DP Physics (last assessment 2024)

Question 20N.2.HL.TZ0.6

Date	November 2020	Marks available	[Maximum mark: 12]	Reference code	20N.2.HL.TZ0.6
Level	HL	Paper	2	Time zone	TZ0
Command term	Calculate, Determine, Estimate, Outline, Show that, State, Suggest, Write down	Question number	6	Adapted from	N/A

[Maximum mark: 12]

20N.2.HL.TZ0.6

One possible fission reaction of uranium-235 (U-235) is

${}_{92}^{235}U + {}_{0}^{1}n \to {}_{54}^{140}{Xe} + {}_{38}^{94}{Sr} + 2 {}_{0}^{1}n$

Mass of one atom of U-235 $= 235 u$
Binding energy per nucleon for U-235 $= 7.59 MeV$
Binding energy per nucleon for Xe-140 $= 8.29 MeV$
Binding energy per nucleon for Sr-94 $= 8.59 MeV$

(a(i))

State what is meant by binding energy of a nucleus.

[1]

Markscheme

energy required to «completely» separate the nucleons
OR
energy released when a nucleus is formed from its constituent nucleons ✓

Allow protons AND neutrons.

Examiners report

Generally, well answered but candidates did miss the mark by discussing the constituents of a nucleus rather than the nucleons, or protons and neutrons. There seemed to be fewer comments than usual about 'the energy required to bind the nucleus together'.

(a(ii))

Outline why quantities such as atomic mass and nuclear binding energy are often expressed in non-SI units.

[1]

Markscheme

the values «in SI units» would be very small ✓

Examiners report

Well answered with some candidates describing the values as too large or small.

(a(iii))

Show that the energy released in the reaction is about $180 MeV$ .

[1]

Markscheme

$140 \times 8.29 + 94 \times 8.59 - 235 \times 7.59$ OR $184 « MeV »$ ✓

Examiners report

Well answered.

A nuclear power station uses U-235 as fuel. Assume that every fission reaction of U-235 gives rise to $180 MeV$ of energy.

(b(i))

Estimate, in $J {kg}^{- 1}$ , the specific energy of U-235.

[2]

Markscheme

see $« energy = » 180 \times 10^{6} \times 1.60 \times 10^{- 19}$ AND $« mass = » 235 \times 1.66 \times 10^{- 27}$ ✓

$7.4 \times 10^{13} « J {kg}^{- 1} »$ ✓

Examiners report

This caused problems for some with mass often correctly calculated but energy causing more difficulty with the eV conversion either being inaccurate or omitted. Candidates were allowed error carried forward for the second mark as long as they were dividing an energy by a mass.

(b(ii))

The power station has a useful power output of $1.2 GW$ and an efficiency of $36 %$ . Determine the mass of U-235 that undergoes fission in one day.

[2]

Markscheme

energy produced in one day $= \frac{1.2 \times 10^{9} \times 24 \times 3600}{0.36} = 2.9 \times 10^{14} « J »$ ✓

mass $= \frac{2.9 \times 10^{14}}{7.4 \times 10^{13}} = 3.9 « kg »$ ✓

Examiners report

Most candidates had the right idea, but common problems included forgetting the efficiency or not converting to days.

(b(iii))

The specific energy of fossil fuel is typically $30 MJ {kg}^{- 1}$ . Suggest, with reference to your answer to (b)(i), one advantage of U-235 compared with fossil fuels in a power station.

[1]

Markscheme

«specific energy of uranium is much greater than that of coal, hence» more energy can be produced from the same mass of fuel / per $kg$
OR
less fuel can be used to create the same amount of energy ✓

Examiners report

HL only. This was well answered.

A sample of waste produced by the reactor contains $1.0 kg$ of strontium-94 (Sr-94). Sr-94 is radioactive and undergoes beta-minus ( $β^{-}$ ) decay into a daughter nuclide X. The reaction for this decay is

${}_{38}^{94}{Sr} \to X + {\bar{v}}_{e} + e$ .