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Date November 2018 Marks available 2 Reference code 18N.3.hl.TZ0.12
Level HL Paper 3 Time zone TZ0
Command term Explain Question number 12 Adapted from N/A

Question

The Sun’s energy is produced by the fusion of hydrogen nuclei.

Uranium-238 produces plutonium-239, which is used as fuel in breeder reactors.

Nuclear energy produces ionizing radiation which leads to the formation of free radicals.

Explain fusion reactions with reference to binding energy.

[2]
a.

Outline why the term breeder is used for the reactors.

[1]
b.i.

Deduce the fission reaction when 239Pu is bombarded with a neutron to produce 133Xe and 103Zr.

[1]
b.ii.

Nuclear disasters release radioactive caesium into the atmosphere, which presents serious health risks.

Cs-137 has a half-life of 30 years.

Calculate the percentage of Cs-137 remaining in the atmosphere after 240 years.

[2]
c.

Deduce a Lewis (electron dot) structure of the superoxide, O2, free radical.

[1]
d.i.

Explain why free radicals are harmful to living cells.

[2]
d.ii.

Markscheme

small/lighter nuclei combine to form larger/heavier nuclei

product has higher binding energy «per nucleon» ✔

 

Accept binding energy curve with explanation.

a.

converts non-fissile «238U» material into fissile «239Pu» material

OR

produces more fissile material than it consumes ✔

b.i.

239Pu + 1n → 133Xe + 103Zr + 41n ✔

 

Accept equation with correct atomic numbers included.

Accept notation for neutrons of “n”.

Accept a correctly described equation in words.

b.ii.

ALTERNATIVE 1:

« 240 30 = » 8 t 1 2 /8 half-lives «required» ✔

% remaining = «0.508 × 100 =» 0.39 «%» ✔

 

ALTERNATIVE 2:

λ = « 0.693 30 = » 0.023 ✔

% remaining = «100 × e–0.023 × 240 =» 0.39 «%» ✔

 

Award [2] for correct final answer.

c.

OR

 

Accept any combination of dots, crosses and lines to represent electrons.

Do not penalize missing brackets.

Penalize missing negative charge.

d.i.

highly reactive

OR

start redox reactions ✔

 

damage/mutate DNA

OR

cause cancer

OR

damage enzymes ✔

d.ii.

Examiners report

[N/A]
a.
[N/A]
b.i.
[N/A]
b.ii.
[N/A]
c.
[N/A]
d.i.
[N/A]
d.ii.

Syllabus sections

Options » C: Energy » C.3 Nuclear fusion and fission
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