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.
State the factor that limits the maximum current that can be drawn from this cell and how electrodes are designed to maximize the current.
Explain how the proportion of 235U in natural uranium is increased.
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]
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]
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]