15.2.1 Entropy

• The entropy (S) of a given system is the number of possible arrangements of the particles and their energy in a given system
  • In other words, it is a measure of how disordered or chaotic a system is

• When a system becomes more disordered, its entropy will increase
• An increase in entropy means that the system becomes energetically more stable
• For example, during the thermal decomposition of calcium carbonate (CaCO₃) the entropy of the system increases:

CaCO₃(s) → CaO(s) + CO₂(g)

• • In this decomposition reaction, a gas molecule (CO₂) is formed
  • The CO₂ gas molecule is more disordered than the solid reactant (CaCO₃), as it is constantly moving around
  • As a result, the system has become more disordered and there is an increase in entropy

• Another typical example of a system that becomes more disordered is when a solid melts
  • For example, melting ice to form liquid water:

H₂O(s) → H₂O(l)

• • The water molecules in ice are in fixed positions and can only vibrate about those positions
  • In the liquid state, the particles are still quite close together but are arranged more randomly, in that they can move around each other
  • Water molecules in the liquid state are therefore more disordered
  • Thus, for a given substance, the entropy increases when its solid form melts into a liquid

• In both examples, the system with the higher entropy will be energetically favourable (as the energy of the system is more spread out when it is in a disordered state)

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