Zadanie 7b, Paper 2, maj 2025, TZ3, Chemia IB SL
Wzorowane na: Chemistry SL Paper 2, May 2025, TZ3, pytanie 7(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
Nitrogen compounds affect the acidity of rain. Nitrogen dioxide, NO2, from vehicle engines is converted into nitric acid, while ammonia, NH3, released from fertilizers partly neutralizes the acid.
Polecenie
Nitrogen dioxide reacts with rain water and oxygen to form nitric acid.
(i) State an equation for this reaction, including state symbols.
(ii) Explain, with reference to the oxidation states of two elements, why this is a redox reaction.
(iii) Describe the effect on buildings made of limestone (calcium carbonate) of acid rain formed in this way. Include a chemical equation in your answer.
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) 4 NO2(g) + 2 H2O(l) + O2(g) → 4 HNO3(aq)
correct product AND state symbols for the reactants and the product ✔
correct balancing ✔
(ii) the oxidation number of nitrogen rises from «+»4 «in NO2» to «+»5 «in HNO3» ✔
the oxidation number of the oxygen atoms from O2 falls from 0 to «−»2 «once they end up in water and nitric acid» ✔
(iii) the limestone surface is pitted / roughened and gradually dissolves away «as the acid attacks it» ✔
CaCO3(s) + 2 HNO3(aq) → Ca(NO3)2(aq) + H2O(l) + CO2(g) ✔
Schemat punktowania
(i) M1: HNO3 as the product with the state symbols (g), (l), (g) and (aq); M2: balanced equation. Accept 2NO2(g) + H2O(l) + ½O2(g) -> 2HNO3(aq). (ii) [1] for the change in oxidation state of nitrogen (accept a stated increase of one unit, from +4 to +5); [1] for the change in oxidation state of oxygen (0 to -2); both an increase and a decrease are needed to justify 'redox'. (iii) [1] for a description of the visible effect on the limestone (surface roughening, loss of detail, dissolving, erosion all accepted); [1] for a correctly balanced equation with correct formulas (state symbols not essential).
Komentarz. One nitrogen atom is oxidized (+4 to +5) while the O2 that is consumed is reduced (0 to -2); both changes are needed to call the reaction redox.