DP Chemistry (first assessment 2025)

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Question 19N.2.SL.TZ0.1

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Date November 2019 Marks available [Maximum mark: 7] Reference code 19N.2.SL.TZ0.1
Level SL Paper 2 Time zone TZ0
Command term Discuss, Distinguish, Draw, Outline, Predict Question number 1 Adapted from N/A
1.
[Maximum mark: 7]
19N.2.SL.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)

Distinguish ultraviolet light from visible light in terms of wavelength and energy.

[1]

Markscheme

«UV» shorter wavelength AND higher energy «than visible» ✔

(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.

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