3.1
Types of Fertilisers
FertiliserA fertiliser is any substance added to soil to supply plant nutrients that are lacking or deficient, thereby increasing crop yield. Fertilisers can be organic (derived from living organisms) or inorganic (synthetic chemicals).
| Type | Source | Examples | N-P-K availability |
|---|---|---|---|
| Organic | Living organisms | Manure, compost, bone meal, blood meal | Slow release — mineralised by soil bacteria |
| Inorganic | Chemical manufacture | Ammonium nitrate, urea, superphosphate | Fast, direct — immediately available to plants |
| Biofertiliser | Microorganisms | Rhizobium, Azotobacter, mycorrhizae | Biological N fixation — sustainable |
3.2
Components of a Fertiliser — N, P, K
Why NPK?
Three macronutrients are almost always needed in significant quantities:
| Nutrient | Role in plants | Deficiency symptom | Common fertiliser source |
|---|---|---|---|
| N (Nitrogen) | Proteins, chlorophyll, DNA — leaf and shoot growth | Yellowing (chlorosis), stunted growth | NH₄NO₃, urea CO(NH₂)₂, (NH₄)₂SO₄ |
| P (Phosphorus) | ATP, DNA, cell membranes — root development | Purple discolouration of leaves, poor root growth | Ca(H₂PO₄)₂ (superphosphate), triple superphosphate |
| K (Potassium) | Enzyme activation, stomatal control, disease resistance | Brown leaf edges, wilting | KCl (muriate of potash), K₂SO₄, KNO₃ |
NPK grade: written as N-P-K ratio by mass of element (e.g. 15-5-10 means 15% N, 5% P, 10% K).
Calculating NPK Values
Mass percentage of N in NH₄NO₃:
M(NH₄NO₃) = 14+4+14+48 = 80 g/mol; 2 N atoms; mass N = 28
%N = 28/80 × 100 = 35%
Mass % of N in urea CO(NH₂)₂:
M = 12+16+2(14+2) = 60; 2N = 28; %N = 28/60 × 100 = 46.7%
Mass % of P₂O₅ equivalent in Ca(H₂PO₄)₂:
Used to express P content as P₂O₅ by convention
2P atoms in Ca(H₂PO₄)₂; M = 40+4(1)+4(2×16+1) = 234
Actually: Ca(H₂PO₄)₂ = 40+2(2+31+64) = 40+194 = 234 g/mol
3.3
Manufacture of Fertilisers
Haber Process — Ammonia (N source)
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH° = −92.4 kJ/mol
Conditions:
Temperature: 400–450°C (compromise — thermodynamics vs kinetics)
Pressure: 200 atm (high P favours product: 4 mol → 2 mol gas)
Catalyst: iron (Fe) with promoters Al₂O₃ and K₂O
Yield: ~15% per pass; unreacted gases recycled
N₂ source: fractional distillation of liquid air
H₂ source: steam reforming of natural gas: CH₄ + H₂O → CO + 3H₂
Ostwald Process — Nitric Acid (for ammonium nitrate)
Step 1: 4NH₃ + 5O₂ → 4NO + 6H₂O (Pt/Rh catalyst, 900°C)
Step 2: 2NO + O₂ → 2NO₂
Step 3: 4NO₂ + O₂ + 2H₂O → 4HNO₃
Manufacture of ammonium nitrate fertiliser:
NH₃ + HNO₃ → NH₄NO₃ (neutralisation; very exothermic)
NH₄NO₃ is granulated or prilled for sale
Contact Process — Sulfuric Acid (for superphosphate)
Step 1: S + O₂ → SO₂ (or 4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂)
Step 2: 2SO₂ + O₂ → 2SO₃ (V₂O₅ catalyst, 450°C, 1–2 atm)
Step 3: SO₃ + H₂SO₄ → H₂S₂O₇ (oleum)
H₂S₂O₇ + H₂O → 2H₂SO₄
Superphosphate manufacture:
Ca₃(PO₄)₂ + 2H₂SO₄ → Ca(H₂PO₄)₂ + 2CaSO₄
(rock phosphate + sulfuric acid → single superphosphate)
Triple superphosphate: Ca₃(PO₄)₂ + 4H₃PO₄ → 3Ca(H₂PO₄)₂
3.4
Advantages and Disadvantages
| Feature | Inorganic fertilisers | Organic fertilisers |
|---|---|---|
| Nutrient availability | Fast, direct uptake | Slow (microbial breakdown first) |
| Application accuracy | Precise NPK ratio | Variable composition |
