Prepare for second-year exams with solved exercises on Common Chemical Industries, based on the PTB curriculum. Cover essential topics such as industrial processes, applications, and key concepts to boost your understanding.
Q. 5 (a) What are fertilizers? Why are they needed?
Fertilizers are chemical or natural substances added to soil to supply essential nutrients to plants and promote growth. They are needed to replenish the soil with nutrients that are depleted due to continuous crop cultivation, ensuring high yields and healthy plant growth.
(b) Discuss the classification of fertilizers and their uses.
Fertilizers are classified into two main types:
- Organic fertilizers: These come from natural sources such as manure, compost, and bone meal. They improve soil structure and water retention while providing nutrients.
- Inorganic (chemical) fertilizers: These are synthesized to provide specific nutrients like nitrogen, phosphorus, and potassium (NPK).
- Nitrogen fertilizers (e.g., urea): Promote leaf and stem growth.
- Phosphorus fertilizers (e.g., superphosphate): Enhance root development and flowering.
- Potassium fertilizers (e.g., potassium chloride): Improve overall plant health and disease resistance.
(c) How is urea manufactured in Pakistan? Describe in detail the process used.
Urea is synthesized from ammonia (NH₃) and carbon dioxide (CO₂) using the Bosch-Meiser process:
- Ammonia synthesis: Nitrogen is obtained from air, and hydrogen is derived from natural gas. These two gases react at high pressure and temperature in the presence of a catalyst to form ammonia.
N₂ + 3H₂ → 2NH₃ - Urea synthesis: Ammonia reacts with carbon dioxide under high pressure to form ammonium carbamate, which is then dehydrated to form urea.
2NH₃ + CO₂ → NH₂CONH₂ + H₂O
The resulting urea is then prilled or granulated for use as fertilizer.
Q. 6 (a) What are the prospects of fertilizer industry in Pakistan?
The fertilizer industry in Pakistan has significant growth potential due to the country’s agrarian economy. Increasing demand for agricultural productivity, government subsidies, and rising export opportunities can further boost the industry. However, challenges include the high cost of raw materials and energy shortages.
(b) What are essential nutrient elements and why are these needed for plant growth?
Essential nutrient elements are chemical elements required for plants to grow and complete their life cycle. They are divided into macronutrients and micronutrients:
- Macronutrients: Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), and Sulfur (S).
- Micronutrients: Iron (Fe), Zinc (Zn), Manganese (Mn), Boron (B), Copper (Cu), Molybdenum (Mo), and Chlorine (Cl).
These nutrients play vital roles in photosynthesis, respiration, enzyme activation, and overall plant health.
(c) Write down the essential qualities of a good fertilizer.
A good fertilizer should:
- Contain the right proportion of essential nutrients.
- Be easily absorbed by plants.
- Be cost-effective and easy to apply.
- Have minimal environmental impact (low toxicity).
- Improve soil fertility over time without causing nutrient imbalance.
Q. 7 (a) Describe the composition of a good Portland cement.
Portland cement is composed of the following main ingredients:
- Calcium oxide (CaO): 60-65%
- Silica (SiO₂): 20-25%
- Alumina (Al₂O₃): 5-10%
- Iron oxide (Fe₂O₃): 2-4%
- Magnesia (MgO): 1-3%
- Sulfur trioxide (SO₃): 1-2%
Additionally, small amounts of gypsum are added to control the setting time.
(b) Discuss the wet process for the manufacturing of cement with the help of flow sheet diagram.
In the wet process, the raw materials (limestone and clay) are ground with water to form a slurry. The steps include:
- Raw material preparation: Limestone and clay are crushed and mixed with water to form slurry.
- Mixing: The slurry is blended to achieve uniform composition.
- Heating in a kiln: The slurry is fed into a rotary kiln and heated to high temperatures (up to 1400-1500°C), resulting in clinker formation.
- Clinker cooling and grinding: The clinker is cooled, mixed with gypsum, and ground to produce cement.
(c) What do you understand by the term “setting of cement”? Also discuss the reactions taking place in the first 24 hours.
The “setting of cement” refers to the process by which the cement paste transforms from a fluid to a solid state upon hydration.
- Initial setting: The hydration of tricalcium silicate (C₃S) and dicalcium silicate (C₂S) produces calcium silicate hydrate (C-S-H) and calcium hydroxide (Ca(OH)₂), resulting in the hardening of the cement.
- Within 24 hours: The initial set occurs as C₃S reacts rapidly, contributing to early strength development, while C₂S reacts slowly, providing long-term strength.
Q. 8 What are the essential non-woody raw materials used in the production of pulp and paper in Pakistan?
The essential non-woody raw materials used in pulp and paper production in Pakistan include:
- Wheat straw
- Bagasse (sugarcane waste)
- Rice straw
- Kenaf (a fibrous plant)
These materials are chosen for their availability and lower environmental impact compared to wood-based raw materials.
Q. 9 (a) What are the principal methods of chemical pulping used for the production of paper?
The principal methods of chemical pulping are:
- Kraft process (sulfate process): Wood chips are cooked in sodium hydroxide (NaOH) and sodium sulfide (Na₂S) to break down lignin and release cellulose fibers.
- Sulfite process: Uses sulfurous acid (H₂SO₃) and bisulfites to dissolve lignin.
- Soda process: Uses sodium hydroxide (NaOH) alone to separate fibers.
(b) Describe the neutral sulphite semi-chemical (NSSC) process for the manufacturing of pulp and paper.
In the NSSC process, wood chips are treated with sodium sulfite (Na₂SO₃) and sodium carbonate (Na₂CO₃) to partially break down lignin, followed by mechanical refining. This process produces a pulp with a balance between strength and quality, suitable for making corrugated boards and packaging material.
Q. 10 (a) What are the common bleaching agents used in the paper industry in Pakistan? Briefly describe the bleaching process.
Common bleaching agents used in the paper industry include:
- Chlorine dioxide (ClO₂)
- Hydrogen peroxide (H₂O₂)
- Oxygen (O₂)
- Ozone (O₃)
Bleaching process:
The bleaching process removes the residual lignin and brightens the pulp. It typically involves multiple stages:
- Lignin removal: Chlorine dioxide or oxygen is used to break down lignin.
- Brightening: Hydrogen peroxide or ozone is used to further whiten the pulp by oxidizing remaining chromophores.
(b) What are the prospects of the paper industry in Pakistan?
The paper industry in Pakistan has significant potential for growth due to the increasing demand for packaging and paper products. However, the industry faces challenges such as dependency on imported raw materials, lack of advanced technology, and environmental concerns related to deforestation. Development of non-wood raw materials and modernizing production processes can improve the industry’s prospects.
