Chemistry in Society

What you'll learn

This topic examines how chemical principles apply to industrial manufacturing, agriculture, and environmental challenges. You'll explore the production of ammonia, sulfuric acid, and fertilisers, alongside extraction methods for metals and the chemistry of water treatment. Understanding these processes demonstrates chemistry's essential role in modern society and sustainable development.

Key terms and definitions

Haber process — The reversible reaction between nitrogen and hydrogen to produce ammonia using an iron catalyst at approximately 450°C and 200 atmospheres pressure.

Contact process — The industrial manufacture of sulfuric acid involving oxidation of sulfur dioxide to sulfur trioxide using a vanadium(V) oxide catalyst.

Electrolysis — The decomposition of an ionic compound when molten or in aqueous solution by passing an electric current through it.

Eutrophication — Excessive nutrient enrichment of water bodies, typically from fertiliser run-off, causing algal blooms and oxygen depletion.

Fractional distillation — The separation of crude oil into fractions based on different boiling points of hydrocarbons.

NPK fertiliser — A compound fertiliser containing nitrogen (N), phosphorus (P), and potassium (K) essential for plant growth.

Sacrificial protection — A method of preventing iron corrosion by attaching a more reactive metal that oxidises preferentially.

Sustainable development — Meeting present needs without compromising the ability of future generations to meet their own needs.

Core concepts

The Haber process for ammonia production

The Haber process manufactures ammonia (NH₃) for fertiliser production and other industrial applications. The equation for this reversible reaction is:

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

Raw materials and their sources:

• Nitrogen obtained from fractional distillation of liquefied air (78% of atmosphere)
• Hydrogen produced from natural gas (methane) reacting with steam, or from cracking of crude oil fractions

Operating conditions and justifications:

The process operates at approximately 450°C and 200 atmospheres pressure with an iron catalyst.

Temperature: 450°C represents a compromise. Although lower temperatures favour the exothermic forward reaction (increasing yield), the reaction rate becomes too slow. Higher temperatures increase rate but decrease yield. 450°C provides an acceptable yield at a reasonable rate.

Pressure: High pressure (200 atm) favours the forward reaction because 4 molecules of reactants produce 2 molecules of product. Higher pressures would increase yield further but require stronger, more expensive equipment and consume more energy.

Catalyst: Iron speeds up the rate of both forward and reverse reactions equally, allowing equilibrium to be reached faster without affecting the position of equilibrium or the yield.

Recycling unreacted gases: Only approximately 15% conversion occurs per pass through the reactor. Unreacted nitrogen and hydrogen are recycled to maximise efficiency and reduce waste, making the process economically viable.

The Contact process for sulfuric acid manufacture

Sulfuric acid (H₂SO₄) is one of the most important industrial chemicals, used in fertiliser production, detergents, paints, and car batteries.

Stage 1: Production of sulfur dioxide

• Sulfur burned in air: S(s) + O₂(g) → SO₂(g)
• Or by roasting sulfide ores like iron pyrites: 4FeS₂(s) + 11O₂(g) → 2Fe₂O₃(s) + 8SO₂(g)

Stage 2: Oxidation to sulfur trioxide

• 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = -196 kJ/mol
• Uses vanadium(V) oxide (V₂O₅) catalyst
• Temperature: approximately 450°C (compromise between rate and yield)
• Pressure: approximately 2 atmospheres (higher pressures unnecessary as yield already exceeds 99%)

Stage 3: Conversion to sulfuric acid

• SO₃ dissolved in concentrated H₂SO₄ to form oleum (H₂S₂O₇), then diluted with water
• Direct addition of SO₃ to water produces dangerous sulfuric acid mist

Fertilisers and their importance

Plants require three primary nutrients for healthy growth:

• Nitrogen (N) — for leaf growth and protein synthesis
• Phosphorus (P) — for root development and energy transfer
• Potassium (K) — for flower and fruit formation, disease resistance

Natural fertilisers include manure and compost, which release nutrients slowly and improve soil structure but have lower, variable nutrient content.

