Industrial Chemistry: Notes, Solved Examples & Exam Questions | Grade 12 Chemistry Unit 3

Grade 12 Chemistry Unit 3: Industrial Chemistry

Welcome, dear student! In this unit, we move from the laboratory into the real world of Industrial Chemistry. Have you ever wondered how the fertilizer that helps grow our food is made? Or how the cement used to build our schools and homes is produced? Industrial chemistry is about turning raw materials into useful products on a large scale. Let’s learn this step by step!

3.2 Natural Resources and Industry

3.2.1 Natural Resources (Raw Materials)

Every industrial process starts with raw materials. These come from natural resources, which can be classified into three main types:

Types of Natural Resources:
Renewable resources: Can be replaced naturally within a short time — e.g., wood, water, air, solar energy, biomass.
Non-renewable resources: Cannot be replaced once used — e.g., fossil fuels (coal, petroleum, natural gas), minerals, metals.
Perpetual resources: Will never run out — e.g., sunlight, wind, tides.

For industrial chemistry, the most important raw materials come from:

The air — nitrogen (N₂) and oxygen (O₂)
Minerals and ores — sulfur, limestone, phosphate rock, bauxite
Fossil fuels — coal, petroleum, natural gas
Water — as a solvent, reactant, and coolant
Biomass — plants and animal products

3.2.2 Industry

An industry is a place where raw materials are converted into finished or semi-finished products on a large scale. Industrial chemistry involves designing, operating, and optimizing chemical processes in factories. Key factors considered include:

Availability of raw materials — Is the resource nearby or affordable to transport?
Energy supply — Many reactions need heat, electricity, or both.
Market demand — Will people buy the product?
Environmental impact — Waste disposal, pollution control.
Transport facilities — Roads, railways, ports for moving materials.

Key Exam Notes — Natural Resources and Industry:

Know the difference between renewable and non-renewable resources with examples.
Industry location depends on raw materials, energy, market, transport, and labor.
Ethiopia has significant natural resources: limestone, natural gas, coal, iron ore, water, geothermal energy.

Practice Question 1: Classify each of the following as renewable or non-renewable: (a) Petroleum (b) Water (c) Natural gas (d) Wood (e) Sunlight (f) Iron ore

Answer:
(a) Petroleum — Non-renewable (formed over millions of years)
(b) Water — Renewable (naturally replenished through the water cycle)
(c) Natural gas — Non-renewable (fossil fuel)
(d) Wood — Renewable (trees can be regrown)
(e) Sunlight — Perpetual (will never run out)
(f) Iron ore — Non-renewable (finite mineral deposit)

Practice Question 2: Explain why most cement factories in Ethiopia are located near limestone deposits.

Answer: Limestone is the main raw material for cement production and it is very heavy and bulky. Transporting it over long distances would be extremely expensive. By locating factories near limestone deposits, transportation costs are minimized, making the process more economical.

3.3 Manufacturing of Valuable Products

3.3.1 Ammonia (NH₃)

Ammonia is one of the most important industrial chemicals in the world. Do you know why? Because it is the starting material for making fertilizers, explosives, and many other chemicals! Ammonia is manufactured by the Haber process.

The Haber Process

Raw materials: Nitrogen from air (N₂) and hydrogen from natural gas (CH₄)

$$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \quad \Delta H = -92 \text{ kJ/mol}$$

Conditions: Temperature: 450–500°C; Pressure: 150–300 atm; Catalyst: iron (Fe) with promoters (Al₂O₃ and K₂O)

Why these conditions? Let’s think carefully:

Low temperature would favor the forward reaction (exothermic), but the reaction would be too slow. So a compromise temperature of 450–500°C is used — fast enough rate with reasonable yield.
High pressure favors the forward reaction (4 moles of gas → 2 moles of gas, so increasing pressure shifts equilibrium to the right by Le Chatelier’s principle). But very high pressure is expensive and dangerous, so 150–300 atm is used.
Iron catalyst speeds up the reaction so that acceptable yields are achieved even at the compromise temperature.

Steps in the Haber Process

Hydrogen production: Natural gas (CH₄) is reacted with steam:
CH₄(g) + H₂O(g) → CO(g) + 3H₂(g)

Then CO is removed: CO(g) + H₂O(g) → CO₂(g) + H₂(g)
Air separation: N₂ is obtained from liquid air by fractional distillation.
Compression: N₂ and H₂ are compressed to 150–300 atm.
Reaction: Gases pass over the iron catalyst at 450–500°C in the reactor.
Cooling: The mixture is cooled; NH₃ condenses (liquefies) and is collected.
Recycling: Unreacted N₂ and H₂ are recycled back into the reactor.

Uses of Ammonia

Manufacture of fertilizers: ammonium sulfate ((NH₄)₂SO₄), ammonium nitrate (NH₄NO₃), urea (CO(NH₂)₂)
Manufacture of nitric acid (Ostwald process)
Refrigerant (liquid ammonia)
Cleaning agent (household ammonia)
Production of explosives, dyes, and pharmaceuticals

Key Exam Notes — Ammonia (Haber Process):

Reaction: N₂ + 3H₂ ⇌ 2NH₃; exothermic; decrease in moles of gas.
Compromise temperature (450–500°C) — low enough for reasonable yield, high enough for reasonable rate.
High pressure (150–300 atm) — favors forward reaction (fewer gas moles).
Iron catalyst with Al₂O₃ and K₂O promoters.
Unreacted gases are recycled to improve efficiency.
Yield is about 15–20% per pass, but recycling increases overall conversion.

Practice Question 3: Explain why a compromise temperature is used in the Haber process instead of a low temperature which would give a higher equilibrium yield.

Answer: The reaction N₂ + 3H₂ ⇌ 2NH₃ is exothermic, so a low temperature would shift equilibrium to the right and give a higher yield. However, at low temperature the reaction rate would be extremely slow — it could take years to reach equilibrium! A compromise temperature of 450–500°C is used: the yield is lower than at room temperature, but the rate is fast enough to make the process economically viable. The iron catalyst further helps increase the rate at this temperature.

Practice Question 4: Using Le Chatelier’s principle, explain why high pressure favors ammonia production in the Haber process.

