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Question 19N.2.hl.TZ0.1

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Date November 2019 Marks available [Maximum mark: 12] Reference code 19N.2.hl.TZ0.1
Level hl Paper 2 Time zone TZ0
Command term Determine, Discuss, Draw, Formulate, Identify, Outline, Predict, Show Question number 1 Adapted from N/A
1.
[Maximum mark: 12]
19N.2.hl.TZ0.1

The equations show steps in the formation and decomposition of ozone in the stratosphere, some of which absorb ultraviolet light.


Step 1    O2 → 2O•

Step 2    O• + O2 → O3

Step 3    O3 → O• + O2

Step 4    O• + O3 → 2O2

(a)

Draw the Lewis structures of oxygen, O2, and ozone, O3.

[2]

Markscheme

NOTES: Coordinate bond may be represented by an arrow.

Do not accept delocalized structure for ozone.

(b)

Outline why both bonds in the ozone molecule are the same length and predict the bond length in the ozone molecule. Refer to section 10 of the data booklet.

Reason: 

Length:

[2]

Markscheme

resonance «structures»
OR
delocalization of «the double/pi bond» electrons ✔
121 «pm» < length < 148 «pm» ✔

NOTE: Accept any length between these two values.

(c)

Predict the bond angle in the ozone molecule.

[1]

Markscheme

any value from 110°–119° ✔

(d)

Discuss how the different bond strengths between the oxygen atoms in O2 and O3 in the ozone layer affect radiation reaching the Earth’s surface.

[2]

Markscheme

«bond» in O2 stronger than in O3 ✔


ozone absorbs lower frequency/energy «radiation than oxygen»
OR
ozone absorbs longer wavelength «radiation than oxygen» ✔

 

NOTE: Accept ozone «layer» absorbs a range of frequencies.

(e(i))

Identify the steps which absorb ultraviolet light.

[1]

Markscheme

steps 1 AND 3 ✔

(e(ii))

Determine, showing your working, the wavelength, in m, of ultraviolet light absorbed by a single molecule in one of these steps. Use sections 1, 2 and 11 of the data booklet.

[2]

Markscheme

ALTERNATIVE 1:
for oxygen:

E=«498 000 J mol−16.02×1023 mol−1=» 8.27×10−19 «J» ✔

λ = «6.63×10−34 J s×3.00×108 m s−18.27×10−19 J=» 2.40×10−7 «m»✔

 

ALTERNATIVE 2:
for ozone:

similar calculation using 200 < bond enthalpy < 400 for ozone, such as

E=«300 000 J mol−16.02×1023 mol−1=» 4.98×10−19«J» ✔

λ = «6.63×10−34 J s×3.00×108 m s−14.98×10−19 J=» 3.99×10−7 «m»✔

 

NOTE: Award [2] for correct final answer.

(f)

Ozone depletion is catalysed by nitrogen monoxide, NO, which is produced in aircraft and motor vehicle engines, and has the following Lewis structure.

Show how nitrogen monoxide catalyses the decomposition of ozone, including equations in your answer.

[2]

Markscheme

•NO + O3 → •NO2 + O2 ✔

•NO2 + O3 → •NO + 2O2 ✔

NOTE: Accept •NO2 → •NO + •O AND •O + O3 → 2O2 for M2.

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