Artificial (synthetic) fertilisers are manufactured compounds with precisely controlled nutrient composition:

• Ammonium nitrate (NH₄NO₃) — made by neutralising ammonia with nitric acid
• Ammonium sulfate ((NH₄)₂SO₄) — from ammonia and sulfuric acid
• Ammonium phosphate ((NH₄)₃PO₄) — contains nitrogen and phosphorus

Manufacturing ammonium sulfate:

2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)

The solution is then evaporated to crystallise the solid fertiliser.

Environmental concerns:

Overuse of fertilisers causes eutrophication:

1. Excess fertiliser washes into rivers and lakes (leaching)
2. Algae grow rapidly due to high nutrient levels
3. Algal bloom blocks sunlight to plants below water surface
4. Plants die and decompose
5. Decomposing bacteria multiply and consume dissolved oxygen
6. Aquatic animals suffocate from oxygen depletion

Extraction of metals

The extraction method depends on metal reactivity:

Electrolysis for reactive metals (potassium to aluminium):

• Reactive metals have strong bonds with oxygen; reduction using carbon is not feasible
• Aluminium extracted from purified bauxite (aluminium oxide, Al₂O₃)
• Bauxite dissolved in molten cryolite (lowers melting point from 2050°C to 950°C, reducing energy costs)
• Electrode reactions:
  • At cathode (−): Al³⁺ + 3e⁻ → Al
  • At anode (+): 2O²⁻ → O₂ + 4e⁻
• Carbon anodes burn away as they react with oxygen, requiring regular replacement

Reduction with carbon for moderate reactivity metals (zinc to iron):

• Carbon (or carbon monoxide) reduces metal oxides
• Iron extraction in blast furnace from haematite (Fe₂O₃)
• Carbon monoxide acts as reducing agent: Fe₂O₃ + 3CO → 2Fe + 3CO₂

Native metals (copper, silver, gold):

• Found uncombined due to very low reactivity
• May require purification only

Titanium extraction uses a different approach:

• TiO₂ converted to TiCl₄ (titanium(IV) chloride)
• Reduced using magnesium or sodium: TiCl₄ + 2Mg → Ti + 2MgCl₂
• Expensive process due to reactive metals required and batch processing

Recycling and sustainability

Advantages of recycling metals:

• Conserves finite ore reserves
• Reduces energy consumption (recycling aluminium uses only 5% of energy needed for extraction)
• Decreases landfill waste
• Reduces environmental damage from mining
• Lowers production costs

Life cycle assessment evaluates environmental impact across:

• Raw material extraction
• Manufacturing and processing
• Product use
• Disposal or recycling

Sustainable approaches balance economic development with environmental protection and resource conservation for future generations.

Rusting and corrosion prevention

Rusting requires both oxygen and water. Iron forms hydrated iron(III) oxide (Fe₂O₃·xH₂O):

4Fe(s) + 3O₂(g) + 2xH₂O(l) → 2Fe₂O₃·xH₂O(s)

Prevention methods:

Barrier methods:

• Painting or coating with plastic
• Oiling or greasing
• Electroplating with unreactive metal (chromium, nickel)

Galvanising:

• Coating iron with zinc
• Provides barrier protection
• Zinc also acts as sacrificial protection (more reactive than iron, so oxidises preferentially)

Sacrificial protection:

• Attaching blocks of more reactive metal (magnesium or zinc)
• Used on ship hulls, underground pipes, steel structures
• Reactive metal corrodes instead of iron, protecting the structure

Water treatment and quality

Potable water is safe to drink and contains acceptable levels of dissolved salts and microbes.

Water treatment stages:

1. Screening — removes large debris (leaves, sticks)
2. Sedimentation — particles settle out in settlement tanks
3. Filtration — water passes through sand/gravel beds to remove smaller particles
4. Chlorination — chlorine gas kills harmful bacteria and microorganisms

Desalination produces fresh water from seawater:

• Distillation — heating seawater and condensing steam (energy-intensive)
• Reverse osmosis — forcing water through semi-permeable membranes under pressure

Both methods are expensive, limiting use to water-scarce regions with sufficient resources.