Answer: In the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), there are 4 moles of gas on the left and 2 moles of gas on the right. Increasing pressure shifts the equilibrium toward the side with fewer moles of gas (to reduce the pressure), which is the right side (ammonia). Therefore, high pressure increases the yield of ammonia.

3.3.2 Nitric Acid (HNO₃)

Nitric acid is another very important industrial chemical. It is used to make fertilizers, explosives, dyes, and plastics. Nitric acid is manufactured by the Ostwald process.

The Ostwald Process

Raw material: Ammonia (NH₃) from the Haber process, and air (O₂)

The process occurs in three main steps:

Step 1: Catalytic oxidation of ammonia

$$4NH_3(g) + 5O_2(g) \xrightarrow{Pt/Rh \text{ catalyst, } 850°C} 4NO(g) + 6H_2O(g) \quad \Delta H < 0$$

Conditions: Platinum-rhodium catalyst, temperature of about 850°C.

Step 2: Oxidation of nitric oxide

$$2NO(g) + O_2(g) \rightarrow 2NO_2(g)$$

This step occurs as the gases cool. NO reacts with excess O₂ to form brown NO₂ gas. No catalyst is needed — this reaction happens spontaneously at room temperature.

Step 3: Absorption in water

$$3NO_2(g) + H_2O(l) \rightarrow 2HNO_3(aq) + NO(g)$$

NO₂ is dissolved in water to produce nitric acid. The NO produced is recycled back to Step 2, where it is re-oxidized to NO₂. This recycling increases efficiency.

The nitric acid produced is about 50–60% concentrated. If higher concentration (98%) is needed, it is concentrated by distillation with concentrated sulfuric acid as a dehydrating agent.

Uses of Nitric Acid

Manufacture of fertilizers: ammonium nitrate (NH₄NO₃)
Manufacture of explosives: TNT, nitroglycerin
Production of dyes, drugs, and plastics
Etching of metals
As a laboratory reagent

Key Exam Notes — Nitric Acid (Ostwald Process):

Three steps: NH₃ → NO → NO₂ → HNO₃
Step 1 needs Pt/Rh catalyst at 850°C (exothermic).
Step 2 is spontaneous (no catalyst), NO + O₂ → NO₂.
Step 3: NO is recycled back to Step 2 for higher efficiency.
Direct product is about 50–60% HNO₃; concentrated to 98% using H₂SO₄.

Practice Question 5: Write balanced equations for all three steps of the Ostwald process and state the catalyst used in Step 1.

Answer:
Step 1: 4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g); Catalyst: Platinum-rhodium (Pt/Rh) at 850°C
Step 2: 2NO(g) + O₂(g) → 2NO₂(g); No catalyst needed
Step 3: 3NO₂(g) + H₂O(l) → 2HNO₃(aq) + NO(g); NO is recycled

Practice Question 6: Why is the NO produced in Step 3 of the Ostwald process recycled?

Answer: The NO produced in Step 3 still contains valuable nitrogen that has already been processed through the expensive catalytic oxidation step. Instead of wasting it, the NO is mixed with air and re-oxidized to NO₂ (Step 2), then absorbed again in water (Step 3). This recycling significantly increases the overall yield and efficiency of the process, reducing waste and cost.

3.3.3 Nitrogen-Based Fertilizers

Ethiopia’s economy depends heavily on agriculture, and fertilizers play a crucial role in increasing crop yields. Fertilizers supply essential elements that plants need to grow: mainly nitrogen (N), phosphorus (P), and potassium (K) — called NPK fertilizers.

Types of Nitrogen Fertilizers

Fertilizer	Formula	% Nitrogen	How Made
Ammonium sulfate	(NH₄)₂SO₄	~21%	NH₃ + H₂SO₄ → (NH₄)₂SO₄
Ammonium nitrate	NH₄NO₃	~35%	NH₃ + HNO₃ → NH₄NO₃
Urea	CO(NH₂)₂	~46%	NH₃ + CO₂ → CO(NH₂)₂ + H₂O

Why is urea the most popular nitrogen fertilizer? It has the highest nitrogen content (46%) and is less hygroscopic (absorbs less water from air) compared to ammonium nitrate, making it easier to store and transport.

Phosphate Fertilizers

Made from phosphate rock (Ca₃(PO₄)₂):

Single superphosphate (SSP): Ca₃(PO₄)₂ + 2H₂SO₄ → Ca(H₂PO₄)₂ + 2CaSO₄
Triple superphosphate (TSP): Ca₃(PO₄)₂ + 4H₃PO₄ → 3Ca(H₂PO₄)₂

Worked Example 1

Question: Calculate the percentage of nitrogen in urea, CO(NH₂)₂. (M_C = 12, M_O = 16, M_N = 14, M_H = 1)

Solution:

Molar mass of CO(NH₂)₂ = 12 + 16 + 2(14 + 2×1) = 12 + 16 + 32 = 60 g/mol

Mass of nitrogen in one mole = 2 × 14 = 28 g

$$\%\text{N} = \frac{28}{60} \times 100\% = 46.67\%$$

Key Exam Notes — Fertilizers:

NPK fertilizers supply Nitrogen, Phosphorus, and Potassium.
Urea has the highest %N (46.67%) among common nitrogen fertilizers.
Ammonium sulfate: NH₃ + H₂SO₄; Ammonium nitrate: NH₃ + HNO₃.
Know how to calculate % composition of a fertilizer.
Excessive fertilizer use causes water pollution (eutrophication).

Practice Question 7: Calculate the percentage of nitrogen in ammonium sulfate, (NH₄)₂SO₄.

Answer:
Molar mass of (NH₄)₂SO₄ = 2(14 + 4×1) + 32 + 4×16 = 2(18) + 32 + 64 = 36 + 32 + 64 = 132 g/mol
Mass of N = 2 × 14 = 28 g
%N = (28/132) × 100% = 21.2%

3.3.4 Sulphuric Acid (H₂SO₄)

Sulfuric acid is often called the “king of chemicals” because it is the most widely used industrial chemical in the world! It is used in making fertilizers, detergents, batteries, and many other products. Sulfuric acid is manufactured by the Contact process.

