DP Chemistry (first assessment 2025)

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Question SPM.2.HL.TZ0.2

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Date May Specimen-2023 Marks available [Maximum mark: 10] Reference code SPM.2.HL.TZ0.2
Level HL Paper 2 Time zone TZ0
Command term Deduce, Explain, Outline, Suggest Question number 2 Adapted from N/A
2.
[Maximum mark: 10]
SPM.2.HL.TZ0.2

Scientific models are used to explain the structure of matter.

(a)

An α-particle is a helium-4 nucleus. In an experiment, α-particles are accelerated towards a thin sheet of gold and their resulting paths are detected, giving evidence of the positive charge of the nucleus.

Angle of detection θ increase from 0° to 180°

The number of α-particles detected at different angles of deflection θ are shown.

(a.i)

Explain why some α-particles follow path II, rebounding from the gold sheet.

[1]

Markscheme

repelled by «hitting/close contact with» gold nucleus  ✔

(a.ii)

Most of the α-particles follow path I and pass straight through undeflected (θ = 0 °). Suggest a conclusion that can be made about the structure of the atom based on this evidence.

[1]

Markscheme

atom is mainly empty space/vacuum
OR
nucleus is very small «compared to the size of the atom»  ✔

(b)

Helium was first identified by analysing spectra of solar radiation.

(b.i)

Outline the appearance of the emission spectrum of helium.

[1]

Markscheme

discrete/series of lines of different frequency/wavelength  ✔

Emission spectra of one-electron systems can be explained using a model with the electron attracted to the nucleus by an electrostatic force.

This model predicts that the electron occupies discrete energy levels. Some energy levels for the He+ ion are shown.

(b.ii)

Explain how the frequencies observed in emission spectra support the idea of the electron occupying discrete energy levels.

[2]

Markscheme

energy of photon relates to a frequency «in the spectrum»  ✔ 
energy of photon depends on difference in energy levels  ✔

(b.iii)

Deduce the ionization energy of the He+ ion from the energy levels shown.

[1]

Markscheme

5250 «kJ mol-1»

(b.iv)

Suggest two reasons why the ionization energy of the hydrogen atom is significantly smaller than the ionization energy of the He+ ion.

[2]

Markscheme

H half/lower nuclear charge/number of protons  ✔
H larger/double radius  ✔

(b.v)

Suggest why the model outlined in (b)(ii) can predict the emission spectrum of He+ but not He.

[1]

Markscheme

electron-electron interactions «need to be taken in account»  ✔

(c)

Outline why models of the atom have evolved over time.

[1]

Markscheme

Any one of:
new evidence
new technology
developments in related models
models incomplete/failed to account for all observations  ✔