12.1. Nitrogen and sulfur
A subsection of Chemistry, 9701, through 12. Nitrogen and sulfur
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The exhaust systems of most modern gasoline‑fuelled cars contain a catalytic converter with three metal catalysts. These metals act as heterogeneous catalysts. Name three metal catalysts used in catalytic converters. Explain what is meant by a heterogeneous catalyst. The exhaust systems of many diesel‑fuelled cars contain an additional system to reduce vehicle emissions. This uses a liquid that is added to the exhaust system. This liquid contains urea, (NH2)2CO, which decomposes on heating to isocyanic acid, HNCO, and ammonia. reaction 1 (NH2)2CO HNCO + NH3 Isocyanic acid reacts with water vapour to form ammonia and carbon dioxide. reaction 2 HNCO+ H2ONH3+ CO2Some values for standard enthalpy changes of formation, ΔHf o, and standard entropies, S o, are given in Table 5.1. Table 5.1 compound ΔHf o / kJ mol–1 S o / J K–1 mol–1 HNCO–101.7 +238.2 H2O–241.8 +188.8 NH3–45.9 +192.8 CO2–393.5 +213.8 Explain what is meant by the term entropy of a system. Use the data in Table 5.1 to calculate ΔG o for reaction 2 at 25 °C. Show your working. ΔG o = kJ mol–1 The ammonia formed in reactions 1 and 2 can be used to remove nitrogen dioxide from exhaust emissions, as shown. reaction 3 8NH3 + 6NO2 7N2 + 12H2O Use the equations for reactions 1, 2 and 3 to construct an overall equation for the reduction of NO2 by (NH2)2CO. Isocyanic acid, HNCO, can form cyanuric acid, C3H3N3O3, under certain conditions. C3H3N3O3 has a cyclic structure containing alternating carbon and nitrogen atoms in the ring system. Suggest a structure for cyanuric acid. Isocyanic acid, HNCO, is a weak acid. HNCO + H2O H3O+ + NCO– pKa = 3.70 at 25 °C Write the mathematical expressions for pKa and pH. pKa = pH = Calculate the pH of 0.120 mol dm–3 HNCO. Give your answer to three significant figures. pH = Calculate the percentage of HNCO molecules that are ionised in 0.120 mol dm–3 HNCO. percentage ionisation of HNCO =
9701_s23_qp_41
THEORY
2023
Paper 4, Variant 1
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The exhaust systems of most modern gasoline‑fuelled cars contain a catalytic converter with three metal catalysts. These metals act as heterogeneous catalysts. Name three metal catalysts used in catalytic converters. Explain what is meant by a heterogeneous catalyst. The exhaust systems of many diesel‑fuelled cars contain an additional system to reduce vehicle emissions. This uses a liquid that is added to the exhaust system. This liquid contains urea, (NH2)2CO, which decomposes on heating to isocyanic acid, HNCO, and ammonia. reaction 1 (NH2)2CO HNCO + NH3 Isocyanic acid reacts with water vapour to form ammonia and carbon dioxide. reaction 2 HNCO+ H2ONH3+ CO2Some values for standard enthalpy changes of formation, ΔHf o, and standard entropies, S o, are given in Table 5.1. Table 5.1 compound ΔHf o / kJ mol–1 S o / J K–1 mol–1 HNCO–101.7 +238.2 H2O–241.8 +188.8 NH3–45.9 +192.8 CO2–393.5 +213.8 Explain what is meant by the term entropy of a system. Use the data in Table 5.1 to calculate ΔG o for reaction 2 at 25 °C. Show your working. ΔG o = kJ mol–1 The ammonia formed in reactions 1 and 2 can be used to remove nitrogen dioxide from exhaust emissions, as shown. reaction 3 8NH3 + 6NO2 7N2 + 12H2O Use the equations for reactions 1, 2 and 3 to construct an overall equation for the reduction of NO2 by (NH2)2CO. Isocyanic acid, HNCO, can form cyanuric acid, C3H3N3O3, under certain conditions. C3H3N3O3 has a cyclic structure containing alternating carbon and nitrogen atoms in the ring system. Suggest a structure for cyanuric acid. Isocyanic acid, HNCO, is a weak acid. HNCO + H2O H3O+ + NCO– pKa = 3.70 at 25 °C Write the mathematical expressions for pKa and pH. pKa = pH = Calculate the pH of 0.120 mol dm–3 HNCO. Give your answer to three significant figures. pH = Calculate the percentage of HNCO molecules that are ionised in 0.120 mol dm–3 HNCO. percentage ionisation of HNCO =
9701_s23_qp_43
THEORY
2023
Paper 4, Variant 3
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