| Cost | Lower cost per kg nutrient | Higher cost, labour-intensive |
| Soil structure | Does not improve soil structure | Improves soil water retention and aeration |
| Soil microbes | Can reduce microbial diversity (salt effect) | Feeds soil microbes, improves biodiversity |
| Leaching risk | High — nitrates very soluble | Lower — nutrients released slowly |
| Eutrophication risk | High (excess N, P run into water) | Moderate |
| Long-term sustainability | Depletes organic matter | Sustainable — returns organic matter |
| Energy | High energy to produce (Haber needs CH₄) | Low energy |
Eutrophication
Excess nitrates (NO₃⁻) and phosphates (PO₄³⁻) run off fields into rivers and lakes. This causes:
- Algal bloom — rapid algae growth (algae use nutrients)
- Algae die and decompose — aerobic bacteria use O₂
- O₂ depleted (BOD increases) — fish and aquatic animals die
- Anaerobic bacteria produce CH₄, H₂S — foul smell
Prevention: slow-release fertilisers, buffer strips, constructed wetlands.
3.5
Dangers of Substandard Fertilisers
Risks
- Incorrect NPK ratio: toxic salt burn to plants; nutrient imbalance inhibits crop growth
- Heavy metal contamination: cadmium (Cd), arsenic (As), lead (Pb) in low-grade phosphate rock accumulate in soil and food chain
- Biuret contamination in urea: formed when urea is overheated; toxic to plants — inhibits protein synthesis
- Moisture content: clumping, uneven spreading, reduced efficacy
- Counterfeit fertilisers: inert fillers sold as nutrient-containing products; farmers lose money, crops fail
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✏️
Exercises
- Calculate the percentage by mass of nitrogen in: (a) NH₄NO₃ (b) urea CO(NH₂)₂ (c) (NH₄)₂SO₄. Which has the highest nitrogen content?
(a) M(NH₄NO₃)=80; N=28; %N=35%
(b) M(urea)=60; N=28; %N=46.7% (highest)
(c) M((NH₄)₂SO₄)=132; N=28; %N=21.2% - Write all steps of the Haber process and explain the choice of conditions (T, P, catalyst).
N₂+3H₂⇌2NH₃, ΔH°=−92.4 kJ/mol. T=400–450°C: lower T gives better yield but too slow; 450°C gives acceptable rate+yield compromise. P=200 atm: high P favours NH₃ (4 mol→2 mol gas); limited by equipment cost/safety. Catalyst=Fe+promoters: lowers Eₐ for acceptable rate at moderate T. Gases recycled to improve overall yield.
- Describe eutrophication. Explain the role of fertilisers and how it can be prevented.
Excess N (NO₃⁻) and P (PO₄³⁻) from fertilisers run into water bodies. Algal bloom → algae die → bacteria decompose them aerobically → O₂ depleted → fish die. Prevention: buffer strips of vegetation near water, slow-release fertilisers, precision application (right rate+time), constructed wetlands to capture runoff.
- Compare inorganic and organic fertilisers under the headings: cost, nutrient availability, effect on soil, and environmental impact.
Cost: inorganic cheaper per kg nutrient; organic more expensive. Availability: inorganic immediate; organic slow (depends on microbial action). Soil: inorganic no benefit to structure; organic improves structure, water retention, microbial diversity. Environment: inorganic higher leaching/eutrophication risk; organic lower but can still contribute if over-applied.
- Write the equations for the Ostwald process and show how ammonium nitrate fertiliser is produced from the products.
