Entropy is a key thermodynamic concept in IB Chemistry Topic 5 and HL Topic 15. It helps explain why some reactions happen naturally while others do not. Although students often associate entropy with “disorder,” the real idea is deeper: entropy measures the number of possible ways particles can be arranged. Understanding entropy is essential for predicting spontaneity, interpreting Gibbs free energy, and analyzing equilibrium behavior.
What Is Entropy?
Entropy (S) is a measure of the disorder or randomness of a system, or more accurately, the number of possible microstates a system can have.
A system with high entropy has:
- More freedom of movement
- More possible arrangements
- Greater randomness
A system with low entropy is more ordered and restricted.
Entropy is a state function and is measured in J K⁻¹ mol⁻¹.
Why Entropy Increases
Entropy increases when the number of possible particle arrangements increases. This can happen due to:
1. Changes in state
Solids → liquids → gases
Entropy increases dramatically from solid to gas because particles gain more freedom.
2. Increase in temperature
Higher temperature means faster motion, more microstates, and higher entropy.
3. Increase in moles of gas
Reactions that produce more gas particles have higher entropy because gases have many possible arrangements.
4. Dissolving a solid in a solvent
Dissolved particles spread out and become more disordered.
5. Mixing substances
Mixing increases randomness.
Entropy naturally tends to increase in isolated systems (Second Law of Thermodynamics).
Examples of Entropy Changes
Melting ice (solid → liquid)
Particles gain freedom → entropy increases.
Boiling water (liquid → gas)
Huge jump in entropy because gas particles move freely.
Dissolving salt in water
Ions disperse → more disorder → positive entropy change.
Decomposition reactions
One reactant producing multiple products increases entropy.
Combustion reactions
Often produce several moles of gas → large entropy increase.
Entropy and the Second Law of Thermodynamics
The Second Law states:
Spontaneous processes increase the total entropy of the universe.
This doesn’t mean every system becomes disordered—local order can increase if the surroundings experience a larger entropy increase.
Example:
Freezing water decreases the entropy of water but releases heat to surroundings, increasing their entropy.
How Entropy Affects Spontaneity
Spontaneity depends on both enthalpy (ΔH) and entropy (ΔS), combined in the Gibbs free energy equation:
ΔG = ΔH − TΔS
A reaction is spontaneous if:
ΔG < 0
Entropy contributes through the term TΔS:
- When ΔS is positive, −TΔS becomes more negative
- This favors spontaneity
Therefore:
- Reactions that increase entropy are more likely to be spontaneous
- Reactions with large negative entropy changes are less favorable
Temperature also matters—high temperatures amplify the effect of entropy.
How to Predict Entropy Changes in IB Exams
Look for these clues:
1. Change in state
Gas has highest entropy.
2. Number of gas particles
More gas particles → higher entropy.
3. Dissolution
Solids dissolving generally increase entropy.
4. Molecular complexity
More atoms in a molecule → more vibrational motions → higher entropy.
Common IB Misunderstandings
“Entropy always increases in a reaction.”
Not true. Some reactions decrease entropy but still occur because enthalpy or temperature favors them.
“Entropy is just disorder.”
This is an oversimplification. Entropy is actually the number of possible microstates.
“A spontaneous reaction always has positive ΔS.”
Incorrect. A reaction can be spontaneous even with negative ΔS if ΔH is sufficiently negative.
“Entropy cannot be measured.”
Entropy values are tabulated and measurable.
Real-World Importance
Entropy explains:
- Why gases diffuse
- Why ice melts at room temperature
- Why some reactions need heat
- How refrigerators and heat pumps work
- Why certain reactions occur spontaneously only at high temperatures
It connects chemistry, physics, and real-world energy processes.
FAQs
Why do gases have the highest entropy?
Because particles move freely with maximum possible arrangements.
Does increasing temperature always increase entropy?
Yes—higher temperature increases particle motion and accessible microstates.
Can entropy decrease in a system?
Yes, but only if the surroundings gain even more entropy.
Conclusion
Entropy measures the level of disorder and the number of possible particle arrangements in a system. It increases with temperature, gas formation, changes in state, and mixing. Entropy plays a major role in determining spontaneity through Gibbs free energy and is essential for understanding many natural processes. Mastering entropy is crucial for success in IB Chemistry thermodynamics.
