Unit 12: Unit 12: Nitrogen and Sulfur — Short Questions

The bond present in nitrogen molecule has a bond enthalpy of +944 kJmol⁻¹. High energy is required to break this bond to form new bonds, making N₂ very unreactive.

The second reason for its lack of reactivity is the nonpolarity of its bond. Both the atoms are the same having zero electronegativity difference. This causes equal sharing of the three bonded electrons between the two atoms making the bond nonpolar

In industrial processes, nitrogen is typically obtained by cooling air until it becomes a liquid. In laboratory settings, nitrogen can be generated by slowly heating a solution of ammonium nitrite.

NH₄NO₂(aq) → N₂(g) + H₂O(l)

Ammonia solution is a weak base due to the low basicity constant (Kb) and the equilibrium position being towards the far left side.

NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq), Kb = 1.8 × 10⁻⁵

N₂(g) + O₂(g) → 2NO(g)
2NO(g) + O₂(g) → 2NO₂(g)
NO₂(g) --hv--> NO(g) + O(g)
O(g) + O₂(g) → O₃(g)
RCH = RCH(g) + O₃(g) →
RCO₂(g) + RCH₂(g) → RC(O)OONO₂

Construction
A catalytic converter is a ceramic or metallic monolith with a honeycomb-like structure. Its inner channels have a layer of alumina to provide a high surface area. Noble expensive metals such as Pt (Platinum), Pd (Palladium), and Rh (Rhodium) are dispersed on the alumina.

Working
These metals catalyze three redox reactions to remove the half harmful exhaust gases. The three-way converter converts harmful CO, NO, and hydrocarbons into CO₂, N₂, and water. These precious metals can also be recycled.

At room temperature it occurs as octasulfur which is quite unreactive and has a high bond strength

At pH = 0 (strongly acidic)
• Most stable oxidation state: +b
• Common sulfur species:
• H₂SO₄
• H₂SO₃
• SO₄²⁻

At pH = 14 (strongly acidic)
• Most stable oxidation state: -2
• Common sulfur species:
• S²⁻
• HS⁻

Sulfur reacts directly with F₂ to form SF₆ and SF₆. Sulfur hexafluoride (SF₆) is a gas and is very unreactive. It is used as an insulator gas in electric devices. Sulfur readily reacts with Cl₂ to form S₂Cl₂ (yellow liquid) which further reacts with Cl₂ to form SCl₂ (red liquid).

S(s) + 3F₂(g) → SF₆(g)
2S(s) + Cl₂(g) → S₂Cl₂(l)
S₂Cl₂(l) + S(s) → 2SCl₂(l)

Cyclo-octasulfur, (S₈) crown molecule

Sulfuric acid
(142.2 pm and 157.4 pm and 97 pm and H bond lengths shown)

Sulphur is used as a cross linker for the rubber molecular chains. This is called vulcanization and it improves the strength of rubber

Gun powder is a coarse blend of 75% potassium nitrate (KNO₃), 15% wood charcoal, and 10% sulfur. Charcoal carbon is the main fuel, nitrate is the oxidizer and sulfur is the additional fuel that burns the powder faster. The following reaction in burning takes place:

10KNO₃(s) + 3S(s) + 8C(s) →
2K₂CO₃(s) + 3K₂SO₄(aq) + 6CO₂(g) + 5N₂(g)

Disulfide bridges are crucial in stability of large organic molecules including biomolecules such as proteins. They provide structural integrity to the molecule.
Sulfur dyes are synthesized by the process of thionation or sulfurization of organic compounds that contain nitro or amino groups. These compounds contain sulfur linkages.
They generally give black, brown, khaki, blue, and green colours.

Self-ionization
It self-ionizes or undergoes autoprotonolysis as follows.

H₂SO₄(l) + H₂SO₄(l) ⇌ H₃SO₄⁺⁺(aq) + HSO⁻(aq)

Ka = 2.7 × 10⁻⁴

The equilibrium constant value is greater than that of water which makes it to be used as a non-aqueous protic solvent. Sulphuric acid is a strong acid as shown by its pKa₁ value:

H₂SO₄(aq) + H₂O ⇌ H₃O⁺(aq) + HSO⁻(aq)
(pKa₁ = -2)

But hydrogen sulfate (HSO⁻) is a far weaker acid due to a positive pKa₂ value:

HSO⁻(aq) + H₂O ⇌ H₃O⁺(aq) + SO₄²⁻(aq)
(pKa₂ = 1.92)

Concentrated sulphuric acid is a powerful dehydrating agent that removes water from many substances such as sucrose, starch, wood, and paper to produce carbon, steam, and heat.

• Dehydration of table sugar
C₁₂H₂₂O₁₁(s) + H₂SO₄(l) →
12C(s) + 11H₂O(g) + H₂SO₄(aq)

e a black porous carbon mass called carbon snake protrudes out of the apparatus.

• Dehydration of alcohol
It also dehydrates ethyl alcohol to ethene or ethoxyethane depending upon the reaction conditions.
C₂H₅OH + H₂SO₄ → C₂H₄(g) + H₂O(l) + H₂SO₄

Purified SO₂ and air, preheated at 420°C– 450°C, are fed to the first converter stage of the contact tower at 1-2 atm pressure. Here, these gases come in contact with vanadium pentoxide (V₂O₅) catalyst.