4NH₃+5O₂→4NO+6H₂O (Pt/Rh, 900°C); 2NO+O₂→2NO₂; 4NO₂+O₂+2H₂O→4HNO₃.
NH₃+HNO₃→NH₄NO₃ (neutralisation; NH₃ from Haber process; HNO₃ from Ostwald process). - A bag of fertiliser is labelled 20-10-5 (N-P-K). What do these numbers mean, and what is the total mass of nutrients in a 50 kg bag?
The numbers mean 20% N, 10% P (as P₂O₅ equivalent), 5% K (as K₂O equivalent) by mass.
Total nutrient mass = (20+10+5)% × 50 kg = 35% × 50 = 17.5 kg. The remaining 32.5 kg is filler, carrier, or other compounds.
🧠
Interactive Quiz
Unit 3 Quiz — Fertilisers & NPK (25 Questions)
Select one answer eachQ1
The Haber process produces:
N₂ + 3H₂ ⇌ 2NH₃. The Haber process synthesises ammonia. This is then used as a raw material for other nitrogen-containing fertilisers (ammonium nitrate via Ostwald/neutralisation, urea, etc.).
Q2
Which macronutrient is most responsible for leaf and shoot growth in plants?
Nitrogen is essential for proteins, chlorophyll (green pigment), and nucleic acids — all needed for rapid vegetative (leaf and shoot) growth. N deficiency causes chlorosis (yellowing of leaves).
Q3
The catalyst used in the Haber process is:
Iron (Fe) with promoters Al₂O₃ (structural promoter) and K₂O (electronic promoter) is the catalyst. V₂O₅ is for the Contact process; Pt/Rh is for the Ostwald process; Ni is for alkene hydrogenation.
Q4
The Contact process manufactures:
The Contact process: S→SO₂→SO₃→H₂SO₄. H₂SO₄ is then used to make superphosphate fertiliser from rock phosphate. Step 2 (2SO₂+O₂→2SO₃) uses V₂O₅ catalyst at ~450°C.
Q5
Eutrophication of water bodies is caused by:
Excess NO₃⁻ and PO₄³⁻ from over-applied fertilisers enter rivers/lakes, causing algal blooms. When algae die and decompose, O₂ is consumed, causing death of fish and aquatic organisms.
Q6
The nitrogen content (%N) of urea CO(NH₂)₂ is:
M(urea) = 12+16+2(14+2×1) = 60 g/mol; mass of 2N = 28. %N = 28/60 × 100 = 46.7%. Urea has the highest N% of common fertilisers.
Q7
Single superphosphate is made by reacting:
Ca₃(PO₄)₂ + 2H₂SO₄ → Ca(H₂PO₄)₂ + 2CaSO₄. Ca(H₂PO₄)₂ is soluble and available to plants. Triple superphosphate uses H₃PO₄ instead, giving higher P content without the CaSO₄ filler.
Q8
In the Ostwald process, the catalyst used in step 1 (oxidation of NH₃) is:
4NH₃ + 5O₂ → 4NO + 6H₂O uses a Pt/Rh (platinum-rhodium) gauze catalyst at ~900°C. This is a short-contact-time reactor to prevent over-oxidation of NO. V₂O₅ is for the Contact process.
Q9
A fertiliser labelled 15-15-15 is called:
15-15-15 means equal proportions of N:P:K — a balanced/complete fertiliser. It provides all three macronutrients in equal amounts, suitable for general application where soil analysis is not available.
Q10
The main advantage of organic fertilisers over inorganic fertilisers is:
Organic fertilisers (manure, compost) add humus to soil, improving structure (aeration, water retention) and feeding soil microbiota. This improves long-term soil health. Inorganic fertilisers are faster-acting but do not improve soil structure.
Q11
Biuret contamination in urea fertiliser is harmful because:
Biuret [NH₂CONHCONH₂] is formed when urea is overheated during manufacture. It is absorbed by plant roots and inhibits enzymes involved in protein synthesis, causing leaf damage and reduced growth. Urea fertiliser must meet biuret content standards (<1.2%).
