Separate chemistry 2

Fertilisers and their production

Fertilisers provide mineral ions (especially nitrogen N, phosphorus P, and potassium K — "NPK") that plants need for growth. Nitrogen-containing fertilisers (e.g. ammonium nitrate NH₄NO₃, ammonium sulfate (NH₄)₂SO₄) are most important.

Why fertilisers are needed: modern intensive farming depletes soil nitrogen faster than natural processes replace it. Without fertilisers, crop yields fall significantly.

Problems with fertilisers: if over-used or applied before rain, they leach into waterways → eutrophication: algae bloom using excess nitrogen → algae die → bacteria decompose algae, using up dissolved O₂ → aquatic animals die from lack of oxygen.

Making nitrogen fertilisers

The Haber process produces ammonia (NH₃), which is then converted to fertilisers:

• Ammonium nitrate: NH₃ + HNO₃ → NH₄NO₃ (nitric acid from Ostwald process)
• Ammonium sulfate: 2NH₃ + H₂SO₄ → (NH₄)₂SO₄

The Haber process

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ/mol (exothermic)

Raw materials: nitrogen from fractional distillation of liquefied air; hydrogen from methane and steam (steam reforming: CH₄ + H₂O → CO + 3H₂).

Industrial conditions: ~450°C, 200 atm, iron catalyst (with promoters K₂O, Al₂O₃).

Yield: only ~15% conversion per pass, so unreacted N₂ and H₂ are recycled. Trade-off: lower temperature gives higher yield but slow rate; higher pressure gives higher yield but costly/dangerous.

The Contact process (sulfuric acid manufacture)

Stage 1: S + O₂ → SO₂ (burning sulfur) Stage 2: 2SO₂ + O₂ ⇌ 2SO₃ (450°C, V₂O₅ catalyst, 1 atm) — equilibrium, forward reaction exothermic Stage 3: SO₃ + H₂SO₄ → H₂S₂O₇ (oleum) then H₂S₂O₇ + H₂O → 2H₂SO₄ (Direct absorption of SO₃ into water is too exothermic and creates acid mist.)

Uses of sulfuric acid: making fertilisers (ammonium sulfate, superphosphates), detergents, paints, fibres, car batteries.

Hydrogen fuel cells

A hydrogen fuel cell converts chemical energy directly into electrical energy using the reaction: H₂ + ½O₂ → H₂O (or: 2H₂ + O₂ → 2H₂O)

At the anode (negative): H₂ → 2H⁺ + 2e⁻ (oxidation) At the cathode (positive): ½O₂ + 2H⁺ + 2e⁻ → H₂O (reduction)

Advantages over combustion engines: no CO₂ emissions at point of use (only H₂O); more efficient (up to 60% vs ~25% internal combustion); quiet. Disadvantages: hydrogen is explosive/hard to store safely; currently mostly produced from natural gas (steam reforming) → still produces CO₂; infrastructure (filling stations) is limited; expensive fuel cells.

Nanoparticles

Nanoparticles are particles with dimensions in the range 1–100 nm (nanometres). 1 nm = 10⁻⁹ m.

Why nanoparticles behave differently: they have an extremely high surface-area-to-volume ratio compared with bulk materials → more atoms on the surface → different chemical and physical properties.

Examples and uses:

• Silver nanoparticles: antibacterial properties (wound dressings, socks, food packaging).
• Titanium dioxide nanoparticles: UV-blocking sunscreen; self-cleaning glass.
• Carbon nanotubes: very strong and conductive; used in composite materials, electronics.
• Gold nanoparticles: catalysis, drug delivery, medical diagnostics.

Concerns about nanoparticles:

• Health risks: can penetrate cell membranes and lung tissue; effects on health not fully understood.
• Environmental persistence: do not break down easily; accumulate in food chains.
• Regulation: not yet well-regulated; long-term effects unknown.

Bulk gold vs nano-gold: bulk gold is yellow and inert; gold nanoparticles can appear red, orange, or blue depending on size, and show catalytic activity at room temperature.