Introduction to Entropy (College Board AP Chemistry)

• 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

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