Q12
The pressure used in the Haber process (200 atm) favours ammonia production because:
N₂+3H₂→2NH₃: 4 moles of gas reactants → 2 moles of gas products. By Le Chatelier's principle, high pressure shifts equilibrium toward fewer moles of gas — favouring NH₃ formation.
Q13
Which process is used to convert N₂ into NH₃ industrially?
The Haber process (Fritz Haber/Carl Bosch): N₂ + 3H₂ ⇌ 2NH₃. Fe catalyst, 400–450°C, 200 atm. NH₃ is the starting material for most N-fertilisers. The Contact, Ostwald, and Bayer processes produce H₂SO₄, HNO₃, and Al₂O₃ respectively.
Q14
Potassium deficiency in plants causes:
K deficiency: brown/scorched leaf margins, wilting (K controls stomata), reduced disease resistance. N deficiency: chlorosis (yellowing). P deficiency: purple leaves and poor roots. K is essential for enzyme activation, osmoregulation, and phloem loading.
Q15
The formula of ammonium nitrate is:
NH₄NO₃ — ammonium nitrate. (NH₄)₂SO₄ is ammonium sulfate; CO(NH₂)₂ is urea; NH₄H₂PO₄ is monoammonium phosphate (MAP). NH₄NO₃ has 35%N and is a common N fertiliser.
Q16
Rhizobium bacteria are used as biofertilisers because they:
Rhizobium live symbiotically in root nodules of legumes (beans, peas, clover). They fix N₂ using nitrogenase enzyme: N₂ + 8H⁺ + 8e⁻ → 2NH₃ + H₂. The plant supplies carbohydrates; Rhizobium supplies fixed N. This reduces the need for synthetic N fertilisers.
Q17
Triple superphosphate has a higher P content than single superphosphate because:
Single superphosphate: Ca₃(PO₄)₂ + 2H₂SO₄ → Ca(H₂PO₄)₂ + 2CaSO₄. ~18% P₂O₅.Triple superphosphate: Ca₃(PO₄)₂ + 4H₃PO₄ → 3Ca(H₂PO₄)₂. No CaSO₄ filler → ~46% P₂O₅. Higher P content per kg.
Q18
The main environmental concern with the Haber process is:
The Haber process requires H₂ (from steam reforming of CH₄: a fossil fuel) and is energy-intensive. It contributes ~1–2% of global CO₂ emissions. Green hydrogen (from electrolysis using renewable energy) could make it carbon-neutral. Green ammonia is an active area of research.
Q19
Why is ammonium nitrate (NH₄NO₃) considered hazardous?
NH₄NO₃ is a powerful oxidiser. Pure dry NH₄NO₄ at >200°C or contaminated NH₄NO₃ can decompose explosively: NH₄NO₃ → N₂O + 2H₂O, or in larger amounts → N₂ + O₂ + H₂O. Several major industrial accidents (Beirut 2020, Texas City 1947) involved NH₄NO₃.
Q20
What is the NPK grade of a fertiliser containing 46% N, 0% P, 0% K?
NPK is stated in order N-P-K. 46% N, 0% P, 0% K = 46-0-0. This is consistent with urea (CO(NH₂)₂) which has ~46.7% N and no P or K. It is a straight N fertiliser.
Q21
Buffer strips near agricultural fields help reduce fertiliser runoff by:
Buffer strips are bands of permanent vegetation (grasses, trees) between fields and water bodies. They trap sediment, absorb nutrients (NO₃⁻, PO₄³⁻) through plant uptake, and allow microbial denitrification, preventing these nutrients from reaching rivers and causing eutrophication.
Q22
Phosphorus deficiency in plants typically causes:
P is needed for ATP, DNA, phospholipids, and root development. P deficiency: dark green then reddish-purple leaves (anthocyanin accumulates), slow root growth, delayed maturity. The purple colouration distinguishes P deficiency from N deficiency (yellowing).