The Contact Process

Raw materials: Sulfur (S) or sulfur dioxide (SO2), air (O2), and water (H2O)

Step 1: Production of SO₂

Sulfur is burned in air:

$$S(l) + O_2(g) \rightarrow SO_2(g)$$

Alternatively, SO₂ can be obtained by roasting metal sulfide ores, e.g.:

$$4FeS_2(s) + 11O_2(g) \rightarrow 2Fe_2O_3(s) + 8SO_2(g)$$

Step 2: Catalytic oxidation of SO₂ to SO₃

$$2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g) \quad \Delta H = -198 \text{ kJ/mol}$$

Conditions: Temperature: 400–450°C; Pressure: 1–2 atm; Catalyst: vanadium(V) oxide (V₂O₅)

Why these conditions? Think about it:

The reaction is exothermic, so low temperature favors SO₃. But a compromise temperature of 400–450°C is needed for a reasonable rate.
2 moles → 2 moles of gas, so pressure has minimal effect on equilibrium. Moderate pressure (1–2 atm) is used mainly for practical reasons.
V₂O₅ catalyst increases the rate at the compromise temperature.

Important: Why not use a higher temperature? Because at higher temperatures, the equilibrium shifts BACK (toward SO₂), reducing the yield. The name “Contact process” comes from the fact that SO₂ and O₂ must “contact” the catalyst surface.

Step 3: Absorption of SO₃

SO₃ is NOT dissolved directly in water (this would create a dangerous mist of H₂SO₄). Instead:

$$SO_3(g) + H_2SO_4(l) \rightarrow H_2S_2O_7(l)$$

Then the oleum (H₂S₂O₇) is diluted with water:

$$H_2S_2O_7(l) + H_2O(l) \rightarrow 2H_2SO_4(l)$$

Uses of Sulfuric Acid

Manufacture of fertilizers (ammonium sulfate, superphosphates)
Manufacture of detergents and soaps
Lead-acid batteries (in cars)
Petroleum refining
Chemical manufacturing (drugs, dyes, pigments)
Steel processing (pickling)

Key Exam Notes — Sulfuric Acid (Contact Process):

Three steps: S → SO₂ → SO₃ → H₂SO₄
Step 2 is the key step: 2SO₂ + O₂ ⇌ 2SO₃; exothermic; V₂O₅ catalyst at 400–450°C.
SO₃ is absorbed in H₂SO₄ (NOT water) to form oleum (H₂S₂O₇).
Oleum is then diluted to get H₂SO₄.
Compromise temperature — same reasoning as Haber process.
Before Step 2, SO₂ must be purified (dust removal, arsenic removal) to prevent catalyst poisoning.

Practice Question 8: Explain why SO₃ is absorbed in concentrated H₂SO₄ rather than directly in water.

Answer: If SO₃ is dissolved directly in water, the reaction is extremely exothermic and produces a fine mist of sulfuric acid droplets. This mist is very difficult to condense and collect — it would escape into the atmosphere as acid fog, causing pollution and loss of product. Instead, SO₃ is absorbed in concentrated H₂SO₄ to form oleum (H₂S₂O₇), which is a liquid that can be safely diluted with water later.

Practice Question 9: Why must SO₂ be purified before entering the contact tower?

Answer: Impurities such as dust particles, arsenic compounds, and other contaminants can “poison” the V₂O₅ catalyst. Catalyst poisoning means the impurities adsorb onto the catalyst surface and block the active sites, making the catalyst ineffective. Purification steps (dust removal by electrostatic precipitators, washing, and arsenic removal) ensure the catalyst remains active and has a long working life.

3.3.5 Some Common Pesticides and Herbicides

Agricultural pests (insects, weeds, fungi) reduce crop yields significantly. Chemicals used to control these pests are called pesticides.

Type	Purpose	Examples
Insecticides	Kill insects	DDT, malathion, rotenone
Herbicides (weed killers)	Kill weeds	2,4-D, paraquat, glyphosate
Fungicides	Kill fungi	Bordeaux mixture (CuSO₄ + Ca(OH)₂)
Rodenticides	Kill rodents	Warfarin, zinc phosphide

Problems with pesticides:

Bioaccumulation — pesticides build up in food chains (e.g., DDT in eagle eggs).
Pollution of water and soil.
Pests can develop resistance over time.
Harmful to non-target organisms (bees, fish, birds).

Key Exam Notes — Pesticides:

Know the four types: insecticides, herbicides, fungicides, rodenticides.
Bordeaux mixture = CuSO₄ + Ca(OH)₂ (fungicide).
DDT is a chlorinated hydrocarbon insecticide — banned in many countries due to bioaccumulation.
Integrated Pest Management (IPM) combines chemical, biological, and cultural methods to reduce pesticide use.

Practice Question 10: What is bioaccumulation? Why is DDT particularly problematic in this regard?

Answer: Bioaccumulation is the process where a chemical substance builds up in an organism’s body over time because it is absorbed faster than it can be eliminated. DDT is fat-soluble (not water-soluble), so it accumulates in fatty tissues. As it moves up the food chain (from water → plankton → small fish → large fish → birds of prey), its concentration increases at each level (biomagnification). This caused thinning of eggshells in birds like eagles, leading to population decline — which is why DDT was banned in many countries.

3.3.6 Sodium Carbonate (Na₂CO₃)

Sodium carbonate (washing soda, Na₂CO₃·10H₂O) is used in glass manufacturing, soap production, water softening, and as a cleaning agent. It is manufactured by the Solvay process.

The Solvay Process

Raw materials: Sodium chloride (NaCl), limestone (CaCO₃), ammonia (NH₃)

Step 1: Ammonia is absorbed in saturated brine (NaCl solution):
NH₃(g) + H₂O(l) → NH₄⁺(aq) + OH⁻(aq)

Step 2: CO₂ (from heating limestone) is passed through the solution:
CaCO₃(s) → CaO(s) + CO₂(g)

The CO₂ reacts with the ammoniated brine:

$$NH_4^+(aq) + HCO_3^-(aq) + Na^+(aq) + Cl^-(aq) \rightarrow NaHCO_3(s) + NH_4^+(aq) + Cl^-(aq)$$

Sodium bicarbonate (NaHCO₃) precipitates because it is less soluble in the cold solution.

