Zadanie 2b, Paper 2, specimen 2025, Chemia IB SL
Wzorowane na: Chemistry SL Paper 2, specimen 2025, pytanie 2(b). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
Wspólne informacje do wiązki zadań
Scientific models of the structure of the atom have been developed and revised as new experimental evidence became available.
Polecenie
Lithium can be identified in the light emitted by stars and by flames containing lithium compounds.
(i) Outline the appearance of the emission spectrum of lithium.
The emission spectrum of a one-electron species, such as the Li2+ ion, can be explained by a model in which the single electron is held by the electrostatic attraction of the nucleus and can occupy only certain discrete energy levels. Some of the energy levels of the Li2+ ion are shown.
Powiększ(ii) Explain how the frequencies of the lines observed in an emission spectrum support the idea that the electron occupies discrete energy levels.
The ionization energy of the Li2+ ion is 11 810 kJ mol−1, whereas the ionization energy of the hydrogen atom is 1312 kJ mol−1.
(iii) Suggest two reasons why the hydrogen atom has a much smaller ionization energy than the Li2+ ion.
(iv) Suggest why the model described above can predict the emission spectrum of Li2+ but not the emission spectrum of the lithium atom, Li.
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Odpowiedź z klucza
(i) a series of separate/discrete lines of different frequencies/wavelengths «on a dark background» ✔
(ii) each line corresponds to photons of one particular frequency/energy «E = hf» ✔
the energy of the photon equals the difference between two energy levels, so only certain frequencies can be emitted ✔
(iii) hydrogen has a smaller nuclear charge / fewer protons «+1 compared with +3» ✔
the electron in hydrogen is further from the nucleus / hydrogen has a larger radius «about three times larger» ✔
(iv) the lithium atom has more than one electron, so repulsion between electrons «must also be taken into account» ✔
Schemat punktowania
(i) [1] discrete lines. (ii) [2]: M1 line corresponds to a photon of a specific frequency (or energy); M2 photon energy equals a difference between two levels. (iii) [2]: one mark for nuclear charge, one for distance/radius. (iv) [1] electron-electron repulsion or more than one electron.
Komentarz. The evidence is in the frequencies (E = hf), not in the colour or brightness of the lines; the value 1312 kJ mol⁻¹ for hydrogen is the one listed in section 9 of the data booklet.