Q23
In the equation Ca₃(PO₄)₂ + 2H₂SO₄ → Ca(H₂PO₄)₂ + 2CaSO₄, the Ca(H₂PO₄)₂ is beneficial because:
Ca(H₂PO₄)₂ (calcium dihydrogen phosphate) is water-soluble, making P available to plant roots. Ca₃(PO₄)₂ (rock phosphate) is very insoluble and unavailable. The reaction with H₂SO₄ converts insoluble phosphate to a soluble, plant-available form.
Q24
Cadmium (Cd) contamination of soils from fertilisers comes mainly from:
Phosphate rock (used to make superphosphate) often contains trace cadmium (Cd) impurities. Repeated application can cause Cd to accumulate in soil and be taken up by crops, entering the food chain. This is why low-Cd phosphate rock sources and Cd limits in fertilisers are regulated.
Q25
Which gas produced in the Ostwald process is also an air pollutant contributing to smog and acid rain?
NO₂ (nitrogen dioxide) is produced in step 2: 2NO + O₂ → 2NO₂. In the atmosphere, NOₓ (NO + NO₂) contributes to smog formation and acid rain (4NO₂ + O₂ + 2H₂O → 4HNO₃). Industrial plants must minimise NOₓ emissions.
📝
Unit Test
ℹ️
InstructionsTotal: 50 marks | Time: 55 minutes | Show all working and write equations where appropriate.
Section A — Short Answer
30 marks
Q1 [5 marks]
(a) Write equations for ALL steps of the Haber process from N₂ and CH₄ to NH₃. Include conditions for each step. [3] (b) Explain the choice of 400–450°C as the operating temperature. [2]
(a) Steam reforming of CH₄: CH₄ + H₂O → CO + 3H₂ (Ni catalyst, 800°C); water-gas shift: CO + H₂O → CO₂ + H₂; N₂ source: fractional distillation of liquid air.
Haber: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH° = −92.4 kJ/mol. Conditions: 400–450°C, 200 atm, Fe catalyst (with Al₂O₃ and K₂O promoters), unreacted gases recycled.
(b) Low T thermodynamically favours NH₃ (exothermic reaction, equilibrium shifts right as T decreases) but rate is too slow at very low T. High T gives faster rate but lower equilibrium yield. 400–450°C is the optimal compromise. With the catalyst, the rate is acceptable at this temperature.
Haber: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH° = −92.4 kJ/mol. Conditions: 400–450°C, 200 atm, Fe catalyst (with Al₂O₃ and K₂O promoters), unreacted gases recycled.
(b) Low T thermodynamically favours NH₃ (exothermic reaction, equilibrium shifts right as T decreases) but rate is too slow at very low T. High T gives faster rate but lower equilibrium yield. 400–450°C is the optimal compromise. With the catalyst, the rate is acceptable at this temperature.
Q2 [5 marks]
Compare organic and inorganic fertilisers with respect to: (a) rate of nutrient release; (b) effect on soil structure; (c) risk of eutrophication; (d) energy requirements for production; (e) suitability for large-scale commercial farming. [5]
(a) Inorganic: immediately available (soluble salts); Organic: slow (microbial mineralisation needed).
(b) Inorganic: no improvement; Organic: improves water retention, aeration, microbial diversity.
(c) Inorganic: high risk (highly soluble NO₃⁻ leaches readily); Organic: lower risk (nutrients bound in organic matter).
(d) Inorganic: high (Haber needs CH₄, high pressure); Organic: low (animal/plant waste).
(e) Inorganic: more suitable (precise NPK, easy to apply, consistent quality); Organic: harder to scale, variable composition, high labour.
(b) Inorganic: no improvement; Organic: improves water retention, aeration, microbial diversity.
(c) Inorganic: high risk (highly soluble NO₃⁻ leaches readily); Organic: lower risk (nutrients bound in organic matter).
(d) Inorganic: high (Haber needs CH₄, high pressure); Organic: low (animal/plant waste).
(e) Inorganic: more suitable (precise NPK, easy to apply, consistent quality); Organic: harder to scale, variable composition, high labour.
