Grade 13 — Term 3

Industrial Chemistry and Environmental Pollution

What you must know

• Which principles does green chemistry emphasize?: Green chemistry principles emphasize atom economy, waste prevention, energy efficiency, safer solvents, and the use of renewable raw materials.
• Greenhouse effect: greenhouse gases (CO₂, CH₄, N₂O, CFCs) absorb outgoing thermal infrared radiation, driving global temperature increase.
• Water pollution parameters: Biochemical Oxygen Demand (BOD) measures dissolved oxygen consumed by microorganisms to decompose organic waste.

Try one question

In the industrial Contact Process for the manufacture of sulfuric acid, sulfur dioxide is converted to sulfur trioxide via the reversible exothermic reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) (ΔH = -196 kJ mol⁻¹). Why is an operating temperature of approximately 450 °C utilized rather than a much higher or much lower temperature?

1. A. Higher temperatures shift the equilibrium toward sulfur trioxide but cause dangerous high-pressure explosions
2. B. 450 °C is an optimum compromise: lower temperatures favor equilibrium yield but have unacceptably slow reaction rates, while higher temperatures shift equilibrium to the left
3. C. 450 °C is the exact boiling point of sulfur trioxide, enabling continuous condensation
4. D. Higher temperatures decompose the vanadium(V) oxide catalyst into inactive vanadium(II) oxide

Show the answer

Answer: B. 450 °C is an optimum compromise: lower temperatures favor equilibrium yield but have unacceptably slow reaction rates, while higher temperatures shift equilibrium to the left. Because the forward reaction is exothermic (ΔH < 0), by Le Chatelier's principle, lower temperatures shift the equilibrium to the right, producing a higher percentage yield of SO₃. However, at low temperatures, the kinetic reaction rate is too slow for commercial viability, even with a V₂O₅ catalyst. At higher temperatures, the reaction proceeds rapidly but the equilibrium shifts left, significantly reducing the yield. An intermediate temperature of ~450 °C is an optimum compromise providing an economic reaction rate with ~98% conversion efficiency.

A model question written by Idasara on the exam pattern. It is not from a past paper.