Step 3: NaHCO₃ is filtered and heated to form Na₂CO₃:

$$2NaHCO_3(s) \xrightarrow{\Delta} Na_2CO_3(s) + CO_2(g) + H_2O(g)$$

The CO₂ produced is recycled back to Step 2.

Step 4: The CaO from Step 2 (heating limestone) is slaked to form Ca(OH)₂, which is used to recover NH₃ from the remaining NH₄Cl solution:

$$2NH_4Cl(aq) + Ca(OH)_2(aq) \rightarrow 2NH_3(g) + CaCl_2(aq) + 2H_2O(l)$$

The recovered NH₃ is recycled. The only by-product is CaCl₂.

Key Exam Notes — Sodium Carbonate (Solvay Process):

Raw materials: NaCl, CaCO₃, NH₃ (only NH₃ is recycled, not consumed).
Key intermediate: NaHCO₃ precipitates and is heated to give Na₂CO₃.
CO₂ and NH₃ are both recycled — this makes the process economical.
By-product: CaCl₂ (used for road de-icing, dust control).
The Solvay process cannot produce potassium carbonate (KHCO₃ is too soluble to precipitate).

Practice Question 11: Why is ammonia recycled in the Solvay process?

Answer: Ammonia is an expensive raw material. If it were not recycled and was lost after one cycle, the process would be economically unviable. By treating the NH₄Cl solution with Ca(OH)₂, ammonia gas is released and can be fed back into the process. This recycling means only a small initial amount of ammonia is needed to keep the process running continuously.

3.3.7 Sodium Hydroxide (NaOH)

Sodium hydroxide (caustic soda) is one of the most important industrial chemicals. It is used in soap making, paper production, aluminum extraction, and many other processes. NaOH is produced by the electrolysis of concentrated NaCl solution (brine) — the chlor-alkali process (which we studied in Unit 2).

Chlor-Alkali Process Summary:

$$2NaCl(aq) + 2H_2O(l) \xrightarrow{\text{electrolysis}} 2NaOH(aq) + Cl_2(g) + H_2(g)$$

Products: NaOH (remains in solution), Cl₂ (at anode), H₂ (at cathode)

Uses of NaOH

Soap and detergent manufacturing (saponification of fats)
Paper and pulp industry
Aluminum extraction (Bayer process: dissolves Al₂O₃)
Textile industry (mercerization of cotton)
Oil refining
Drain cleaner

Key Exam Notes — Sodium Hydroxide:

NaOH is produced by electrolysis of brine (chlor-alkali process).
Three products: NaOH, Cl₂, and H₂.
NaOH is a strong base and is highly corrosive.
Used in soap making (reacts with fats/oils to form soap + glycerol).

3.4 Some Manufacturing Industries in Ethiopia

Ethiopia has been developing its industrial sector rapidly. Let’s learn about some of the important manufacturing industries in our country.

3.4.1 Glass Manufacturing

Glass is made by heating a mixture of raw materials until they fuse together. The main ingredient is silica sand (SiO₂), but pure silica needs very high temperature to melt. So other chemicals are added to lower the melting point.

Raw materials for glass:
Silica sand (SiO2) — the main component (~70%)
Sodium carbonate (Na2CO3) — lowers melting point (acts as a flux)
Limestone (CaCO3) — adds strength and durability
Cullet (recycled glass) — reduces energy needed and waste

Manufacturing process:

Raw materials are mixed in correct proportions and finely ground.
The mixture is melted in a furnace at about 1500°C.
The molten glass is shaped by blowing, pressing, or drawing.
The glass is annealed (slowly cooled) to remove internal stresses and prevent cracking.

Types of glass: soda-lime glass (windows, bottles), borosilicate glass (lab glassware — Pyrex), colored glass (with metal oxides), safety glass (laminated).

Key Exam Notes — Glass:

Main components: SiO₂ + Na₂CO₃ + CaCO₃
Na₂CO₃ acts as a flux (lowers melting point).
CaCO₃ adds strength/chemical stability.
Annealing = slow cooling to remove stress.
Borosilicate glass has B₂O₃ added — resistant to thermal shock.

Practice Question 12: What is the role of sodium carbonate in glass manufacturing?

Answer: Sodium carbonate (Na₂CO₃) acts as a flux — it lowers the melting point of silica (SiO₂) from about 1700°C to about 1500°C. This significantly reduces the energy needed for the melting process, making glass production more economical. Without a flux, the furnace would need much higher temperatures, increasing fuel costs.

3.4.2 Manufacturing of Ceramics

Ceramics are made from clay and other non-metallic minerals that are shaped and then hardened by heating. Ethiopia has a long tradition of ceramics.

Raw materials: Clay (kaolin, Al2O3·2SiO2·2H2O), feldspar, quartz, water

Process:

Preparation: Clay is purified, ground, and mixed with water to form a plastic mass.
Forming: Shaped by hand, on a potter’s wheel, or by casting into molds.
Drying: Slowly dried to remove water (prevents cracking).
Firing (biscuit firing): Heated in a kiln at about 900–1000°C to harden.
Glazing: A glassy coating is applied and the item is fired again at higher temperature (~1100–1300°C).

Uses: pottery, tiles, sanitary ware, electrical insulators, refractory bricks (for furnace linings).

3.4.3 Cement

Cement is one of Ethiopia’s most important industrial products — think of all the buildings, roads, and dams being constructed! Cement is mainly Portland cement.

Raw materials: Limestone (CaCO3) and clay (containing SiO2, Al2O3, Fe2O3)

Manufacturing Process (Dry Process):

Crushing and grinding: Raw materials are ground into a fine powder.
Blending: Mixed in correct proportions (about 60–65% CaO, 17–25% SiO₂, 3–8% Al₂O₃, 1–6% Fe₂O₃).
Preheating: Fed into a preheater tower to reduce energy consumption.
Burning in rotary kiln: Heated to about 1450°C. Chemical reactions form clinker — hard, grey nodules.
Cooling: Clinker is cooled quickly.
Adding gypsum: 2–5% gypsum (CaSO₄·2H₂O) is added to clinker and ground into fine powder — this is cement.