Q3 [6 marks]
Calculate: (a) the %N in ammonium sulfate (NH₄)₂SO₄. [2] (b) the mass of ammonium nitrate needed to supply 50 kg of pure nitrogen. [2] (c) the %P₂O₅ equivalent in Ca(H₂PO₄)₂ (show working). [2] [Ar: N=14, H=1, S=32, O=16, Ca=40, P=31]
(a) M((NH₄)₂SO₄)=2(14+4)+32+64=132. 2N=28. %N=28/132×100=21.2%
(b) NH₄NO₃: M=80; %N=28/80=35%. Mass needed = 50/0.35 = 142.9 kg
(c) Ca(H₂PO₄)₂: M=40+2(2+31+64)=40+194=234. Contains 2P, which as P₂O₅ is equivalent to 1 mole P₂O₅=142 g. So P₂O₅ in 234g = 142g. %P₂O₅ = 142/234×100 = 60.7%
(b) NH₄NO₃: M=80; %N=28/80=35%. Mass needed = 50/0.35 = 142.9 kg
(c) Ca(H₂PO₄)₂: M=40+2(2+31+64)=40+194=234. Contains 2P, which as P₂O₅ is equivalent to 1 mole P₂O₅=142 g. So P₂O₅ in 234g = 142g. %P₂O₅ = 142/234×100 = 60.7%
Q4 [5 marks]
Describe the process of eutrophication, explaining (a) how it begins, (b) how it leads to depletion of dissolved oxygen, (c) the effect on aquatic life, and (d) two measures to prevent it. [5]
(a) Excess NO₃⁻ and PO₄³⁻ from fertiliser run-off enter still or slow-moving water bodies. These nutrients stimulate rapid growth of algae (algal bloom), which covers the water surface.
(b) Algae die (blocked from light below; nutrients used up) and are decomposed by aerobic bacteria. The huge amount of organic matter causes explosive bacterial growth, which rapidly consumes dissolved O₂ (biological oxygen demand, BOD, rises dramatically).
(c) Fish and other aquatic organisms suffocate as O₂ levels drop below survival threshold. Anaerobic bacteria then dominate, producing toxic CH₄ and H₂S, killing remaining organisms. Biodiversity collapses.
(d) Measures: (i) buffer strips of permanent vegetation along field margins absorb runoff; (ii) precision/slow-release fertilisers applied at correct rates and times to prevent excess; (iii) constructed wetlands to filter agricultural runoff.
(b) Algae die (blocked from light below; nutrients used up) and are decomposed by aerobic bacteria. The huge amount of organic matter causes explosive bacterial growth, which rapidly consumes dissolved O₂ (biological oxygen demand, BOD, rises dramatically).
(c) Fish and other aquatic organisms suffocate as O₂ levels drop below survival threshold. Anaerobic bacteria then dominate, producing toxic CH₄ and H₂S, killing remaining organisms. Biodiversity collapses.
(d) Measures: (i) buffer strips of permanent vegetation along field margins absorb runoff; (ii) precision/slow-release fertilisers applied at correct rates and times to prevent excess; (iii) constructed wetlands to filter agricultural runoff.
Q5 [4 marks]
State four dangers of using substandard or counterfeit fertilisers, and explain the consequences for farmers and the environment. [4]
1. Incorrect NPK ratio: crops receive wrong nutrients — excess N causes soft, disease-prone growth; insufficient P causes poor roots; all reduce yield.
2. Heavy metal contamination (Cd, As, Pb) from low-grade phosphate rock: accumulates in soil, absorbed by crops, enters human food chain — long-term health risks.
3. Biuret contamination in overheated urea: inhibits plant enzymes, causes leaf scorch, reduces yield.
4. Inert filler sold as fertiliser: farmers pay for nutrients they don't receive — direct financial loss; crops fail, food insecurity.
2. Heavy metal contamination (Cd, As, Pb) from low-grade phosphate rock: accumulates in soil, absorbed by crops, enters human food chain — long-term health risks.
