Zadanie 1a
Wzorowane na: Chemistry HL Paper 2, May 2023, TZ1, pytanie 1(a). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Informacja do zadań 1a-1b.
Solutions of hydrobromic acid, HBr(aq), and methanoic acid, HCOOH(aq), of the same concentration were compared in a school laboratory.
Polecenie
(i) Deduce the ionic equation, including state symbols, for the reaction of hydrogen bromide gas with water.
(ii) Calculate the pH of 0.25 mol dm−3 hydrobromic acid.
(iii) Explain why a solution of methanoic acid has a higher pH than hydrobromic acid of the same concentration.
(iv) A pH probe can be used to distinguish between the two acids. Identify another simple instrumental method that could be used in a school laboratory to tell them apart.
(v) Outline how the method identified in part (iv) distinguishes between the two acids.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) HBr(g) + H2O(l) → H3O+(aq) + Br−(aq) «correct species, balanced» ✔
correct state symbols ✔
(ii) «pH = −log10[H+] = −log10 0.25 =» 0.60 ✔
(iii) methanoic acid ionizes only partially / has a lower [H+] than the fully ionized hydrobromic acid ✔
(iv) conductivity meter / conductivity probe
OR
ammeter «with a power supply» ✔
(v) HBr(aq) has the higher conductivity «because it has the higher concentration of ions» ✔
Schemat punktowania
(i) [2]: one mark for the equation, one for the state symbols. Do not accept an equilibrium arrow. Accept H+(aq) in place of H3O+(aq), with water then omitted from the left-hand side. (ii) [1] for 0.60 (two decimal places). (iii) [1]: 'weak acid' alone is not enough; accept the converse argument about hydrobromic acid. (iv) [1]: ignore any reference to indicators or chemical methods; accept an ion-selective probe. (v) [1]: accept the explanation that matches the method given in (iv); apply ECF for an incorrect method.