Why is gypsum added? Without gypsum, cement would harden immediately when water is added (flash setting). Gypsum delays the setting time, allowing time for mixing, transporting, and placing the concrete.

Chemical composition of clinker:

Tricalcium silicate (3CaO·SiO₂) — hardens quickly, gives early strength
Dicalcium silicate (2CaO·SiO₂) — hardens slowly, gives long-term strength
Tricalcium aluminate (3CaO·Al₂O₃) — generates heat during setting
Tetracalcium aluminoferrite (4CaO·Al₂O₃·Fe₂O₃)

When cement is mixed with water, it undergoes hydration reactions and hardens into a strong solid — this is concrete (when mixed with sand and gravel).

Key Exam Notes — Cement:

Raw materials: limestone + clay.
Key step: burning in rotary kiln at ~1450°C to form clinker.
Gypsum (2–5%) is added to control setting time (prevent flash setting).
Cement + water + sand + gravel = concrete.
Ethiopia has large limestone deposits — good for cement industry.

Practice Question 13: Explain the role of gypsum in cement manufacturing.

Answer: Gypsum (CaSO₄·2H₂O) is added to the clinker before final grinding (about 2–5% by weight). Its role is to retard (slow down) the setting time of cement. Without gypsum, the tricalcium aluminate (3CaO·Al₂O₃) in cement would react violently with water and cause “flash setting” — the cement would harden almost immediately, leaving no time for mixing, transporting, or placing. Gypsum forms a protective layer around the cement particles, slowing the hydration reaction and allowing workable time.

3.4.4 Sugar Manufacturing

Ethiopia is one of Africa’s major sugar producers. Sugar is manufactured mainly from sugarcane (grown in Wonji, Metehara, Fincha, and Tendaho sugar estates).

Manufacturing process:

Harvesting and crushing: Sugarcane is crushed to extract juice.
Purification: Juice is treated with lime (CaO) and heated — impurities precipitate and are filtered out.
Evaporation: Clear juice is concentrated by evaporating water in multiple-effect evaporators.
Crystallization: Concentrated syrup is further heated under vacuum until sugar crystals form.
Centrifugation: Crystals are separated from remaining syrup (molasses) by spinning in a centrifuge.
Drying and packaging: Sugar crystals are dried and packaged.

By-products of the sugar industry:

Molasses — used to make ethanol, animal feed
Bagasse (cane fiber) — used as fuel for boilers, paper pulp, animal feed
Press mud — used as fertilizer

Key Exam Notes — Sugar:

Raw material: sugarcane (Ethiopia: Wonji, Metehara, Fincha, Tendaho).
Key steps: crushing → purification (lime) → evaporation → crystallization → centrifugation.
By-products: molasses (for ethanol), bagasse (fuel/fiber), press mud (fertilizer).
Lime (CaO) is added to neutralize acidity and precipitate impurities.

3.4.5 Paper and Pulp

Paper is made from cellulose fibers, which come from wood, bagasse (sugarcane residue), or recycled paper. Ethiopia produces paper using both wood pulp and bagasse.

Process:

Pulping: Raw material is broken down into fibers by chemical (kraft process using NaOH + Na₂S) or mechanical means.
Bleaching: Pulp is bleached with chlorine or hydrogen peroxide to make white paper.
Paper making: Pulp is mixed with water, spread on a wire mesh, water drains away, and the sheet is pressed and dried.

3.4.6 Tannery

Ethiopia has the largest livestock population in Africa, making the leather industry very important. Tanneries convert animal hides and skins into leather.

Main stages:

Curing: Hides are treated with salt to prevent decomposition.
Soaking and liming: Rehydrated and treated with lime (Ca(OH)₂) and sodium sulfide to remove hair and flesh.
Deliming: Lime is removed using ammonium salts.
Tanning: Hides are treated with chromium(III) sulfate (chrome tanning) or vegetable tannins to preserve and strengthen the leather.
Finishing: Leather is dyed, dried, and polished.

Key Exam Notes — Tannery:

Lime (Ca(OH)₂) + Na₂S removes hair (unhairing).
Chrome tanning uses Cr₂(SO₄)₃ — gives soft, flexible leather.
Vegetable tanning uses tannins from tree bark — gives firm, brown leather.
Ethiopia’s large livestock population supports a strong leather industry.

3.4.7 Food Processing and Preservation

Food processing transforms raw agricultural products into preserved, safe, and convenient food. Methods include:

Thermal processing: Canning (heating food in sealed containers), pasteurization (milk, juice)
Dehydration/drying: Sun drying, spray drying (milk powder)
Cold preservation: Refrigeration, freezing
Chemical preservation: Using salt, sugar, vinegar, or food preservatives (sodium benzoate, sorbic acid)
Fermentation: Injera (teff + fermentation), tella, tej

3.4.8 Manufacturing of Ethanol

Ethanol (ethyl alcohol, C₂H₅OH) can be produced by fermentation or synthetic methods.

Fermentation method:

Sugars from molasses, grains, or fruits are converted to ethanol by yeast (Saccharomyces cerevisiae):

$$C_6H_{12}O_6(aq) \xrightarrow{\text{yeast}} 2C_2H_5OH(aq) + 2CO_2(g)$$

The ethanol produced is about 8–15% concentrated. It is then purified by fractional distillation to about 95% ethanol.

Synthetic method:

Ethylene (from petroleum cracking) is hydrated:

$$CH_2=CH_2(g) + H_2O(g) \xrightarrow{H_3PO_4 \text{ catalyst}} CH_3CH_2OH(g)$$

Uses of ethanol: Beverages, solvent, fuel (bioethanol), antiseptic, chemical feedstock.

3.4.9 Soap and Detergent

Soap has been made for thousands of years. It is produced by saponification — the reaction of fats or oils with sodium hydroxide.

Saponification Reaction:$$\text{Fat/Oil (triglyceride)} + 3NaOH \rightarrow \text{Soap (sodium salt of fatty acid)} + \text{Glycerol}$$

Soap manufacturing process:

Saponification: Fat or oil is heated with NaOH solution.
Salting out: NaCl is added to precipitate (separate) the soap from the mixture.
Purification: Soap is washed, dried, and molded into bars.