SO₂(g) + ½O₂(g) ⇌ SO₃(g) ΔH = -98.98 kJmol⁻¹

The catalyst works in two steps:

Oxidation of SO₂ (into SO₃ by V⁵⁺)
SO₂(g) + V₂O₅(s) → SO₃(g) + V₂O₄(s)

Oxidation of V⁴⁺ back into V⁵⁺ by oxygen (catalyst regeneration)
V₂O₄(s) + ½O₂(g) → V₂O₅(s)

Mixing SO₃ with water is not feasible because the reaction is extremely exothermic and acidic vapour or mist is produced rather than a liquid solution. Mainly, sulfur trioxide is dissolved in recalculating hot 98.5% sulphuric acid. The term fuming sulphuric acid or oleum is used for the mixtures of sulfur trioxide with 100 percent sulphuric acid.

• The high concentration of nitrogen in the air serves to dilute oxygen, preventing every spark in our atmosphere from igniting a massive fire.
• In the case of large shipments of hydocarbons or edible oils, it is crucial to utilize blankets of Nitrogen or any other inert gas on ships to safegard them from oxygen and moisture.
• It is also used in laboratory to carry out the reactions which require inert atmosphere.

Ammonia molecule has pyramidal shape due to lone pair of nitrogen. But when nitrogen atom in ammonia utilizes this lone pair of electrons to form ammonium, this ion adopts a tetrahedral shape in which all the bonds are of equal length and strength.

Natural Sources
Natural sources include lightning, volcanos, biological decay, forest fires, soil microorganisms, oceans, etc.
• NO is produced when N₂ and O₂ in the air react during lightning
• It is produced by microorganism using air N₂

Anthropogenic (Man-made) Sources
• The main anthropogenic sources of NOₓ are the combustion of fossil fuels in vehicles and power plants.
• Other sources include chemical plants, biomass burning, welding, etc.

Lahore smog consists of
• Volatile organic compounds (VOCs),
• NOₓ,
• Ground level ozone (O₃),
• Particulate matter PM2.5,
• CO
• SO₂.

Nitrogen is a significant component of the air, known for its low reactivity due to its small size, symmetrical electronic cloud, and nonpolar triple bond.
With an electronic configuration of 1s² 2s² 2p³, nitrogen requires three electrons to complete its octet, forming a triple bond by sharing three electrons with another nitrogen atom.

The reduction reaction in catalytic converter is:
2NO(g) + 2CO(g) →(Pt/Rh) N₂(g) + 2CO₂(g)

Ammonia NH₃ / NH₄⁺ is converted into nitrite (NO₂⁻) and nitrate (NO₃⁻).
Nitrifying bacteria aerobic conditions, pH 6.5 – 8.0, optimum temperature 20 °C–30°C.

Iron pyrite often called "Fool's gold" because of its gold like appearance. Many people considered it gold when it is mined. It's formula is FeS₂.

pH: In general, under acidic conditions, reduced forms (like H₂S, -2 oxidation state) are more stable, while under basic or neutral conditions, oxidized forms (like SO₄²⁻, +6 oxidation state) become more stable in water.

Thermodynamics and kinetics Although sulfur (+6), as in SO₃, is thermodynamically the most stable kinetic limitations can prevent it from forming readily at standard temperatures, making sulfur (+4), as in SO₂, the more frequent form.

Sulfuric acid has melting point of 10°C and boiling point of 290°C between the range of boiling and melting point any substance will be liquid state therefore sulfuric acid is liquid at room temperature. As it is a strongly polar molecule it has a high boiling point and low vapour pressure

It oxidizes SO₂ to SO₃ in the process and itself is reduced to NO

2SO₂ + 2NO₂ → 2SO₃ + 2NO

2NO + O₂ → NO₂

NO₂ acts as a catalyst in its reaction industrially catalyst like vanadium pentoxide is used.

Because high temperature increases the rate of reaction but lowers the yield while low temperature has high yield but the reaction is very slow. Therefore high temperature and catalysts are used and yield is increased by adding oxygen into the reaction chamber.

Oxidation of V⁴⁺ back into V⁵⁺ by oxygen (catalyst regeneration):
V₂O₄(s) + ½O₂(g) → V₂O₅(s)

Sulphuric acid is a strong acid as shown by its pKa₁ value:
H₂SO₄(aq) + H₂O ⇌ H₃O⁺(aq) + HSO⁻(aq) (pKa₁ = -2)

But hydrogensulfate (HSO⁻) is a far weaker acid due to a positive pKa₂ value
HSO⁻(aq) + H₂O(aq) ⇌ H₃O⁺ + SO₄²⁻(aq) (pKa₂ = 1.92)

As shown in the structure two oxygen atoms have double bonds with sulfur while two have single double bonds, acidic bond have less bond length than single bond and hence greater bind energy, therefore two different bond lengths are observed for sulfur and oxygen bond in sulfuric acid.

Sulfur reacts directly with F₂ to form SF₄ and SF₆. Sulfur hexafluoride (SF₆) is a gas and is very unreactive. It is used as an insulator gas in electric devices. Sulfur readily reacts with Cl₂ to form S₂Cl₂ (yellow liquid) which further reacts with Cl₂ to form SCl₂ (red liquid).

S(s) + 3F₂(g) → SF₆(g)
2S(s) + Cl₂(g) → S₂Cl₂(l)
Cl₂(g) + S₂Cl₂(l) → 2SCl₂(l)

2Mg + N₂O → MgO + N₂

This is a highly exothermic reaction and causes the strip to burn brightly.

See Q.1 from theory.

See Q.3 from theory.

See Q.8 from theory.

See Q.9 from theory.