Zadanie 2d
Wzorowane na: Chemistry SL Paper 2, November 2024, pytanie 2(d). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Polecenie
Calcium reacts with water to form calcium hydroxide solution and hydrogen gas.
Ca(s) + 2 H2O(l) → Ca(OH)2(aq) + H2(g)
(i) Use the thermochemical equations in the table to calculate the enthalpy change, in kJ, when 8.00 g of calcium reacts completely with water.
| Thermochemical equation | ΔH / kJ mol-1 |
|---|---|
| −635.1 | |
| CaO(s) + H2O(l) → Ca(OH)2(aq) | −81.9 |
| 2 H2(g) + O2(g) → 2 H2O(l) | −571.6 |
(ii) Describe how the reaction of lithium with water differs from the reaction of potassium with water.
(iii) Demonstrate the acid-base character of CaO(s) by means of an equation.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) «third equation reversed and halved: , ΔH = +285.8 kJ mol−1» ✔
«ΔH = −635.1 + (−81.9) + 285.8 =» −431.2 «kJ mol−1 of Ca» ✔
«»
«enthalpy change = 0.1996 × (−431.2) =» −86.1 «kJ» ✔
(ii) lithium reacts more slowly / less vigorously / releases less heat «than potassium» ✔
(iii) CaO(s) + H2O(l) → Ca(OH)2(aq)
OR
CaO(s) + H2O(l) → Ca2+(aq) + 2 OH−(aq)
OR
CaO(s) + 2 H+(aq) → Ca2+(aq) + H2O(l) ✔
Schemat punktowania
(i) M1: the water-formation equation reversed and halved (+285.8), or an equivalent cycle; M2: enthalpy change per mole of calcium; M3: enthalpy change for 8.00 g. Award [3] for a correct final answer in the range −86.0 to −86.2 kJ. Apply ECF. (ii) Accept lithium: hydrogen does not ignite AND potassium: lilac flame / hydrogen ignites. A bare statement such as "lithium is less reactive" is not enough; the answer must say how the reaction with water differs. (iii) [1]; ignore state symbols.