Introduction to Entropy (College Board AP® Chemistry)

Study Guide

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Entropy (S) in a system reflects the variety of ways particles and their energy can be arranged
- It is a measure of the system's disorder or randomness
When a system becomes more disordered, its entropy goes up
- This rise in entropy signals increased energetic stability
Take the thermal decomposition of calcium carbonate (CaCO₃) as an example:

CaCO₃(s) → CaO(s) + CO₂(g)

The creation of a gas molecule (CO₂) adds disorder to the system
The production of two product molecules compared to one reactant molecule adds disorder to teh system
The constant movement of CO₂ molecules makes them more disordered than the original solid (CaCO₃)
This results in a higher overall entropy
Consider another example: when a solid transitions to a liquid, like the melting of ice:

H₂O(s) → H₂O(l)

In the solid state, water molecules in ice are fixed in position with limited movement
In the liquid state, these particles are more randomly arranged, allowing for freer motion
As a result, the liquid state is more disordered, leading to an increase in entropy compared to the solid state
In both cases, systems with higher entropy are energetically preferred because a disordered state allows for a more evenly spread distribution of energy

Melting a solid will cause the particles to become more disordered resulting in a higher entropy state

For reactions involving reactants and products that are both in the gas phase, the entropy generally increases when the total number of moles of gas-phase products is greater than the number of moles of gas-phase reactants

In which reaction would you expect the largest decrease in entropy?

Answer:

All four reactions have the same number of molecules as reactants and products, which means that the number of molecules will not be a consideration for this question
Reaction 1 will show an increase in entropy as one of the reactants is a liquid but all products are gases
- Therefore, reaction 1 cannot be the correct answer
Reaction 2 will show a decrease in entropy as the three gaseous reactants form one solid and two gaseous products
Reaction 3 will show no change in entropy as all of the reactants and products are gases
- Therefore, reaction 3 cannot be the correct answer
Reaction 4 will show a decrease in entropy as the three gaseous reactants form one gaseous and two liquid products
Between reactions 2 and 4:
- Reaction 4 will have the biggest decrease in entropy as there is a bigger change in the disorder of the particles for this reaction

the (exam) results speak for themselves:

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