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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 Draw 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:

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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 C3H6O.

[2]
c.

The equilibrium constant, Kc, for the conversion of A to B is 1.0×108 in water at 298K.

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«cm1» 
OR
B AND absence of OH /32003600«cm1 absorption» ✔


Accept any value between 17001750 cm1
.

b.

Accept any two C3H6O isomers except for propanone and propen-2-ol:

✔✔

 

Penalize missing hydrogens in displayed structural formulas once only.

c.

B AND Kc 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 10: Organic chemistry » 10.1 Fundamentals of organic chemistry
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