9701 · 12.1
Nitrogen and sulfur flashcards
Revision flashcards for Cambridge 9701 Nitrogen and sulfur (syllabus 12.1). Flip, recall, then mark a real past-paper question.
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Why is nitrogen gas (N₂) so unreactive?
It has a very strong covalent triple bond (N≡N) with a high bond enthalpy of +945 kJ mol⁻¹, requiring a large amount of energy to break.
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What are the conditions for the Haber process?
Pressure: ~200 atm (20,000 kPa), Temperature: ~400–450 °C, Catalyst: Iron (Fe).
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Write the balanced equation for the Haber process.
$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$
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How are nitrogen oxides (NOx) formed in a car engine?
The high temperature and pressure cause atmospheric nitrogen and oxygen to react: $N_2(g) + O_2(g) \rightarrow 2NO(g)$. NO is then further oxidised to $NO_2$.
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What is the primary source of atmospheric sulfur dioxide (SO₂)?
The combustion of fossil fuels (like coal and crude oil) that contain sulfur impurities. $S(in~fuel) + O_2(g) \rightarrow SO_2(g)$.
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What are the conditions for the key step in the Contact process?
Oxidation of $SO_2$ to $SO_3$. Catalyst: Vanadium(V) oxide ($V_2O_5$), Temperature: ~450 °C, Pressure: ~1-2 atm.
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How does sulfur dioxide contribute to acid rain?
$SO_2$ is oxidised in the atmosphere to $SO_3$. $SO_3$ then dissolves in water droplets to form sulfuric acid ($H_2SO_4$). $SO_3(g) + H_2O(l) \rightarrow H_2SO_4(aq)$.
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Describe the chemical test for ammonia gas (NH₃).
Ammonia is alkaline. It will turn damp red litmus paper blue. $NH_3(g) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$.
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Describe the chemical test for sulfur dioxide (SO₂).
Bubble the gas through acidified potassium dichromate(VI) solution. If $SO_2$ is present, the orange solution turns green as the dichromate(VI) ion ($Cr_2O_7^{2-}$) is reduced to the chromium(III) ion ($Cr^{3+}$).
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What is eutrophication?
The enrichment of bodies of water with nitrates and phosphates, often from fertiliser run-off. This causes excessive algal growth (algal bloom), which deoxygenates the water when it dies and decomposes, killing aquatic life.
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What is the role of a catalytic converter?
It converts harmful pollutants from car exhausts into less harmful substances. E.g., $2NO(g) + 2CO(g) \rightarrow N_2(g) + 2CO_2(g)$. The catalysts are typically platinum, rhodium, and palladium.