Lesson Notes By Weeks and Term v5 - Grade 12

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Performance objectives

• Explain the Haber-Bosch and Ostwald processes.
• Describe the production of phosphate fertilisers.
• Discuss the environmental impacts of fertiliser use.
• Apply stoichiometry to fertiliser production calculations.

Lesson summary

Fertilisers are essential for modern agriculture, especially in South Africa, where we strive for food security. Our population relies heavily on agricultural produce, making the efficient production and use of fertilisers critical. The fertiliser industry is a cornerstone of our economy, providing jobs and contributing to export earnings. Understanding the chemical processes involved in fertiliser production allows us to appreciate the complexities of this industry, its environmental impact, and potential for sustainable development.

Lesson notes

2.1 The Haber-Bosch Process for Ammonia Production Ammonia (NH₃) is a crucial ingredient in many nitrogen-based fertilisers. The Haber-Bosch process is the primary industrial method for synthesising ammonia from nitrogen and hydrogen.

Reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH < 0 (Exothermic)

Explanation: Source of Nitrogen: Nitrogen is obtained directly from the air via fractional distillation.

Source of Hydrogen: Hydrogen can be obtained from various sources, including steam reforming of natural gas (CH₄) or coal gasification. A common process involves reacting methane with steam: CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g) The carbon monoxide produced is then reacted with steam in the water-gas shift reaction to produce more hydrogen: CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g) The carbon dioxide is removed via absorption.

Reaction Conditions: The Haber-Bosch process requires specific conditions to achieve a reasonable yield of ammonia: Temperature: A moderately high temperature (around 400-450°C) is used. Although lower temperatures favor the formation of ammonia (due to the exothermic nature of the reaction, as predicted by Le Chatelier's Principle), the reaction rate is too slow at lower temperatures.

Therefore, a compromise is reached by using a moderately high temperature to balance equilibrium yield and reaction rate.

Pressure: High pressure (200-400 atm) is employed. According to Le Chatelier's Principle, increasing the pressure favors the side with fewer moles of gas. In this case, the forward reaction (forming ammonia) reduces the number of gas molecules (4 moles of reactants → 2 moles of product).

Catalyst: An iron catalyst, often with promoters like potassium oxide (K₂O) and aluminum oxide (Al₂O₃), is used to increase the reaction rate. The catalyst provides a surface for the reactants to adsorb and react, lowering the activation energy.

Le Chatelier's Principle: This principle states that if a change of condition is applied to a system in equilibrium, the system will shift in a direction that relieves the stress.

In the Haber-Bosch process: Increasing the concentration of reactants (N₂ and H₂) will shift the equilibrium to the right, favoring the formation of ammonia. Increasing the pressure will shift the equilibrium to the right, favoring the formation of ammonia. Decreasing the temperature will shift the equilibrium to the right, favoring the formation of ammonia.

However, as mentioned above, kinetics must be considered.

Example Calculation: Consider the Haber-Bosch reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) If 100 kg of nitrogen gas reacts completely with excess hydrogen, what mass of ammonia is produced? Calculate moles of N₂: Molar mass of N₂ = 28 g/mol = 0.028 kg/mol Moles of N₂ = mass / molar mass = 100 kg / 0.028 kg/mol = 3571.43 mol Calculate moles of NH₃: From the balanced equation, 1 mole of N₂ produces 2 moles of NH₃. Moles of NH₃ = 2 moles of N₂ = 2 3571.43 mol = 7142.86 mol Calculate mass of NH₃: Molar mass of NH₃ = 17 g/mol = 0.017 kg/mol Mass of NH₃ = moles molar mass = 7142.86 mol 0.017 kg/mol = 121.43 kg Therefore, approximately 121.43 kg of ammonia is produced. 2.2 The Ostwald Process for Nitric Acid Production Nitric acid (HNO₃) is essential for producing nitrate-based fertilisers. The Ostwald process is the industrial method for its manufacture, involving the oxidation of ammonia in two steps.

Reactions: Step 1: Oxidation of Ammonia 4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g) (Platinum-rhodium catalyst, 900°C)

Step 2: Oxidation of Nitric Oxide 2NO(g) + O₂(g) → 2NO₂(g)

Step 3: Absorption in Water 3NO₂(g) + H₂O(l) → 2HNO₃(aq) + NO(g)

Explanation: Step 1: Ammonia is mixed with air (source of oxygen) and passed over a platinum-rhodium catalyst at a high temperature (around 900°C). This oxidizes ammonia to nitric oxide (NO) and water. This reaction is highly exothermic, providing heat for the subsequent steps.

Step 2: The nitric oxide is cooled and further oxidized with more oxygen to form nitrogen dioxide (NO₂).

Step 3: The nitrogen dioxide is absorbed in water to form nitric acid. A small amount of nitric oxide is also produced as a byproduct, which is recycled back into the process to maximize the yield. 2.3 Production of Phosphate Fertilisers Phosphate rock, primarily consisting of calcium phosphate (Ca₃(PO₄)₂), is insoluble in water and therefore cannot be directly used by plants. It needs to be converted into a soluble form.

Reactions: Production of Superphosphate: Ca₃(PO₄)₂(s) + 2H₂SO₄(aq) → 2CaSO₄(s) + Ca(H₂PO₄)₂(aq) Phosphate rock is treated with sulfuric acid (H₂SO₄) to produce superphosphate, which is a mixture of calcium sulfate (CaSO₄) and monocalcium phosphate (Ca(H₂PO₄)₂). Monocalcium phosphate is water-soluble and can be absorbed by plants.

Production of Triple Superphosphate: Ca₃(PO₄)₂(s) + 4H₃PO₄(aq) → 3Ca(H₂PO₄)₂(aq) Phosphate rock is treated with phosphoric acid (H₃PO₄) to produce triple superphosphate, which is almost entirely monocalcium phosphate.