3. Biuret contamination in overheated urea: inhibits plant enzymes, causes leaf scorch, reduces yield.
4. Inert filler sold as fertiliser: farmers pay for nutrients they don't receive — direct financial loss; crops fail, food insecurity.
Q6 [5 marks]
Write equations for all steps of the Ostwald process and show how the product is combined with ammonia to produce ammonium nitrate. State the conditions for step 1 and state the importance of Pt/Rh catalyst. [5]
Step 1: 4NH₃ + 5O₂ → 4NO + 6H₂O (Pt/Rh gauze, 900°C, 8 atm). Short contact time prevents re-oxidation of NO back to NO₂ on catalyst.
Step 2: 2NO + O₂ → 2NO₂ (no catalyst; cooling; gas phase).
Step 3: 4NO₂ + O₂ + 2H₂O → 4HNO₃ (absorption in water). Dilute HNO₃ (~60–68%) formed.
Fertiliser: NH₃(g) + HNO₃(aq) → NH₄NO₃(aq) (strong neutralisation); then evaporated, granulated/prilled. Pt/Rh catalyst: gives extremely high selectivity for NO formation (>95%); resists high temperature; can be recovered (expensive). Without catalyst, NH₃ would mainly be oxidised to N₂ (not NO).
Step 2: 2NO + O₂ → 2NO₂ (no catalyst; cooling; gas phase).
Step 3: 4NO₂ + O₂ + 2H₂O → 4HNO₃ (absorption in water). Dilute HNO₃ (~60–68%) formed.
Fertiliser: NH₃(g) + HNO₃(aq) → NH₄NO₃(aq) (strong neutralisation); then evaporated, granulated/prilled. Pt/Rh catalyst: gives extremely high selectivity for NO formation (>95%); resists high temperature; can be recovered (expensive). Without catalyst, NH₃ would mainly be oxidised to N₂ (not NO).
Section B — Extended Response
20 marks
Q7 [10 marks]
Describe in detail the industrial manufacture of ammonia (Haber process) and nitric acid (Ostwald process). For each process, state: (i) the raw materials and their sources; (ii) the chemical equations and conditions; (iii) the catalyst used and its function; (iv) the economic and environmental significance. [10]
Haber process: Raw materials: N₂ (from fractional distillation of liquid air), H₂ (from steam reforming of natural gas: CH₄+H₂O→CO+3H₂; water-gas shift: CO+H₂O→CO₂+H₂).
Reaction: N₂+3H₂⇌2NH₃ ΔH°=−92.4 kJ/mol.
Conditions: 400–450°C, 200 atm, Fe catalyst+Al₂O₃+K₂O promoters; yield ~15% per pass; gases recycled.
Catalyst: Fe lowers activation energy; allows acceptable rate at moderate T; Al₂O₃ maintains Fe surface area (structural); K₂O increases electron density (electronic promoter).
Economics: basis of 80%+ of world nitrogen fertiliser production; cheap N source. Environment: ~1-2% global CO₂; uses fossil fuel CH₄; green H₂ alternative being developed.
Ostwald process: Raw material: NH₃ (from Haber process), air (O₂).
Step 1: 4NH₃+5O₂→4NO+6H₂O (Pt/Rh, 900°C). Step 2: 2NO+O₂→2NO₂. Step 3: 4NO₂+O₂+2H₂O→4HNO₃.
Catalyst: Pt/Rh gauze — high selectivity for NO, resists high T, recoverable.
Economics: HNO₃ used to make NH₄NO₃ fertiliser, explosives (TNT, nitroglycerin), nylon.
Environment: NOₓ releases can contribute to acid rain; modern plants use tail-gas treatment to reduce emissions.
Reaction: N₂+3H₂⇌2NH₃ ΔH°=−92.4 kJ/mol.
Conditions: 400–450°C, 200 atm, Fe catalyst+Al₂O₃+K₂O promoters; yield ~15% per pass; gases recycled.
Catalyst: Fe lowers activation energy; allows acceptable rate at moderate T; Al₂O₃ maintains Fe surface area (structural); K₂O increases electron density (electronic promoter).
