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Date May 2018 Marks available 3 Reference code 18M.3.hl.TZ1.14
Level HL Paper 3 Time zone TZ1
Command term Explain Question number 14 Adapted from N/A

Question

Modern electric cars store their energy in lithium ion batteries.

The diagram represents a cell in such a battery delivering a current.

The carbon footprint of electric cars depends on how the electricity is produced. Nuclear fission of 235U is one source of electrical energy that has a minimal carbon footprint.

Complete the half-equations on the diagram and identify the species moving between the electrodes.

M18/4/CHEMI/HP3/ENG/TZ1/14.a.i

[3]
a.i.

State the factor that limits the maximum current that can be drawn from this cell and how electrodes are designed to maximize the current.

[2]
a.ii.

Explain how the proportion of 235U in natural uranium is increased.

[3]
b.ii.

Markscheme

 

Accept any balanced equation which shows Li oxidized to Li+ for M3, such as

LiC6 → Li+ + C6 + e–     or

LixC6 → xLi+ + 6C + xe

[3 marks]

a.i.

Limiting factor:

internal resistance «of the cell»

Electrodes design:

large surface area

 

Accept “time it takes ions to diffuse between electrodes”.

Accept specific ways of increasing surface area, such as “porous electrodes”.

Accept “close together/small separation”.

[2 marks]

a.ii.

uranium converted to uranium hexafluoride/UF6 gas

 

ALTERNATIVE 1:

gas «allowed to» diffuse

lower mass isotope/235U passes through more rapidly

 

ALTERNATIVE 2:

use of centrifuge

 

higher mass isotope/238U moves/closer to outside of centrifuge

OR

lower mass isotope/235U stays in/removed from middle of centrifuge

 

[3 marks]

b.ii.

Examiners report

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

Syllabus sections

Options » C: Energy » C.7 Nuclear fusion and nuclear fission (HL only)
Options » C: Energy
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