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4.4.4Unit 4. Rates, Equilibria and Further Organic Chemistry

Equilibrium Constant and Temperature — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 4 parts Lesson 32 of 56 Unit quiz

📚 IAL Edexcel Linear (Chemistry) Hub / Topical Revision
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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Part 1 of 4The Direction of Change

Temperature is the only condition that changes the numerical value of K.

Exothermic reaction
  • Exothermic reaction Raising the temperature favours the reverse, endothermic direction, so K decreases and less product is present at equilibrium.
Endothermic reaction
  • Endothermic reaction Raising the temperature favours the forward reaction, so K increases and the equilibrium mixture contains more product.
Lowering temperature
  • Lowering temperature The opposite changes occur: K rises for an exothermic reaction and falls for an endothermic reaction.
Rate versus yield
  • Rate versus yield A higher temperature always raises rates, so an exothermic process needs a compromise temperature.
Reaction typeTemperature riseEffect on KProduct yield
Exothermic (ΔH negative)IncreaseDecreasesDecreases
Endothermic (ΔH positive)IncreaseIncreasesIncreases

Part 2 of 4Why It Happens

Energy considerations explain the shift.

1
When the temperature rises, the system absorbs heat to reduce the effect of the change, opposing the rise.
2
Absorbing heat means the endothermic direction is favoured, whichever way that is written in the equation.
3
The new balance between forward and reverse reactions gives a different ratio of products to reactants.
4
Because K is defined as that ratio at equilibrium, the value of K itself changes with temperature.

Part 3 of 4Applying the Idea

Applying the Idea
Ammonia synthesis
  • Ammonia synthesis The Haber reaction has ΔH = −92 kJ mol⁻¹, so a lower temperature gives more ammonia at equilibrium.
Industrial compromise
  • Industrial compromise About 450 °C is used because lower temperatures would make the rate far too slow to be economical.
Sketching trends
  • Sketching trends For an exothermic reaction a graph of K against temperature falls steadily as the temperature increases.
Stating answers
  • Stating answers Always state the reaction is exothermic or endothermic, the direction favoured, and then the effect on K.

Part 4 of 4Exam Checklist & Tips

4.4.4 – Key Points for the Exam

  • Exothermic A reaction that releases heat to the surroundings.
  • Endothermic A reaction that absorbs heat from the surroundings.
  • Equilibrium constant K Value that changes only when temperature changes.
  • Haber process Industrial synthesis of ammonia from nitrogen and hydrogen.

Common Mistakes

  • Saying K does not change when temperature changes.
  • Mixing up the effect on K for exothermic and endothermic reactions.
  • Assuming a high yield means a high rate, or the opposite.
  • Forgetting to state which direction is endothermic when explaining a shift.

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3 & IGCSE —

Kc

  • : Kc! (ΔH < 0) Kc. (ΔH > 0) Kc.
  • : ( ).
  • : Kc .
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