Nitrogen and Sulfur (Cambridge (CIE) A Level Chemistry): Flashcards
Exam code: 9701
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Why is nitrogen gas very unreactive?
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Why is nitrogen gas very unreactive?
Nitrogen forms a triple covalent bond (N≡N) with a bond enthalpy of 1000 kJ mol-1. This very strong bond requires a large amount of energy to break. Nitrogen is also nonpolar, making it unlikely to react with other molecules.
True or False?
Nitrogen gas is polar because it contains a triple bond.
False.
Nitrogen gas (N≡N) is nonpolar. The two identical nitrogen atoms have the same electronegativity, so the electrons are shared equally and there is no overall dipole.
Ammonia is manufactured using the Haber process: N2 (g) + 3H2 (g) ⇌ .......... (g). The reaction is ...........
Ammonia is manufactured using the Haber process: N2 (g) + 3H2 (g) ⇌ 2NH3 (g). The reaction is reversible.
Define Brønsted-Lowry base.
A Brønsted-Lowry base is a proton (H+) acceptor. Ammonia acts as a Brønsted-Lowry base, using its lone pair on nitrogen to accept a proton and form the ammonium ion (NH4+).
Describe the shape of ammonia (NH3) and the ammonium ion (NH4+), and explain the difference.
NH3 has a pyramidal shape due to the lone pair of electrons on nitrogen repelling the three bonding pairs. NH4+ has a tetrahedral shape because the lone pair has been donated to form a dative bond with H+, leaving four equal bonds.
True or False?
Ammonia is a strong base because it reacts completely with water.
False.
Ammonia is a weak base. The equilibrium NH3 (aq) + H2O (l) ⇌ NH4+ (aq) + OH- (aq) lies well to the left, so only a small proportion of NH3 molecules ionise.
Ammonia turns damp .......... litmus paper .......... because it is an alkaline gas.
Ammonia turns damp red litmus paper blue because it is an alkaline gas.
Write the equation for the preparation of ammonia gas from ammonium chloride and calcium hydroxide.
2NH4Cl (s) + Ca(OH)2 (s) → CaCl2 (s) + 2H2O (l) + 2NH3 (g) (heat required)
NH4+ acts as the acid (proton donor) and OH- as the base (proton acceptor).
How is the presence of ammonium ions in an unknown solution confirmed?
The unknown solution is warmed with calcium hydroxide. If ammonium ions are present, ammonia gas is released. The gas turns damp red litmus paper blue, confirming the presence of NH4+ ions.
How are nitrogen oxides formed in car engines?
Inside a car engine, the high temperature and pressure cause atmospheric nitrogen and oxygen to react, forming nitrogen oxides (NO and NO2). These are released into the atmosphere through exhaust fumes.
True or False?
Catalytic converters convert nitrogen oxides into harmless nitrogen gas.
True.
In a catalytic converter, NO is reduced on the surface of a hot metal catalyst (e.g. platinum): 2CO (g) + 2NO (g) → 2CO2 (g) + N2 (g). N2 is released as a harmless gas.
VOCs react with nitrogen oxides in sunlight to form .......... (PAN), a harmful component of .......... smog.
VOCs react with nitrogen oxides in sunlight to form peroxyacetyl nitrate (PAN), a harmful component of photochemical smog.
Define secondary pollutant.
A secondary pollutant is one that is not emitted directly from a source but is formed in the atmosphere through chemical reactions involving primary pollutants. Peroxyacetyl nitrate (PAN) is an example, formed from nitrogen oxides and VOCs in sunlight.
Write the equation showing how NO2 and water contribute to the formation of acid rain.
4NO2 (g) + 2H2O (l) + O2 (g) → 4HNO3 (aq)
The nitric acid dissolves in cloud water droplets. When these fall as rain, the result is acid rain.
True or False?
NO2 acts as a catalyst in the conversion of SO2 to SO3.
True.
NO2 catalyses the oxidation of SO2: NO2 (g) + SO2 (g) → SO3 (g) + NO (g). The NO is then re-oxidised to NO2, regenerating the catalyst. SO3 reacts with water to form H2SO4, contributing to acid rain.
NO2 catalyses the oxidation of SO2 to .......... , which reacts with water to form .......... acid in acid rain.
NO2 catalyses the oxidation of SO2 to SO3, which reacts with water to form sulfuric acid in acid rain.
Distinguish between primary and secondary pollutants, giving one example of each.
A primary pollutant is emitted directly from a source (e.g. NO from car exhausts). A secondary pollutant forms through atmospheric reactions between primary pollutants (e.g. PAN, formed from VOCs and nitrogen oxides in sunlight).
Write the equation for the reaction of NO with oxygen in the atmosphere and explain its environmental significance.
2NO (g) + O2 (g) ⇌ 2NO2 (g)
NO2 is a more reactive pollutant than NO. It contributes to acid rain by reacting with water and oxygen to form HNO3, and to photochemical smog by reacting with VOCs in sunlight.
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