Economics: basis of 80%+ of world nitrogen fertiliser production; cheap N source. Environment: ~1-2% global CO₂; uses fossil fuel CH₄; green H₂ alternative being developed.
Ostwald process: Raw material: NH₃ (from Haber process), air (O₂).
Step 1: 4NH₃+5O₂→4NO+6H₂O (Pt/Rh, 900°C). Step 2: 2NO+O₂→2NO₂. Step 3: 4NO₂+O₂+2H₂O→4HNO₃.
Catalyst: Pt/Rh gauze — high selectivity for NO, resists high T, recoverable.
Economics: HNO₃ used to make NH₄NO₃ fertiliser, explosives (TNT, nitroglycerin), nylon.
Environment: NOₓ releases can contribute to acid rain; modern plants use tail-gas treatment to reduce emissions.
Q8 [10 marks]
(a) Explain the role of each of N, P, and K in plant growth, the symptoms of deficiency, and name one fertiliser source for each. [6] (b) Discuss the environmental impact of excessive fertiliser use, including eutrophication and the risks associated with nitrogen compounds entering water supplies. [4]
(a) N: component of amino acids, proteins, chlorophyll, nucleic acids — essential for vegetative growth (leaves, shoots). Deficiency: chlorosis (yellowing) starting from older leaves; stunted growth. Source: NH₄NO₃, urea, (NH₄)₂SO₄.
P: component of ATP (energy transfer), DNA, phospholipids (cell membranes) — critical for root development, flower/seed formation, energy metabolism. Deficiency: purple/red leaves (anthocyanin accumulation), poor root system, delayed maturity. Source: superphosphate Ca(H₂PO₄)₂, triple superphosphate, MAP.
K: enzyme activator (>60 enzymes), controls stomatal aperture (water loss regulation), phloem loading, disease resistance. Deficiency: brown leaf margins (scorch), wilting, reduced pest resistance. Source: KCl (muriate of potash), K₂SO₄, KNO₃.
(b) Eutrophication: NO₃⁻ and PO₄³⁻ run off into rivers/lakes → algal bloom → O₂ depletion → death of fish and aquatic biodiversity → anaerobic decomposition → H₂S and CH₄ → foul water. Economic damage to fishing, tourism, water supply.
Nitrate in drinking water: high NO₃⁻ (>50 mg/L, WHO limit) converted to NO₂⁻ in infant gut by bacteria → binds haemoglobin → methemoglobinaemia (blue baby syndrome). Adults: possible link to stomach cancer with chronic exposure. EU Nitrates Directive limits N application in vulnerable zones.
N₂O (nitrous oxide) from denitrification is a potent greenhouse gas (298× CO₂ over 100 years) and stratospheric ozone depleter.
P: component of ATP (energy transfer), DNA, phospholipids (cell membranes) — critical for root development, flower/seed formation, energy metabolism. Deficiency: purple/red leaves (anthocyanin accumulation), poor root system, delayed maturity. Source: superphosphate Ca(H₂PO₄)₂, triple superphosphate, MAP.
K: enzyme activator (>60 enzymes), controls stomatal aperture (water loss regulation), phloem loading, disease resistance. Deficiency: brown leaf margins (scorch), wilting, reduced pest resistance. Source: KCl (muriate of potash), K₂SO₄, KNO₃.
(b) Eutrophication: NO₃⁻ and PO₄³⁻ run off into rivers/lakes → algal bloom → O₂ depletion → death of fish and aquatic biodiversity → anaerobic decomposition → H₂S and CH₄ → foul water. Economic damage to fishing, tourism, water supply.
Nitrate in drinking water: high NO₃⁻ (>50 mg/L, WHO limit) converted to NO₂⁻ in infant gut by bacteria → binds haemoglobin → methemoglobinaemia (blue baby syndrome). Adults: possible link to stomach cancer with chronic exposure. EU Nitrates Directive limits N application in vulnerable zones.
N₂O (nitrous oxide) from denitrification is a potent greenhouse gas (298× CO₂ over 100 years) and stratospheric ozone depleter.