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Date May 2019 Marks available 1 Reference code 19M.2.sl.TZ2.3
Level SL Paper 2 Time zone TZ2
Command term Outline Question number 3 Adapted from N/A

Question

Dinitrogen monoxide, N2O, causes depletion of ozone in the stratosphere.

Different sources of N2O have different ratios of 14N:15N.

Outline why ozone in the stratosphere is important.

[1]
a.

State one analytical technique that could be used to determine the ratio of 14N:15N.

[1]
b(i).

A sample of gas was enriched to contain 2 % by mass of 15N with the remainder being 14N.

Calculate the relative molecular mass of the resulting N2O.

[2]
b(ii).

Predict, giving two reasons, how the first ionization energy of 15N compares with that of 14N.

[2]
b(iii).

Suggest why it is surprising that dinitrogen monoxide dissolves in water to give a neutral solution.

[1]
c.

Markscheme

absorbs UV/ultraviolet light «of longer wavelength than absorbed by O2»  [✔]

a.

mass spectrometry/MS  [✔]

b(i).

«  ( 98 × 14 ) + ( 2 × 15 ) 100 = » 14.02  [✔]

«Mr = (14.02 × 2) + 16.00 =» 44.04  [✔]

b(ii).

Any two:
same AND have same nuclear charge/number of protons/Zeff  [✔]

same AND neutrons do not affect attraction/ionization energy/Zeff
OR
same AND neutrons have no charge [✔]

same AND same attraction for «outer» electrons [✔]

same AND have same electronic configuration/shielding [✔]

 

Note: Accept “almost the same”.
“same” only needs to be stated once.

b(iii).

oxides of nitrogen/non-metals are «usually» acidic  [✔]

c.

Examiners report

60 % of the candidates were aware that ozone in the atmosphere absorbs UV light. Some candidates did not gain the mark for not specifying the type of radiation absorbed.

a.

Well answered. More than half of the candidates stated mass spectrometry is used to determine the ratio of the isotopes.

b(i).

Many candidates successfully calculated the relative atomic mass of nitrogen in the sample. M2 was awarded independently of M1, so candidates who calculated the relative molecular mass using the Ar of nitrogen in the data booklet (14.01) were awarded M2. Many candidates scored both marks.

b(ii).

This was a challenging question for many candidates, while stronger candidates often showed clarity of thinking and were able to conclude that the ionization energies of the two isotopes must be the same and to provide two different reasons for this. Some candidates did realize that the ionization energies are similar but did not give the best reasons to support their answer. Many candidates thought the ionization energies would be different because the size of the nucleus was different. Some teachers commented that the question was difficult while others liked it because it made students apply their knowledge in an unfamiliar situation. The question had a good discrimination index.

b(iii).

Only a quarter of the candidates answered correctly. Some simply stated that N2O forms HNO3 with water which did not gain the mark.

c.

Syllabus sections

Core » Topic 5: Energetics/thermochemistry » 5.3 Bond enthalpies
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