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8.2.3The Periodic Table

The Effect of Changing Temperature — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 4 parts Lesson 8 of 23 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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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.
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Part 1 of 4Exothermic and Endothermic Directions

The effect of temperature depends on which direction of the reaction releases heat.

Opposite directions
  • Opposite directions If the forward reaction is exothermic, the reverse reaction is endothermic, and the two directions are linked.
Raising temperature
  • Raising temperature A higher temperature favours the endothermic reaction, so the yield of the exothermic product falls.
Lowering temperature
  • Lowering temperature A lower temperature favours the exothermic reaction, so the yield of the exothermic product rises.
Opposing change
  • Opposing change The system opposes a temperature rise by absorbing the extra heat in the endothermic direction.

Part 2 of 4Worked Example: Nitrogen Dioxide

Nitrogen dioxide and dinitrogen tetroxide show the effect of temperature clearly through colour.

1
Nitrogen dioxide and dinitrogen tetroxide are in equilibrium: 2NO₂(g) ⇌ N₂O₄(g).
2
Forming N₂O₄ from NO₂ is exothermic, so the reverse reaction, which breaks N₂O₄ back into NO₂, is endothermic.
3
Heating the mixture favours the endothermic reverse reaction, which produces more brown nitrogen dioxide.
4
The mixture therefore becomes darker brown when heated and paler when cooled.

Part 3 of 4Answering Temperature Questions

A clear method earns the marks in an exam.

Answering Temperature Questions
Step one
  • Step one Decide whether the forward reaction is exothermic or endothermic from the information given.
Step two
  • Step two Decide which direction the temperature change favours, using the endothermic reaction for a rise.
Step three
  • Step three Link the direction to the product formed more, and to any colour or yield change.
Marks
  • Marks Mark schemes reward an explanation of which reaction is favoured and why, not just the word opposes.

Part 4 of 4Exam Checklist & Tips

8.2.3 – Key Points for the Exam

  • Exothermic A reaction that releases energy to the surroundings.
  • Endothermic A reaction that takes in energy from the surroundings.
  • Yield The amount of product obtained from a reaction.
  • Dinitrogen tetroxide A colourless gas, N₂O₄, that is in equilibrium with brown nitrogen dioxide.

Common Mistakes

  • Saying the equilibrium moves to lower the temperature without naming the reaction that absorbs heat.
  • Assuming the reverse of an exothermic forward reaction is also exothermic; it is endothermic.
  • Stating that heating always increases the yield of the product; it favours the endothermic direction.
  • Forgetting to explain which reaction is favoured and why, which is where the marks are given.

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