How soap works: Soap molecules have two parts — a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. The tail attaches to grease/oil, while the head interacts with water. This allows grease to be lifted off surfaces and washed away as emulsified droplets.

Soap vs. Detergent:

Feature	Soap	Detergent
Source	Natural fats/oils + NaOH	Synthetic (from petroleum)
Works in hard water?	No (forms scum with Ca²⁺/Mg²⁺)	Yes (does not form scum)
Biodegradability	Highly biodegradable	Some types are not biodegradable
Example	Na stearate	Sodium dodecyl benzene sulfonate

Why doesn’t soap work well in hard water? Hard water contains Ca²⁺ and Mg²⁺ ions. These react with soap to form insoluble calcium/magnesium salts of fatty acids — a gray, sticky substance called scum. This wastes soap and leaves stains. Detergents don’t have this problem because their calcium salts are soluble.

Key Exam Notes — Soap and Detergent:

Soap = fat/oil + NaOH (saponification).
Soap has hydrophilic head + hydrophobic tail — this is how it cleans.
Soap forms scum in hard water (with Ca²⁺/Mg²⁺); detergents do not.
Detergents are synthetic, work in hard water, but some are non-biodegradable.
By-product of soap making: glycerol (used in cosmetics, pharmaceuticals).

Practice Question 14: Explain with a chemical equation why soap does not work well in hard water.

Answer: Hard water contains Ca²⁺ and Mg²⁺ ions. Soap (sodium stearate, C₁₇H₃₅COONa) reacts with these ions to form insoluble salts (scum):
2C₁₇H₃₅COONa + Ca²⁺ → (C₁₇H₃₅COO)₂Ca↓ + 2Na⁺
The calcium stearate (scum) is insoluble in water, so it precipitates as a gray, sticky substance. This wastes soap (less soap available for cleaning) and leaves stains on clothes and surfaces. Detergents avoid this problem because their calcium salts remain soluble.

Practice Question 15: What is glycerol and how is it obtained during soap manufacturing?

Answer: Glycerol (glycerin, C₃H₅(OH)₃) is a thick, sweet-tasting liquid that is a by-product of the saponification reaction. When a triglyceride (fat/oil) reacts with NaOH, it is broken down into soap (sodium salts of fatty acids) and glycerol. The glycerol remains dissolved in the water layer and is separated from the soap by “salting out” (adding NaCl precipitates the soap while glycerol stays in solution). Glycerol is used in cosmetics, pharmaceuticals, food, and as an antifreeze.

Revision Notes — Exam Focus

Summary of Major Industrial Processes

Process	Product	Key Reaction	Key Conditions
Haber	NH₃	N₂ + 3H₂ ⇌ 2NH₃	Fe catalyst, 450–500°C, 150–300 atm
Ostwald	HNO₃	4NH₃ + 5O₂ → 4NO + 6H₂O	Pt/Rh catalyst, 850°C
Contact	H₂SO₄	2SO₂ + O₂ ⇌ 2SO₃	V₂O₅ catalyst, 400–450°C
Solvay	Na₂CO₃	2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O	NaHCO₃ precipitates from cold solution
Chlor-alkali	NaOH, Cl₂, H₂	2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂	Electrolysis of brine

Important Definitions

Renewable resource: Can be naturally replenished within a short time (e.g., water, wood).
Non-renewable resource: Cannot be replaced once used (e.g., coal, petroleum, minerals).
Flux: A substance added to lower the melting point (e.g., Na₂CO₃ in glass).
Slag: Impurities removed as molten waste during metal extraction.
Clinker: Hard nodules produced in the cement kiln before grinding with gypsum.
Saponification: Reaction of fats/oils with NaOH to produce soap + glycerol.
Bioaccumulation: Build-up of substances in organisms over time.
Bioremediation: Use of living organisms to clean up pollution.
Eutrophication: Excessive nutrients in water causing algal bloom and oxygen depletion.
Annealing: Slow cooling of glass to remove internal stresses.

Key Equations to Remember

$$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \quad \text{(Haber)}$$
$$4NH_3(g) + 5O_2(g) \rightarrow 4NO(g) + 6H_2O(g) \quad \text{(Ostwald Step 1)}$$
$$2NO(g) + O_2(g) \rightarrow 2NO_2(g) \quad \text{(Ostwald Step 2)}$$
$$3NO_2(g) + H_2O(l) \rightarrow 2HNO_3(aq) + NO(g) \quad \text{(Ostwald Step 3)}$$
$$2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g) \quad \text{(Contact)}$$
$$SO_3 + H_2SO_4 \rightarrow H_2S_2O_7 \quad \text{(oleum formation)}$$
$$2NaHCO_3(s) \xrightarrow{\Delta} Na_2CO_3(s) + CO_2(g) + H_2O(g) \quad \text{(Solvay)}$$
$$C_6H_{12}O_6 \xrightarrow{\text{yeast}} 2C_2H_5OH + 2CO_2 \quad \text{(Fermentation)}$$

Common Mistakes to Avoid

Mistake 1: Writing the Haber process reaction as non-reversible (no ⇌). It IS reversible — equilibrium is important!
Mistake 2: Saying SO3 is dissolved directly in water. NO — it goes into H2SO4 first to make oleum.
Mistake 3: Confusing catalysts: Haber = Fe; Ostwald = Pt/Rh; Contact = V2O5.
Mistake 4: Saying NH3 is consumed in the Solvay process. NH3 is RECYCLED, not consumed.
Mistake 5: Forgetting that the Contact process reaction has equal moles of gas on both sides (2:1→2:2), so pressure has minimal effect.
Mistake 6: Saying cement = clinker alone. Gypsum MUST be added (2–5%) to control setting.
Mistake 7: Confusing soap and detergent — soap comes from natural fats + NaOH; detergents are synthetic from petroleum.
Mistake 8: Saying urea has lower %N than ammonium sulfate. Urea has 46.7% N vs. 21.2% for ammonium sulfate — urea is HIGHER.
Mistake 9: Forgetting to mention NO recycling in the Ostwald process.
Mistake 10: Confusing the temperatures: Haber (450–500°C), Ostwald (850°C), Contact (400–450°C).

