}$ , for the conversion of A to B is $1.0 \times 10^{8}$ in water at $298 K$ .

Deduce, giving a reason, which compound, A or B, is present in greater concentration when equilibrium is reached.

[1]

d.

Markscheme

Electron domain geometry: tetrahedral ✔

Molecular geometry: bent/V-shaped ✔

a.

B AND $C = O$ absorption/ $1750 « {cm}^{- 1} »$
OR
B AND absence of $O - H / 3200 - 3600 « {cm}^{- 1} absorption »$ ✔

Accept any value between $1700 - 1750 c m^{- 1}$ .

b.

Accept any two $C_{3} H_{6} O$ isomers except for propanone and propen-2-ol:

✔✔

Penalize missing hydrogens in displayed structural formulas once only.

c.

$B$ AND $K_{c}$ is greater than $1$ /large ✔

d.

Examiners report

Half of the candidates answered correctly. The rest of the candidates often answered the question in terms of the carbon atom indicating that they did not read the question carefully.

a.

About 50% of the candidates answered correctly. Quite a few, however, gave compounds other than A or B, indicating not reading the question properly or being confused by the skeletal formulas given in the question.

b.

Nearly half of the candidates gave two correct isomers. Propanal was often given as one of the isomers. Some candidates repeated the compounds given in the question and a few gave structures with 5 bonds on a carbon atom.

c.

Half of the candidates answered correctly. A common mistake was K > 0 instead of K > 1.

d.

Syllabus sections

Core » Topic 4: Chemical bonding and structure » 4.3 Covalent structures

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Core » Topic 4: Chemical bonding and structure