Zadanie 2a, Paper 2, listopad 2025, TZ1, Chemia IB SL
Wzorowane na: Chemistry SL Paper 2, November 2025, TZ1, pytanie 2(a). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
Polecenie
Methanoic acid, HCOOH(aq), and hydrochloric acid, HCl(aq), are both acids. A 0.100 mol dm−3 solution of HCOOH(aq) has a pH of 2.39 while a solution of HCl(aq) of the same concentration has a pH of 1.00.
(i) Explain the difference in pH.
(ii) Describe what would be observed if an excess of magnesium, Mg, was added to equal volumes of the same concentration of methanoic acid and hydrochloric acid.
(iii) A solution of methanoic acid contains two conjugate acid–base pairs. State the formulas of the conjugate acid and conjugate base in each pair.
(iv) State the oxidation state of carbon in methanoic acid, HCOOH.
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Odpowiedź z klucza
(i) HCl is a strong acid and ionizes/dissociates completely, so [H+] equals the acid's concentration «0.100 mol dm−3» ✔
HCOOH is a weak acid and only partially ionizes «an equilibrium is set up», so [H+] is lower than the acid's concentration, giving a higher pH ✔
(ii) both acids react «completely, since Mg is in excess» to give the same final volume of gas «and the same temperature rise», because both solutions contain the same amount, in mol, of acid ✔
bubbles of gas form faster / more vigorously with HCl than with HCOOH, because the initial [H+] is higher in the strong acid ✔
(iii) conjugate acid HCOOH«(aq)» AND conjugate base HCOO−«(aq)» ✔
conjugate acid H3O+«(aq)» AND conjugate base H2O«(l)» ✔
(iv) +2 ✔
Schemat punktowania
(i) M1: HCl fully ionizes so [H+] = concentration; M2: HCOOH only partially ionizes so [H+] is lower, giving a higher pH. (ii) M1: same final volume of gas / amount of product, because equal moles of acid are present; M2: faster / more vigorous reaction with HCl because of its higher initial [H+]. (iii) [1] for each pair; both members of the pair are needed for the mark. (iv) [1] for +2; accept (II); do not accept 2+ or 2.
Komentarz. The rate of reaction with Mg depends on [H+] at that instant, which is higher for the strong acid, but the total amount of H2 produced depends only on the total amount, in mol, of acid, which is the same for both solutions of equal concentration and volume.