8.2.4The Periodic Table
The Effect of Changing Pressure — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 4 parts Lesson 9 of 23 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Effect of Pressure
Pressure changes only affect systems containing gases.
Gases only
- Gases only Changing pressure has no effect on solids or liquids in an equilibrium, only on gases.
Higher pressure
- Higher pressure Increasing the pressure favours the side of the equation with fewer gas molecules.
Lower pressure
- Lower pressure Decreasing the pressure favours the side of the equation with more gas molecules.
No change
- No change If both sides have the same number of gas molecules, pressure has no effect on the position of equilibrium.
Section 2 of 4Worked Example: Nitrogen Dioxide
Dinitrogen tetroxide formation shows how pressure changes the colour of the mixture.
1
Consider 2NO₂(g) ⇌ N₂O₄(g), where the forward reaction turns 2 gas molecules into 1 gas molecule.
2
Increasing the pressure favours the forward reaction because it produces fewer gas molecules.
3
More colourless N₂O₄ forms, so the brown colour of the mixture fades and it becomes paler.
4
For H₂(g) + I₂(g) ⇌ 2HI(g), both sides have two gas molecules, so changing the pressure would not move the equilibrium.
Section 3 of 4Counting Molecules and Trade-offs
Pressure is chosen carefully in industry.
Counting Molecules and Trade-offs
Molecules
- Molecules Count molecules, not atoms, on each side of the balanced equation when deciding the effect of pressure.
Yield
- Yield A higher pressure gives a higher yield for reactions with fewer gas molecules on the product side.
Cost
- Cost High pressure needs strong, expensive equipment and more energy, which increases the running cost.
Safety
- Safety High-pressure plant is hazardous, so a pressure that is safe to handle is usually chosen.
Section 4 of 4Exam Checklist & Tips
8.2.4 – Key Points for the Exam
- Pressure The force exerted by gas molecules per unit area of a container wall.
- Equilibrium position The relative amounts of reactants and products present once equilibrium is reached.
- Dinitrogen tetroxide A colourless gas, N₂O₄, formed from two molecules of nitrogen dioxide.
- Compromise conditions Conditions chosen to balance yield, rate, cost and safety.
Common Mistakes
- Counting atoms rather than molecules when comparing the two sides of an equation.
- Stating that pressure changes the position of equilibrium when both sides have the same number of gas molecules.
- Describing pressure effects on solids or liquids within an equilibrium.
- Forgetting that high pressure raises cost and safety concerns, which is why a compromise is used.
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