Zadanie 6b, Paper 2, maj 2025, TZ1, Chemia IB HL
Wzorowane na: Chemistry HL Paper 2, May 2025, TZ1, pytanie 6(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
The rate of the reaction between acidified potassium manganate(VII) and sodium ethanedioate, Na2C2O4, can be measured by timing the colour changes. When ethanedioate ions are in excess, an unstable red-brown manganese(III) complex forms before the colour fades.
Polecenie
Both ionic and covalent bonding are present in sodium ethanedioate, Na2C2O4.
(i) Describe both types of chemical bonding present in this compound.
Ionic bonding:
Covalent bonding:
(ii) Deduce a Lewis formula of the ethanedioate ion, C2O42−.
(iii) State the geometry of the bonds around each carbon atom in the ethanedioate ion.
(iv) Using section 11 of the data booklet, predict what the carbon-oxygen bond lengths in the ethanedioate ion should be, and justify your prediction.
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Odpowiedź z klucza
(i) Ionic bonding: electrostatic attraction between oppositely charged ions / between Na+ cations and C2O42− anions ✔
Covalent bonding: electrostatic attraction between a shared pair of electrons and the positively charged nuclei «of the carbon and oxygen atoms» ✔
(ii) «» two carbon atoms joined by a single bond; each carbon bonded to one oxygen by a double bond «oxygen with two lone pairs» and to one oxygen by a single bond «oxygen with three lone pairs and a negative charge»; 17 electron pairs in total AND the 2− charge shown ✔
(iii) trigonal planar ✔
(iv) all four C-O bonds have the same length AND between 122 and 143 «×10−12 m» / about 125-130 «×10−12 m» ✔
because of delocalized electrons / resonance «the double bond can be in either position on each carbon; bond order 1.5» ✔
Schemat punktowania
(i) [1] per description; penalize a missing «electrostatic attraction» once only; «transfer of electrons» alone does not describe the ionic bond. (ii) [1] for a formula with all lone pairs and the charge; accept lines, dots or crosses; do not accept a resonance hybrid with dashed bonds. (iii) [1]; accept triangular planar; ECF from an incorrect Lewis formula. (iv) M1: equal lengths within the given range; M2: delocalization or resonance.