Testing water purity:

• pH testing ensures neutrality (pH 6.5-8.5 acceptable)
• Dissolved oxygen levels indicate biological health
• Nitrate testing detects fertiliser contamination
• Heavy metal analysis prevents toxic contamination

Worked examples

Example 1: Haber process calculation

Question: In the Haber process, 28 tonnes of nitrogen react with hydrogen. Calculate the maximum theoretical mass of ammonia that could be produced. (Relative atomic masses: N = 14, H = 1) [3 marks]

Solution: N₂ + 3H₂ → 2NH₃

Molar mass N₂ = 28 g/mol Molar mass NH₃ = 17 g/mol

From equation: 1 mole N₂ produces 2 moles NH₃ Therefore: 28 g N₂ produces 34 g NH₃ [1 mark]

28 tonnes N₂ produces (34/28) × 28 tonnes = 34 tonnes NH₃ [1 mark]

Maximum mass of ammonia = 34 tonnes [1 mark]

Example 2: Fertiliser percentage composition

Question: Calculate the percentage by mass of nitrogen in ammonium sulfate, (NH₄)₂SO₄. (Relative atomic masses: N = 14, H = 1, S = 32, O = 16) [3 marks]

Solution: Molar mass (NH₄)₂SO₄ = (2 × 14) + (8 × 1) + 32 + (4 × 16) = 132 g/mol [1 mark]

Mass of nitrogen = 2 × 14 = 28 g [1 mark]

Percentage nitrogen = (28/132) × 100 = 21.2% [1 mark]

Example 3: Electrolysis of aluminium oxide

Question: Aluminium is extracted by electrolysis of aluminium oxide dissolved in molten cryolite. (a) Write the half-equation for the reaction at the negative electrode. [1 mark] (b) Explain why the carbon anodes need regular replacement. [2 marks]

Solution: (a) Al³⁺ + 3e⁻ → Al [1 mark]

(b) Oxygen gas is produced at the positive electrode/anode [1 mark]. The oxygen reacts with the carbon anode forming carbon dioxide, which burns away the electrode [1 mark].

Common mistakes and how to avoid them

• Confusing conditions in Haber and Contact processes — Both use approximately 450°C but different pressures. Haber uses ~200 atm (to shift equilibrium toward fewer molecules), while Contact uses ~2 atm (already achieving >99% yield).

• Incorrectly explaining catalyst function — Catalysts speed up the rate of reaching equilibrium; they do NOT increase the yield or change the position of equilibrium. Always state that catalysts affect both forward and reverse reactions equally.

• Mixing up electrode reactions — Remember "PANIC": Positive Anode, Negative Is Cathode. Oxidation occurs at the anode (loss of electrons), reduction at the cathode (gain of electrons).

• Forgetting both requirements for rusting — Iron rusts only when BOTH oxygen AND water are present. Stating just one is incomplete and loses marks.

• Incomplete eutrophication sequences — Exam answers must include: nutrient addition → algal bloom → light blocked → plant death → bacterial decomposition → oxygen depletion → aquatic animal death. Missing steps lose marks.

• Vague environmental statements — Avoid general phrases like "bad for the environment." Specify exact impacts: habitat destruction, greenhouse gas emissions, resource depletion, water pollution with named substances.

Exam technique for "Chemistry in Society"

• Command word precision — "Explain" requires reasons (because/so that/this causes); "Describe" needs observations or steps; "State" requires brief facts only. "Suggest" indicates applying knowledge to unfamiliar contexts.

• Numbered processes — For multi-stage processes (Contact process, water treatment, eutrophication), number your points clearly. This ensures logical sequence and prevents omitting stages worth marks.

• Justifying industrial conditions — Always link conditions to rate AND yield, explaining the compromise. State what happens at higher/lower values, then justify the chosen value. This demonstrates understanding beyond memorisation.

• Using equations effectively — Include state symbols where specified. Balanced equations often carry marks independently, so write them even if your calculation contains errors.

Quick revision summary

Chemistry in society examines industrial processes including the Haber process (ammonia from nitrogen and hydrogen at 450°C, 200 atm with iron catalyst) and Contact process (sulfuric acid via SO₂ oxidation using vanadium(V) oxide). Fertilisers provide NPK nutrients but excess causes eutrophication through oxygen depletion. Metal extraction methods depend on reactivity: electrolysis for reactive metals, carbon reduction for moderate reactivity. Recycling conserves resources and reduces energy consumption. Rusting requires oxygen and water; prevention uses barriers or sacrificial protection. Water treatment involves sedimentation, filtration, and chlorination.