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Question 19M.2.HL.TZ2.11

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Date May 2019 Marks available [Maximum mark: 10] Reference code 19M.2.HL.TZ2.11
Level HL Paper 2 Time zone TZ2
Command term Calculate, Comment, Estimate, Suggest Question number 11 Adapted from N/A
11.
[Maximum mark: 10]
19M.2.HL.TZ2.11
(a)

Suggest why de Broglie’s hypothesis is not consistent with Bohr’s conclusion that the electron’s orbit in the hydrogen atom has a well defined radius.

[2]

Markscheme

«de Broglie’s hypothesis states that the» electron is represented by a wave ✔

therefore it cannot be localized/it is spread out/it does not have a definite position ✔

Award MP1 for any mention of wavelike property of an electron.

Examiners report

This question could have simply asked for the differences but did not puzzle the students who, when scoring, referred successfully to the differences between Bohr's postulate and de Broglie quantification of the wave like characteristics. Examiners marked this question without reference to particular physicist or what individuals suggested. As a result many scored the first mark for suggesting that electrons have wave like properties. The rest of the answers then commonly restated the stem of the question about a well-defined radius.

In an experiment to determine the radius of a carbon-12 nucleus, a beam of neutrons is scattered by a thin film of carbon-12. The graph shows the variation of intensity of the scattered neutrons with scattering angle. The de Broglie wavelength of the neutrons is 1.6 × 10-15 m.

(bi)

Estimate, using the graph, the radius of a carbon-12 nucleus.

[2]

Markscheme

« d sin ⁡ θ = λ ⇒ »  d = 1.6 × 10 − 15 sin ⁡ 17 ∘ / 5.47 × 10 − 15 «m» ✔

R = d 2 ≈ 2.7 / 2.8 × 10 − 15 «m» ✔

Examiners report

Many candidates answered this well but calculated the diameter rather than the radius.

(bii)

The ratio volume of a nucleaus of mass number  A volume of a nucleon is approximately A.

Comment on this observation by reference to the strong nuclear force.

[2]

Markscheme

this implies that the nucleons are very tightly packed/that there is very little space in between the nucleons ✔

because the nuclear force is stronger than the electrostatic force ✔

Examiners report

Many scored the first mark but it was rare to see answers that talked about the strong nuclear and electrostatic forces.

A pure sample of copper-64 has a mass of 28 mg. The decay constant of copper-64 is 5.5 × 10-2 hour–1.

(ci)

Estimate, in Bq, the initial activity of the sample.

[2]

Markscheme

number of nuclei is  28 × 10 − 3 64 × 6.02 × 10 23 / 2.63 × 10 20 ✔

A = « λ N = 2.63 × 10 20 × 5.5 × 10 − 2 3600 » 4.0 × 10 15 «Bq» ✔

Examiners report

Many scored the first mark for calculating the number of nuclei but neglected to convert λ to s-1.

(cii)

Calculate, in hours, the time at which the activity of the sample has decreased to one-third of the initial activity.

[2]

Markscheme

1 3 = e − λ t ✔

t = 20«hr» ✔

Examiners report

This was very well answered with the majority of candidates scoring full marks. This is a good indication that candidates weren’t short of time in this paper.