Ethiopian Industries — Quick Reference

Industry	Main Raw Materials	Key Locations in Ethiopia
Cement	Limestone + clay	Mugher, Diredawa, Mekelle
Sugar	Sugarcane	Wonji, Metehara, Fincha, Tendaho
Leather/Tannery	Animal hides/skins	Modjo, Kombolcha, Bahir Dar
Glass	Silica sand + Na₂CO₃ + CaCO₃	Addis Ababa
Textile	Cotton	Awasa, Bahir Dar, Kombolcha
Paper	Wood/bagasseAwasa, Addis Ababa
Chemical/Fertilizer	Natural gas, minerals	Awash Melkasa (fertilizer factory)

Challenge Exam Questions

Test yourself with these difficult questions! Try each one before checking the answer.

Section A: Multiple Choice Questions

Question 1: Which catalyst is used in the Contact process for sulfuric acid manufacture?
(a) Iron (Fe) (b) Platinum-rhodium (Pt/Rh) (c) Vanadium(V) oxide (V₂O₅) (d) Manganese(IV) oxide (MnO₂)

Answer: (c) Vanadium(V) oxide (V₂O₅)
V₂O₅ is the catalyst used in the Contact process for the oxidation of SO₂ to SO₃. Iron is used in the Haber process. Pt/Rh is used in the Ostwald process. MnO₂ is used as a catalyst in the laboratory preparation of chlorine from HCl.

Question 2: The function of adding gypsum to cement clinker is to:
(a) Increase the strength of cement (b) Speed up the setting time
(c) Slow down the setting time (d) Lower the melting point of the mixture

Answer: (c) Slow down the setting time
Gypsum (CaSO₄·2H₂O) is added to prevent flash setting — the rapid hardening that would occur due to tricalcium aluminate reacting with water. Gypsum forms a protective layer, retarding the hydration and giving workers enough time to mix, transport, and place the concrete.

Question 3: Which nitrogen fertilizer has the highest percentage of nitrogen?
(a) Ammonium sulfate (b) Ammonium nitrate (c) Urea (d) Calcium ammonium nitrate

Answer: (c) Urea
Urea (CO(NH₂)₂) has 46.7% nitrogen, which is the highest among common nitrogen fertilizers. Ammonium sulfate has ~21%, ammonium nitrate has ~35%, and calcium ammonium nitrate has about 26% nitrogen.

Question 4: In the Solvay process, ammonia is recovered by treating NH₄Cl solution with:
(a) H₂SO₄ (b) Ca(OH)₂ (c) NaOH (d) HCl

Answer: (b) Ca(OH)₂
In the Solvay process, the NH₄Cl remaining after NaHCO₃ precipitation is treated with Ca(OH)₂ (slaked lime, obtained from heating limestone): 2NH₄Cl + Ca(OH)₂ → 2NH₃ + CaCl₂ + 2H₂O. The NH₃ is recycled. This Ca(OH)₂ comes from the same limestone that provides CO₂, making the process efficient.

Question 5: SO₃ from the Contact process is absorbed in concentrated H₂SO₄ rather than water because:
(a) Water is too expensive (b) Direct dissolution creates an acid mist
(c) H₂SO₄ reacts faster (d) Water would reduce SO₃ to SO₂

Answer: (b) Direct dissolution creates an acid mist
When SO₃ reacts directly with water, the reaction is so exothermic that it produces a fine mist of H₂SO₄ droplets that are difficult to condense and would escape as atmospheric pollution. Absorbing in concentrated H₂SO₄ to form oleum is safer and more efficient.

Section B: Fill in the Blanks

Question 6: In the Haber process, a __________ temperature is used because low temperature gives a high yield but a very slow __________.

Answer: compromise (450–500°C); rate of reaction.

Question 7: In the Ostwald process, Step 2 (NO to NO₂) does not require a __________ because the reaction occurs __________ at room temperature.

Answer: catalyst; spontaneously.

Question 8: The main raw materials for glass manufacturing are silica sand, sodium __________, and __________.

Answer: carbonate (Na₂CO₃); limestone (CaCO₃).

Question 9: In sugar manufacturing, the by-product __________ can be used as fuel for boilers, and __________ can be fermented to produce ethanol.

Answer: bagasse; molasses.

Question 10: Soap does not work well in hard water because it forms insoluble __________ with Ca²⁺ and Mg²⁺ ions, known as __________.

Answer: salts (calcium/magnesium salts of fatty acids); scum.

Section C: Short Answer Questions

Question 11: Compare the Haber process and the Contact process in terms of: (a) nature of the reaction (exothermic/endothermic), (b) effect of pressure on equilibrium, (c) catalyst used.

Answer:
(a) Both are exothermic (Haber: ΔH = −92 kJ/mol; Contact: ΔH = −198 kJ/mol).
(b) Haber: 4 mol gas → 2 mol gas — increasing pressure significantly favors the forward reaction. Contact: 2 mol gas → 2 mol gas — pressure change has minimal effect on equilibrium.
(c) Haber: Iron (Fe) with Al₂O₃/K₂O promoters. Contact: Vanadium(V) oxide (V₂O₅).

Question 12: Why is it important for Ethiopia to develop its fertilizer manufacturing industry rather than importing all fertilizers?

Answer: Developing local fertilizer manufacturing has several advantages: (1) Ethiopia’s economy depends heavily on agriculture, and fertilizers are essential for increasing crop yields to ensure food security. (2) Importing fertilizers costs large amounts of foreign currency (foreign exchange). (3) Ethiopia has the necessary raw materials — natural gas (for hydrogen in Haber process) and phosphate rock. (4) Local manufacturing creates jobs and develops industrial skills. (5) It reduces dependence on international supply chains which can be disrupted. The fertilizer factory at Awash Melkasa is an example of this effort.

Question 13: Describe the environmental problems associated with the fertilizer industry and suggest possible solutions.

