12. Nitrogen and Sulfur

Question 1

Why is nitrogen relatively inert?

Answer 1

• It is normally found as N₂
• There is a very strong triple bond between the two nitrogen atoms with a bond enthalpy of 1000 kJ mol⁻¹
• As the two atoms are homogenous, a nitrogen molecule has no polarity
• As a consequence, nitrogen only reacts under extreme conditions

Question 2

Why is ammonia a base?

Answer 2

• Ammonia’s basicity (shown by how it turns damp red litmus paper blue) comes from its ability to accept a proton
• It is able to donate its lone pair to a hydrogen ion, forming a coordinate bond and becoming an ammonium ion

Question 3

How is the ammonium ion formed by an acid-base reaction?

Answer 3

• Ammonia reacts with water in the reversible reaction seen below
• NH₃ (aq) + H₂O (l) ⇌ NH₄⁺ (aq) + OH⁻ (aq)
• Since the equilibrium lies to the left, ammonia is considered a weak base

Question 4

How can ammonia gas be prepared from ammonium salts, using the example of ammonium chloride?

Answer 4

• React ammonium chloride with a salt with a more reactive cation like calcium hydroxide
• In the reaction, the ammonium ion loses a hydrogen to the hydroxide ion, so it acts as an acid while the hydroxide acts as a base
• 2NH₄Cl (s) + Ca(OH)₂ (s) –> CaCl₂ (s) + 2H₂O (l) + 2NH₃ (g)

Question 5

What are the natural and artificial occurences of oxides of nitrogen?

Answer 5

• Naturally, lightning can trigger the oxidation of nitrogen to form nitric oxide and nitrogen dioxide (N₂ (g) + O₂ (g) → 2NO (g) and N₂ (g) + 2O₂ (g) → 2NO₂ (g))
• Artifically, nitrogen can react with oxygen in a combustion engine to form nitrogen oxides (with the same equations)

In both situations, vast amounts of energy are supplied to initiate the reactions

Question 6

How are oxides of nitrogen removed from car engines?

Answer 6

• Through catalytic conversion
• The catalyst is usually a rare metal like platinum
• The nitrogen oxides are reduced to form nitrogen gas
• 2CO (g) + 2NO (g) → 2CO₂ (g) + N₂ (g)

Question 7

How is PAN produced in the atmosphere?

Answer 7

• Nitrogen oxides, a type of primary pollutant, react with other substances in the atmosphere to form more dangerous pollutants
• These other substances include volatile organic compounds (VOCs), which are unburned organic compounds (usually hydrocarbons) released by engines or power plants
• VOCs react with nitrogen oxides to form peroxyacetyl nitrate (PAN), a secondary pollutant harmful to life
• Sunlight provides the energy necessary for this reaction, so PAN forms part of what is known as photochemical smog

Question 8

How do nitrogen oxides directly contribute to acid rain?

Answer 8

• Nitric oxide (NO) oxidises to form nitrogen dioxide (NO₂), or NO₂ forms directly from nitrogen as a result of lightning or artificial combustion
• Nitrogen dioxide dissolves in water droplets and reacts with oxygen to form nitric acid
• This nitric acid falls as acid rain, damaging plant life and acidifying bodies of water

Question 9

How do nitrogen oxides catalytically contribute to acid rain?

Answer 9

• Nitrogen dioxide catalyses the oxidation of sulfur dioxide (found in the atmosphere as a result of burning sulfur impurities in fossil fuels) into sulfur trioxide
• NO₂ (g) + SO₂ (g) → SO₃ (g) + NO (g)
• The sulfur trioxide is able to dissolve in water to form sulfuric acid
• The nitrogen dioxide is considered a catalyst as it the produced nitric oxide reacts with oxygen to form nitrogen dioxide again