Zadanie 2c
Wzorowane na: Chemistry SL Paper 2, May 2023, TZ1, pytanie 2(c). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Polecenie
(i) State the electron domain geometry of the water molecule.
(ii) Deduce the Lewis (electron dot) structure of water and sketch its 3D molecular shape.
(iii) Explain, with reference to the forces between molecules, why water has a higher boiling point than hydrogen sulfide, H2S.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) tetrahedral «four electron domains» ✔
(ii) Lewis structure: O bonded to two H atoms by single bonds, with two lone pairs on O ✔
3D molecular shape: bent / V-shaped «H-O-H angle about 104.5°» ✔
(iii) water molecules form hydrogen bonds with each other «hydrogen bonding between H on one molecule and the lone pair on O of another»; H2S does not form hydrogen bonds ✔
both substances have London (dispersion) forces and dipole-dipole forces ✔
hydrogen bonds are stronger «than the forces between H2S molecules», so more energy is needed to separate the water molecules ✔
Schemat punktowania
(i) [1]. (ii) [2]: M1 Lewis structure (accept any combination of dots, crosses and lines; lone pairs required), M2 bent shape (lone pairs not required in the sketch). (iii) [3]: M1 hydrogen bonds in water but not in H2S; M2 London / dipole-dipole forces in both; M3 hydrogen bonds stronger. Accept the converse argument for H2S. Award M3 also for a statement that water is more polar so its dipole-dipole forces are stronger.
Komentarz
Students often write that hydrogen bonds are 'bonds' broken on boiling; only intermolecular forces are overcome, and the covalent O-H bonds stay intact.