Entropy & Phase Changes
- 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)
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- 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)
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- 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
Increasing entropy in a system
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
Worked example
In which reaction would you expect the largest decrease in entropy?
- CH4 (l) + 2O2 (g) → CO2 (g) + 2H2O (g)
- CH4 (g) + 2O2 (g) → CO2 (s) + 2H2O (g)
- CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)
- CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
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