Zadanie 3e
Wzorowane na: Chemistry HL Paper 2, November 2024, pytanie 3(e). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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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) Calculate the enthalpy change, in kJ, when 5.00 g of calcium reacts completely with water. Use section 13 of the data booklet. ΔHf⊖ of Ca(OH)2(aq) is −1002.8 kJ mol−1.
(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) ΔHf⊖(H2O(l)) = −286 kJ mol−1 ✔
«ΔH⊖ = ΔHf⊖(products) − ΔHf⊖(reactants)»
«ΔH⊖ = (−1002.8 + 0) − (0 + 2 × (−286)) =» −430.8 «kJ mol−1 of Ca» ✔
«»
«enthalpy change = 0.12475 × (−430.8) =» −53.7 «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: enthalpy of formation of water from the data booklet (may be awarded from working); M2: enthalpy change per mole of calcium; M3: enthalpy change for 5.00 g. Award [3] for a correct final answer in the range −53.6 to −53.8 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.