4.2.1Unit 4. Rates, Equilibria and Further Organic Chemistry
Entropy – Introduction — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 19 of 56 Unit quiz
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.
Part 1 of 5What Entropy Means
Entropy helps explain why some reactions happen even though they absorb energy.
Definition
- Definition Entropy, S, measures how many ways the particles and their energy can be arranged, often described as the disorder of a system.
Units
- Units Entropy is measured in J K⁻¹ mol⁻¹, a smaller unit than the kJ used for enthalpy.
Natural direction
- Natural direction Systems tend to change to states with more ways of arranging energy, which means higher entropy.
Standard entropy
- Standard entropy S° is the entropy of one mole of a substance at 298 K and 100 kPa, and is always positive for substances at this temperature.
Part 2 of 5Entropy and State
Entropy and State
Solids
- Solids Particles are fixed in an ordered lattice, so solids have the lowest entropy.
Liquids
- Liquids Particles can move and are more disordered, so liquids have higher entropy than solids.
Gases
- Gases Particles move freely over a large volume, so gases have the highest entropy.
Example
- Example Water has S° of about 70 J K⁻¹ mol⁻¹ as a liquid and about 189 J K⁻¹ mol⁻¹ as a gas.
| Process | Entropy change |
|---|---|
| Melting | Increases |
| Boiling | Increases greatly |
| Dissolving a solid | Usually increases |
| Condensing | Decreases |
Part 3 of 5Other Factors
1
Raising the temperature gives particles more energy and more ways of sharing it, so entropy increases.
2
A reaction that increases the number of gas moles, such as the decomposition of calcium carbonate to CaO and CO₂, increases the entropy.
3
A larger or more complex molecule has more ways to store energy than a small one, so entropy is generally higher.
4
Mixing two gases or dissolving a solute increases entropy because particles can be spread over a larger volume.
5
At 0 K a perfect crystal has an entropy of zero, which is the reference point for standard values.
Part 4 of 5Predicting Signs
Gas formed
- Gas formed If a reaction produces more gas moles than it uses, ΔS is positive.
Fewer particles
- Fewer particles If a reaction ends with fewer particles, such as 2H₂ + O₂ → 2H₂O(l), ΔS is negative.
Same state
- Same state If the number of gas moles is unchanged, the entropy change is small and hard to predict.
Link to enthalpy
- Link to enthalpy Entropy is one part of the question of whether a reaction is feasible, and enthalpy is the other.
Part 5 of 5Exam Checklist & Tips
4.2.1 – Key Points for the Exam
- Entropy A measure of the disorder of a system, or the number of ways of arranging its particles and energy.
- Standard entropy The entropy of one mole of a substance under standard conditions.
- Disorder The degree to which particles are randomly arranged and energy is spread out.
- ΔS The change in entropy in J K⁻¹ mol⁻¹.
- Spontaneous change A change that has a natural tendency to occur.
Common Mistakes
- Giving entropy units of kJ K⁻¹ mol⁻¹ and then combining it with enthalpy without conversion.
- Saying that elements have zero standard entropy; only a perfect crystal at 0 K does.
- Assuming entropy always increases in an exothermic reaction.
- Describing entropy only as 'messiness' without linking it to arrangements of particles and energy.
Last updated:
\n