Answer:
Problems:
(1) Eutrophication: Excess fertilizer (especially nitrates and phosphates) washes into rivers and lakes, causing algal blooms that deplete oxygen and kill aquatic life.
(2) NO_x and SO_x emissions: From the Haber and Contact processes, contributing to acid rain and air pollution.
(3) Ammonia leakage: Can contaminate soil and water, harm aquatic organisms.

Solutions:
(1) Educate farmers on proper fertilizer application rates and timing.
(2) Use slow-release fertilizers that reduce leaching.
(3) Install pollution control equipment (scrubbers, catalytic converters) in factories.
(4) Promote organic farming and composting to reduce chemical fertilizer dependency.
(5) Monitor water quality near fertilizer plants.

Question 14: Explain why the Solvay process cannot be used to manufacture potassium carbonate (K₂CO₃).

Answer: In the Solvay process, sodium bicarbonate (NaHCO₃) precipitates from the cold ammoniated brine because it has low solubility. However, potassium bicarbonate (KHCO₃) is significantly MORE soluble in cold water and does NOT precipitate under the same conditions. Since the precipitation step is essential for separating the product, the Solvay process cannot be adapted for potassium carbonate. K₂CO₃ must be produced by other methods (e.g., from potassium chloride and magnesium carbonate).

Section D: Step-by-Step Calculation Questions

Question 15: Calculate the percentage of phosphorus in single superphosphate (SSP), Ca(H₂PO₄)₂. (M_Ca = 40, M_H = 1, M_P = 31, M_O = 16)

Answer:
Molar mass of Ca(H₂PO₄)₂ = 40 + 4(1) + 2(31) + 8(16) = 40 + 4 + 62 + 128 = 234 g/mol
Mass of P in one mole = 2 × 31 = 62 g
%P = (62/234) × 100% = 26.5%

Question 16: An ammonia plant produces 340 kg of NH₃ per hour. (a) What volume of N₂ (at STP) is consumed per hour? (b) What volume of H₂ (at STP) is consumed per hour? (M_N = 14, M_H = 1; molar volume at STP = 22.4 L/mol)

Answer:
Molar mass of NH₃ = 14 + 3 = 17 g/mol
Moles of NH₃ produced per hour = 340,000 / 17 = 20,000 mol

From N₂ + 3H₂ → 2NH₃:
(a) Moles of N₂ needed = 20,000/2 = 10,000 mol
Volume of N₂ at STP = 10,000 × 22.4 = 224,000 L = 224 m³

(b) Moles of H₂ needed = 3 × 10,000 = 30,000 mol
Volume of H₂ at STP = 30,000 × 22.4 = 672,000 L = 672 m³

Question 17: A cement factory produces 1000 tonnes of clinker per day. How much gypsum (in kg) should be added per day to make cement? (Assume 4% gypsum by mass)

Answer:
Mass of gypsum needed = 4% of clinker mass
= 0.04 × 1000 tonnes = 40 tonnes per day
= 40 × 1000 = 40,000 kg per day

Question 18: In the fermentation of glucose, 180 g of glucose (C₆H₁₂O₆) produced 46 g of ethanol (C₂H₅OH). Calculate the percentage yield. (M_glucose = 180, M_ethanol = 46)

Answer:
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
1 mol glucose (180 g) → 2 mol ethanol (2 × 46 = 92 g)
Theoretical yield of ethanol from 180 g glucose = 92 g
Actual yield = 46 g
% yield = (46/92) × 100% = 50%

Question 19: Write balanced chemical equations for the complete Solvay process, starting from NaCl, CaCO₃, and NH₃, and ending with Na₂CO₃. Show how NH₃ and CO₂ are recycled.

Answer:
1. Limestone decomposition: CaCO₃(s) → CaO(s) + CO₂(g)
2. Slaking: CaO(s) + H₂O(l) → Ca(OH)₂(aq)
3. Ammoniation: NH₃(g) + H₂O(l) → NH₄⁺(aq) + OH⁻(aq)
4. Carbonation: NH₄⁺(aq) + HCO₃⁻(aq) + Na⁺(aq) → NaHCO₃(s)↓ + NH₄⁺(aq)
(From: NH₃ + CO₂ + H₂O → NH₄HCO₃; then NH₄HCO₃ + NaCl → NaHCO₃ + NH₄Cl)
5. Calcination: 2NaHCO₃(s) → Na₂CO₃(s) + CO₂(g) + H₂O(g) — CO₂ is recycled to Step 4
6. Ammonia recovery: 2NH₄Cl(aq) + Ca(OH)₂(aq) → 2NH₃(g) + CaCl₂(aq) + 2H₂O(l) — NH₃ is recycled to Step 3
Net result: NaCl + CaCO₃ → Na₂CO₃ + CaCl₂ (NH₃ and CO₂ are not consumed)

Question 20: Explain why bioaccumulation of DDT is a greater problem for top predators (like eagles) than for organisms lower in the food chain.

Answer: DDT is fat-soluble and not easily excreted by organisms. When it enters a food chain, each organism at a higher trophic level eats many organisms from the level below. Since DDT accumulates in body fat and is not eliminated, the concentration increases at each level — this is called biomagnification. For example: if water has 0.001 ppb DDT, plankton might have 0.04 ppb, small fish 0.5 ppb, large fish 2 ppb, and eagle tissues 25 ppb. Top predators like eagles eat many large fish, so they receive a massive dose. The accumulated DDT interferes with calcium metabolism, causing thin eggshells that break during incubation — leading to population decline. This is why DDT was banned in many countries.

Question 21: A student claims that both the Haber process and the Contact process use a compromise temperature for the same reason. Is this claim correct? Explain with reference to Le Chatelier’s principle.

Answer: Yes, the claim is correct in principle. Both processes involve exothermic reactions, so low temperature would give higher equilibrium yield (Le Chatelier’s principle: decreasing temperature favors the exothermic direction). However, low temperature also means very slow reaction rate (kinetic limitation). Therefore, in BOTH processes, a compromise temperature is chosen — high enough for a reasonable rate but low enough for acceptable yield. The catalyst (Fe in Haber, V₂O₅ in Contact) allows the compromise temperature to be used by increasing the rate without needing even higher temperatures.